BE537802A - - Google Patents

Description

       

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   The present invention relates to the roasting of metal sulphide concentrates by the fluidized bed technique, and relates to improvements made to the method and apparatus for treating slurries based on metal sulphides in heating furnaces. grilling which work according to the principle of fluidized layers or beds.



   More particularly, this invention relates to the introduction into the interior of a fluidized bed grilling oven of slurries of metal sulphides which may advantageously contain particles of sludge and / or water-soluble materials in such a manner that the agglomerating action of sludge and water-soluble material becomes an advantage, rather than a disadvantage, for the treatment based on the fluidised bed technique. In other words, a particular object of the invention is to prevent the formation of solid masses of exaggerated dimensions which might otherwise accumulate in the fluidized bed and form, instead of such masses. , very small aggregates, which has many process and processing advantages.



   Until now, the metal sulphide slurries have normally been forced inside the fluidized bed grilling oven through a horizontal feed tube disposed in the wall of the oven at a point either below. the surface of the turbulent bed, i.e. just above this surface. Since, in either case, the dehydration of the slurry must take place in the fluidized bed itself, it is on the turbulence of the bed that the disintegration of the individual particles on which depends. the masses of slurry as the drying of these masses takes place.

   In other words, as long as water is the only agent of union of the slurry, its disappearance by evaporation in the roasting bed leaves the individual particles free to place themselves in the bed like separate elements, thereby providing the bed with new material whose particle size distribution is essentially the same as that of the slurry.

   Thus, when the normal substantially sludge-free sulphide float concentrates are fed to the roasting bed in the form of aqueous slurries in which the content of water-soluble matter does not exceed that which normally occurs in the form of reactants. of residual float and impurities, the conversion of the slurry into separate solid particles is uninterrupted and, although this results in relatively smooth operation of the broiler, dust entrainment is often excessive.



   On the contrary, if the slurry subjected to the treatment contains certain types of sludge-giving material and / or water-soluble matter, such as one or more salts, the slurry, even after complete drying, does not disintegrate. not in the manner described above and a high proportion of the new material thus introduced agglomerates; the aggregates thus formed tend to increase in volume and to remain in the bed in the form of too large aggregates which reduce its fluidity and render the treatment inoperative.

   In other words, sludge and water-soluble matter behave like adhesive substances binding the grains and aggregates together in the slurry masses, thus preventing the particles from being broken up. by the turbulent action of the fluidized bed, even after complete dehydration and subsequent scorching.



  Not only does the adhesive action of the above-mentioned materials take place between the grains or particles of the fresh slurry, but it can also cause agglomeration between the freshly introduced slurry and the particles of which the fluidized bed in which this slurry is composed. is added. It follows that such an uninterrupted supply of slurries of this kind to the bed causes the formation of increased proportions of agglomerates in the form of too large aggregates, as opposed to the desirable size of the par-

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 ticles of fluidized beds used for sulphide roasting, namely about 4.75 mm.

   It is thus evident that the formation, within the fluidized bed, of increasing proportions of aggregates having more than about 4.75 mm can only result in a final reduction in the fluidity of the bed. , which ends the toasting operation.



   The formation of large lumps agglomerated in the manner described above is further aggravated by either of the two conventional methods by which the feeding of slurries of the type referred to is effected. invention. For example, when metal sulphide slurries containing sludges or salts are introduced into a fluidized bed at a point below the surface of that bed, sulphides tend to settle and form growths. hard and partially burnt on the feed nozzle. Such growths frequently cause clogging of the feed tube and obstruct the smooth operation of the grill oven. A more serious drawback, however, lies in the effect of these growths on the fluidity of the layer.

   This is because they break and accumulate in the bed with the other pieces that are too large, which tends to reduce the fluidity of the bed. On the one hand, if the slurry is introduced horizontally into the furnace at a point above the surface of the fluidized bed, a considerable proportion of the slurry simply falls or drips or runs off. from the orifice of the feed tube, which is necessarily placed near the wall of the grill oven. The part of the bed adjacent to this zone is a little less turbulent than the center of the layer, due to a so-called "wall" effect, and it is therefore particularly disadvantageous for the slurry to fall there.

   In addition, as the gases surrounding the supply nozzle have a temperature close to that prevailing in the bed during roasting, the slurry during dripping causes the gradual deposition of a mass of solid and hard material on the surface. the nozzle, thus forming a solidified mass or protuberance which falls into the bed, the turbulence and fluidity of which are further reduced. Over time, repeating this process removes any turbulence from the bed in the area adjacent to the feed nozzle, and the result is the formation of a large crude sulphide bank which extends from the bed to the bottom. feed tube and increasing in volume from the wall, or edge of the batch, to the center of the bed, as operation continues. This inactive slope decreases the effective area of the hearth and, consequently, the working capacity of the furnace.

   Introducing compressed air together with the slurry into the feed tube does little to remedy this state of affairs and often has the additional disadvantage of causing the slurry to move at high speed. sufficient for it to strike the opposite wall of the furnace, where it forms hard outgrowths of crude sulphides which, finally, fall into the bed and effect a further reduction in its fluidity.



   The deleterious effects of sludge or water soluble materials, such as salts contained in the sulphide slurry introduced into the fluidized bed grilling oven, are cumulative, since the turbulence of the bed is the only factor. which would allow the growths or agglomerates initially having exaggerated dimensions to be broken up when the slurry is introduced directly into the bed. The presence of these growths in the fluidized bed has the effect of reducing or even destroying its fluidity and turbulence, which not only results in an increased tendency to form a greater quantity of agglomerates. , but also by a reduction in the possibilities of disintegration of such agglomerates once formed.

   Such cumulative effects ultimately suppress the fluidity of the bed, and it becomes necessary to stop the oven to clean it.

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   Research has led to the discovery that it is possible to largely overcome such difficulties experienced in furnace operation, and to obtain other advantages, by proceeding in accordance with the present invention.



   The invention contemplates roasting the metal sulfide concentrates in a fluidized bed in which a slurry is prepared by mixing the metal sulfide concentrates and slurries with an aqueous liquid. The slurry is projected a sufficient distance into and through the hot free space above the fluidized bed to produce a large number of very small wetted aggregates, formed of concentrate particles bound by the sludge and liquid in the slurry. If the excess aqueous liquid in the slurry separates from it as a result of the formation of the wetted aggregates, this liquid vaporizes completely in the free space above the fluidized bed, and its vapor, entrained by the hot gases rising from said bed prevent the liberated liquid from reaching and wetting the fluidized material.

   The freshly formed aggregates follow a fairly long path, inside the free space surmounting the bed so that their residence time in this space ensures the elimination of their humidity, which separates from said aggregates by evaporation. while they travel the path in question through the free space above the bed, in contact with hot gases, which has the effect of drying the agglomerates and preventing them from reaching and humidifying the layer or fluidized bed .

   The resulting mixture of vapors and gas escapes from the upper part of the free space above the bed, and the dried aggregates fall onto the bed to be roasted there. The aggregates of the fluidized bed are thus continuously maintained at the bed. dry state, which prevents them from uniting with each other to give too large pieces which could reduce the fluidity of the beds The dried aggregates are roasted in an autogenous manner in the fluidized bed and generally retain the same size and the same physical form throughout the roasting operation, which ensures uniform fluidity of the bed.



  The resulting calcined aggregates are removed from the fluidized bed.



   According to a presently preferred embodiment of the invention, a slurry feed tube passes through an opening in the top wall of the fluidized bed roasting furnace and is directed towards the center of the surface of this layer. It is thus possible, if necessary by having recourse to a stream of compressed air in the same direction, to impart to the stream of slurry such speeds as to result in the formation of a finely divided jet, without risk of it. crude sulphurous growths form on the walls of the furnace. The aggregates thus formed from the finely divided slurry then have a very large surface area relative to their volume, during their descent through the hot gases rising in countercurrent of the free space of the furnace surmounting the bed. .



  These factors promote the dehydration of the slurry aggregates during their fall and exclude any tendency for these aggregates to unite with each other to form too large pieces by the time they reach the bed. The dried aggregates are thus obtained in the form of particles varying in size within the limits of a large scale, resulting in a new fluidized bed material which is extremely satisfactory in maintaining fluidity. In addition, when the slurry feed tube is held in an upright position, the slurry does not tend to drip and thus form deposits in the form of hard material growths on the end of the slurry. tube, which thus remains clean at all times, allowing regular and continuous operation of the toasting oven.



   The drying characteristic of the slurry aggregates obtained according to the invention will be better understood when compared with a two-stage process which would consist in subjecting a sulphide slurry to

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 a distinctly distinct drying treatment, before feeding it into the broiler as dry solids, One way to achieve drying would be to spray the slurry in a finely divided state vertically downwardly inside of a drying tower, by spraying, in countercurrent with the hot gases rising inside the tower, and to collect the dried material at the base of the tower. The dried material would then be transferred to the fluidized bed roasting oven using a mechanical or pneumatic conveyor.

   According to the present invention, the head space of the fluidized bed grilling oven is used as a drying tower, which is advantageous from the viewpoint of simplicity of operation.



   The binder property possessed by the slurry or water-soluble matter contained in the slurry can be exploited when sulphide slurries containing such materials are introduced into the furnace from its top, in that the slurries particles of the sprayed slurry in the divided state and the aggregates newly formed during the descent of said particles through the free space of the furnace behave as collecting cores to which the dust which is conveyed by the rising gases adheres, these gases thus returning said dust to the fluidized bed.

   This mutual action which occurs between the particles of the sprayed slurry in the divided state and the freshly formed aggregates on the one hand, and the dust conveyed by the gases of the oven on the other hand, seems to play a doubly advantageous role in the roasting process, as the adhesion of dusts to slurry and wet aggregates not only effectively remove these dusts, as such, from the plant, but also form relatively dry surfaces on the aggregates slurry, preventing them from agglomerating further before they have been completely dehydrated as they descend through the headspace.



   Although the invention relates to the fluidized bed roasting of metal sulphide concentrates in general, research has established that the present process is particularly applicable to the treatment of concentrates obtained from the ores of nickel, copper, zinc, gold, iron, etc. ,,,
One of the major sources of nickel today is Canada's well-known Sudbury ore. The metals of this ore, and more especially nickel, cobalt, copper and iron, are present in the form of sulphides. The nickeliferous constituents of the ore consist largely of a pyrrhotite which contains about 0.5 to 1% nickel (based on the weight of the pyrrhotite), in a form which cannot be separated from pyrrhotite by known mechanical treatments.



  The presence of this pyrrhotite fraction necessarily reduces the nickel analysis of a batch for grilling ovens to such a low value that it becomes unusable and causes a corresponding increase in the amount of iron that is present. necessary to remove in the form of slag in the smelting operation,
In one of the applications of the present invention, most - if not all - of the nickel, cobalt and copper contained in a concentrate consisting primarily of the above-mentioned type of pyrrhotite is brought to a state which allows the extracting the metal as an extremely useful water soluble sulfate leaving a residue of iron as a water insoluble oxide which is also very useful and which can, if desired,

   be of sufficient purity to be suitable for use as a source of high quality iron ore. Iron can be extracted separately by known methods, as can nickel, cobalt and copper.

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   Pyrrhotite concentrates can be obtained from nickel-bearing ore by any of the well-known methods, but the particle size of the nickel-bearing sulphide must be such that the so-called "roasting effect" is substantially avoided. core ", which will be described later. Research has shown that this effect can be eliminated to a great extent by making use in the process of a concentrate of which at least 90% passes through. a 74 micron mesh screen.



   Research and trials have also established that in order for the nickel contained to be brought to a water soluble state, it is necessary that a salt of an alkali metal such as sodium be in contact with the particles. of pyrrhotite while their roasting takes place. Sodium sulfate appears to be the most suitable reagent to be used for this purpose, due to its abundance.



   A slurry is prepared by mixing the salt solution and the concentrate particles to obtain a paste of the consistency most suitable to facilitate feeding the slurry to the roasting oven. The slurry is loaded into a roasting oven. fluidized bed. Although the concentrate particles are dispersed throughout the slurry charged to the oven, they tend to clump together as small aggregates.



   This occurs as follows: the slurry is delivered under pressure, from a reservoir, using an injection tube buttressing into the free space of the upper portion of the furnace. The slurry is caused to penetrate into the furnace in the form of a divided jet, obtained either by making use of an injection tube terminated by a nozzle or spray nozzle of suitable construction, or by mixing. compressed air with the mixture in the injection tube. In either case, it is necessary for the slurry to penetrate the upper part of the free space present in the oven in the form of a shower of droplets of slurry, the largest of which must not. not exceed the maximum tolerable size of the particles of which the fluidized bed is composed.



   The desired size of the aggregates can be achieved by suitably adjusting the variable factors involved at this stage, including the consistency of the slurry, the diameter of the injection tube, the pressure and the volume of compressed air injected into the tube. . During the extremely rapid drying stage, the saline solution is brought to a concentration such that it behaves as a binder capable of forming in situ agglomerates of concentrate particles in the form of small aggregates.



  The thus newly formed aggregates are kept in suspension in. free space for a sufficient time to be perfectly dried, - before reaching the fluidized bed. The agitation or turbulence of the bed, and consequently of the aggregates, is sufficient to prevent them from adhering or sticking to each other, and it is thus certain that the size of the particles of the bed will be included within the interior of the bed. most favorable scale, not exceeding about 3 mm.



   The grilling of the fluidized bed of the dry aggregates formed in situ is carried out in a favorable temperature scale. Excellent results have been obtained at a temperature above about 600 C and not exceeding about 700 C. The reactions of the roasting process are carried out in an autogenous manner and the desired temperature is obtained by adjusting the rate of d. 'feeding and, if necessary, injecting water into the free area.



  The quantity of air admitted to the fluidized bed should be at least that required by the flow rate of the feed corresponding to the desired stoichiometry, and tests indicate that satisfactory calcined products can be obtained when the rate of air intake exceeds this amount by up to 300%.

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   The particular advantages of applying aggregates of pyrrhotite concentrate particles agglomerated by a sodium sulphate backbone are evident when considering the behavior of such aggregates in a fluidized bed. . With regard to the fluidified bed from the mechanical point of view, each of the aggregates is a separate entity and, as such, is subjected to the uniformity of temperature and of the gaseous medium resulting from the constant turbulence of a fluidized bed. On the other hand, the particles of the aggregate remain in contact with the same salt skeleton constituting the union agent throughout the duration of the treatment, such prolonged contact being essential to allow the sodium salt to play a satisfactory role. as a chemical reagent.

   Despite the effectiveness of the salt as a binder of particles in an individual aggregate, the aggregates do not tend to adhere to each other during the treatment of fluidized bed roasting. In other words, the aggregates do not clump together in masses, and thus a reduction in the fluidity of the lite is avoided.
The selective sulphate roasting of nickeliferous pyrrhotite is based on the thermodynamic relationships that exist between the sulphates of metals present in a concentrate rich in this mineral. When roasting is carried out under practical conditions, the thermodynamic stabilities of the metal sulphates decrease in the following order: cobalt, nickel, copper and iron.

   As these stabilities are direct functions of both the temperature and the partial pressures of the sulfur oxides present in the system, proper adjustment of these factors can cause the formation of a system in which the sulfates of cobalt, nickel and copper are present. thermodynamically stable, whereas those of iron are not. In addition, if the pyrrhotite and other metal sulphide minerals contained in the ore are made to be subjected to the roasting reactions at a satisfactory rate, in the same system the end products of said reactions are calcined products composed mainly of iron oxide and sulphates of cobalt, nickel and copper. In other words, iron is made insoluble in water, while nickel, cobalt and copper are made soluble in water.



   Cobalt and copper sulphides in pyrrhotite concentrates can very easily be converted to their sulphates, and thus made soluble in water, when such concentrates are roasted under appropriate conditions of temperature and gas medium. Nickel sulphide shows a strong tendency to avoid the formation of sulphates, as has been mentioned. Its refractory nature appears to be due to causes such as the following
1) The ability of nickel sulphide to give rise to relatively stable nickel fer- rites of varying composition;
2) The "core roasting" effect whereby the nickel of an iron-nickel sulfide particle tends to concentrate in the residual sulfide core of the hot particle as the roasting takes place.



  With the gradual increase in the proportion of nickel in the residual sulphide, there is a decrease in the tendency of the nickel sulphide to participate in the necessary roasting reactions. This is due, in part, to the dense and more strongly protective nature of the iron oxide which forms during the later stages of the oxidation of the particle. The effect is a direct function of the size of the particle. In other words, the smaller the particle of concentrate is, the weaker the "core scorch" effect, and this assures a substantially complete conversion of the nickel sulfide contained in the particle to sodium sulfate. desired nickel, to the mesh.

   Such conversion is ensured if it is

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 makes use of extremely fine concentrates, the particle size of which is, for example, of the order of that previously specified.



   Although the principle of particle size reduction is generally applied when it is desired to increase the rate of the reaction which takes place between solids and gases, the object of the present invention is not so much to increase the rate of reaction between solids and gases. reaction rate than to prevent the selective oxidation of nickel-containing iron sulphides, which results in "core enrichment".

   The heart enrichment effect is cumulative. This is because when the iron in a concentrate particle is removed from the nickel sulfide to form an iron oxide layer, the core of the particle tends to concentrate the nickel so as to produce a ratio of nickel to nickel. iron approaching the ratio which, during oxidation, would give rise to dense protective layers of nickel ferrite between the nickel sulphide and the nickel oxide layer.



  In addition, sulphides rich in nickel are much more difficult to oxidize than sulphides poor in nickel.



   An even more important advantage of applying pyrrhotite particles in an extremely divided state is their ability to be brought to a state conducive to their agglomeration, which is essential to the practice of this invention. As will be seen later, this tendency of finely ground pyrrhotite concentrates to form agglomerates is enhanced by the binding action of the added sodium sulfate.



   In general, tests indicate that the concentration of salt additions required appears to depend to some degree on the nickel content of the pyrrhotite. The main chemical function of sodium sulphate is to make nickel ferrites unstable. This result is probably effected by an exchange between sodium sulphate and nickel ferrite, in a way according to the reaction expressed by the following hypothetical equation
NiFe2O4 + Na2SO4 = Na2Fe204 + NiS04
It emerges from this equation that the effective behavior of sodium sulphate depends to a great extent on the intimate contact between this salt and the sulphide particles during roasting,

   as well as maintaining a gas-temperature relationship which ensures the conditions under which nickel sulfate remains stable. Thus, although the necessary amount of sodium sulfate added to the pyrrhotite will depend on the concentration of nickel, the portion of the sodium sulfate which is effective in the above reaction is limited by the extent to which this portion remains in contact with the ore particles during roasting. As will be seen later, this limit is reached with additions of about 5% Na2SO4, based on the weight of the dry pyrrhotite concentrate. Beyond these proportions, further additions are ineffective due to particle surface limitations.



   Sodium sulfate also behaves as a support or, in a sense, a catalyst, for the sulfation reaction. The sodium sulphate Na2S0, contained in the oven, borrows the SO3 resulting from the oxidation of SO2 as it passes mixed with excess air through the bed.
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  Na2S04 + S03 Na80?
Sodium pyrosulphate is thus formed, which in itself constitutes a strong sulphating agent.



   The stability of pyrosulfate is an inverse function of the temperature.

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 rature and a direct function of the SO3 content of the surrounding gases.



  From a qualitative point of view, the same conditions apply for nickel, cobalt and copper sulphates. For the sake of simplicity, we can consider the following two equilibrium reactions:
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 (1) Na2SO4 + SO3. > Na2S207 (2) N10 S03> - NiS04
Nickel sulfate is more stable than sodium pyrosulfate indicated by reactions. It can be assumed that one of the agglomerates, for example during its roasting treatment, is subjected to varying conditions of temperature and SO 3 concentration or, in other words, to conditions which are not. always favorable to the formation of sulphates.

   It seems reasonable to assume that, although the agglomerate is located in a zone favorable to sulfation, reactions (1) and (2) both progress to the right to give rise to both sodium pyrosulfate and nickel sulfate. When such an agglomerate arrives in a less favorable zone, reaction (1) is reversed, which restores the SO3 content necessary for reaction (2) to progress to the right. In this way, the sodium sulfate acts as SO3 would act, that is, as a reserve or "stabilizer" which ensures an adequate local concentration of the sulfating agent. Of course, this action is in addition to the role of sodium in preventing the formation of fer- rites.



   The fines resulting from the roasting can advantageously be returned to the fluidized bed. The fines themselves can be separated in any suitable manner from the gas mixture rising from the bed, for example by passing the mixture through one or more cyclones. The fines collected in the cyclones can be brought back inside the bed, which is obtained, for example, by pushing them back inside the bed using a wrapped screw conveyor. Returning fines to bed offers several important benefits. First, it helps to maintain in the bed a small but essential proportion of fine material to ensure conditions favorable to maintaining fluidity. Secondly, it makes it possible to enrich the fluidized bed with additional quantities of sulphation gas.

   These gases result from the decomposition, at bed temperature, of iron sulphates which have been generated during the passage of dust through the colder zones of the system, such as the headspace of the furnace above the bed, and in the conduits leading to the dust collection device and passing through this device. Third, the dust particles which have been removed from the treatment by the fluidized bed due to their premature entrainment by the gases are returned to this bed for further scorching and sulphation treatment.



   As previously indicated, the calcined products resulting from selective sulphate roasting are agglomerates which essentially retain their original geometric identity. These agglomerates can be subjected to subsequent handling operations, including washing with water, without losing their shape or hardness. All of the water soluble material can be removed therefrom by aqueous treatment, without the need to subject them to any further treatment, such as grinding or other form of disintegration.

   This constitutes a distinct advantage over the conventional type of calcined product obtained from finely ground nickel sulphides, in that in all subsequent operations of the process, such as leaching, filtration and drying , are greatly facilitated by the presence of relatively low proportions of fine material.

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   These characteristics of the invention, and others, will be better understood by the description given below with reference to the accompanying drawings, in which:
Figure 1 is a schematic side view, in partial vertical section, of one form of apparatus illustrating one embodiment of the invention;
Figure 2 is a section taken on line 2-2 of Figure 1, through the oven floor;
Figure 3 is a section through line 3-3 of Figure 2 and shows, seen from above, the surface of the fluidized bed resting on the hearth;
Figure 4 is a section on line 4-4 of Figure 2 showing a wind box placed below the sole and a series of air distributors disposed above the sole;

   
Figure 5 is an enlarged detail of Figure 4 showing some of the air distributors immediately adjacent to an exhaust pipe extending from the hearth and terminating in a wind box.



   Reference will first be made to FIG. 1. The apparatus shown comprises a pipe 10 with a slurry supply valve, opening into the upper wall of a constant level tank 12 which is provided in its upper part with 'an overflow pipe 14 serving to ensure the con- stance of the level 16 of the slurry 18 inside the tank, From the bottom of the tank 12 leaves a discharge pipe 20 ¯ on which a pump is mounted. slurry 22 forcing the slurry into a pipe 24 with a tap provided with a connector 25. An air pipe 26 connects an air compressor 28 to the slurry supply pipe. The slurry feed pipe is provided with a vertical branch which passes through a removable cover 30 of a hood 32 mounted centrally around a manhole 33 at the top of a roasting oven 34.



   The fluidized bed roasting oven shown schematically is generally rectangular in elevation and cross section, this apparatus comprising side heat-insulated walls (Figures 2 and 3), indicated at 36 and 38, extremes 40 and 42, and a upper wall 44 (Figure 1).



  The hearth 46 of the toasting oven (Figures 1 and 2) comprises three removable plates 48, 50 and 52, on the upper face of which are secured a series of air nozzles spaced from one another and extending upwards. .



   The bottom of the furnace ends with a wind box or air distributor 56 (Figures 1 and 4) which has a generally triangular shape in vertical section (Figure 4). This box is composed of side walls 58 and 60 and end walls 62 and 64. An air distribution pipe 70 (Figures 1 and 2), provided with a valve 72, extends along the opposite side. of the box. Outlet connections 74, 76 and 78 connect the air distribution pipe 70 to three compartments 82, 84 and 86 formed by partitions 88 and 90 (Figures 1 and 2). The inlet connections are provided with tap counters 92, 94 and 96 (figure 2) for regulating and measuring the quantities of air introduced into the compartments.



   The sole plates 48, 50 and 52 (figure 2) extend across the compartments and are respectively provided with discharge pipes 100, 102 and 104, which are arranged centrally and the upper end of which rises slightly above the floor. above the plates (figures 4 and 5). The discharge pipes descend vertically through compartments 82, 84 and 86, and below said compartments, where they are provided with valves 106, 108 and 110 respectively. Near its lower end, each of the com-

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 Partiments is provided with a discharge pipe 114 with a tap (Figure 4) through which the discharge of solids takes place.



   The roasting apparatus is provided (Figure 1) with a pipe 116 for discharging the calcined products, which passes through the end wall 40 and occupies a position suitable for maintaining a favorable predetermined level 110 of a fluidized bed 120 formed. of the agglomerated material subjected to the treatment below the head space 122. A fine and gas discharge pipe 124 connects the upper portion of the apparatus to a fine and gas separator 126, such as a cyclone , the latter being able to be advantageously connected to a second separator 128 of fines and of gas, such as another cyclone, by a pipe 130. A chimney 132 starts from the second cyclone and opens into the open air.



   It is advantageous to return the fines, separated from the gases of the apparatus in the cyclones, to said apparatus, with a view to a new treatment.



  For this purpose (Figure 1), downpipes 134 and 136 run from cyclones 126 and 128, respectively, to a side feed tube 138 provided with a mechanical conveyor screw 140. The discharge end of the tube connects to the lower portion of fluidized bed 120.



   The air nozzles 54 can be of any suitable construction.



   Those of the present apparatus shown in more detail in Fig. 5 take the form of ordinary pipe couplings or fittings 144, mounted on the upper ends of tubes 146 attached in the manner shown to plates 48, 50 and 52. Their lower ends are fixed in the holes 148 distributed at axis to axis distances of 10 cm in the plates 48, 50 and 52, as shown. The upper ends of the tubes 146 fit into vertical portions 150 of the fittings.



  The upper ends of said vertical portions are closed by plugs 152; andthe lower ends of the inclined portions 154 of the fittings are provided with like plugs 156, each of these plugs however having a small hole or central jet passage 168 allowing very fine streams or jets of air to escape downwardly into it. a direction making an acute angle with the vertical. Although this has not been shown in the drawing, the air nozzles all point in the same direction in current practice.



   As the passages 158 face downward, fines from the fluidized bed of material cannot fall into and obstruct these passages.



  In addition, as the plugs 156 are placed above the base plates, the material located near said plates is not influenced by the fluidizing gases and remains static when the calcined products are discharged through the pipe d. The drain 116. This results in the formation of a dead layer 160. This is an advantage because this layer acts to insulate the plates. As will be indicated later, this layer can moreover be considered as composed of an upper layer 162 made of aggregates of normal size, and of a lower layer 164 containing larger aggregates, if such aggregates are here. The two layers tend to vary with respect to each other during operations both in size and contour.



   The oven is equipped with one or more burners 180, 182, such as oil or gas burners, serving to heat the chamber to bring it to the temperature suitable for starting the grilling operation. . These burners are advantageously placed just above the surface of the bed 120,
It is extremely desirable to be able to regulate the temperature of the operation, both inside the bed and in the free space above.

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 both the lite As the roasting takes place autogenously, the bed gives off exothermic heat which rises with the roasting gases within the free space. Some control of the bed temperature can be made by varying the rate of slurry feed from above and the rate of air introduction below the furnace floor.

   In order to reduce the temperature of the bed, it has been proposed to wet it also with water, a stream of water being poured onto the bed for this purpose or introduced inside the bed. This has the disadvantage, however, that the wet material tends to clump together and hence reduce the fluidity of the bed.



   This drawback can be avoided by introducing one or more streams of water into the free space and causing all the water thus introduced to evaporate before it has reached the bed.



   To this end, the apparatus (Figure 1) may comprise, for example, a main water supply pipe 190, provided with a tap and a fitting 192 which connects the pipe to a lateral distribution pipe 194, itself provided with tap-type supply connections 196, 198 and 200. The side connections 196 and 200 extend vertically through the peripheral portion of the manhole cover 30 with their free ends 202 and 204 facing towards the end. 'exterior so that streams of water 206 and 208 projected from these pipes can be directed away from, and generally along the sides, of the tapered slurry stream projected from the feed pipe 24 The water or liquid composition (and hence the viscosity) of the slurry is not significantly changed when operated in this manner.

   The middle branch pipe 198 is connected to the slurry supply pipe 24 so that, if desired, additional water can be added to the slurry to cool the head space. The construction shown has the advantage that the fittings 25 and 192 allow the slurry and water branch pipes to be dismantled as a block with the cover. Water supply pipes may open into the free space at any other suitable place from its upper portion, provided that the projected water has sufficient time, during its descent, to evaporate before it drains. 'reach the fluidized bed.



   Closable openings (not shown) are provided in the walls of the chamber, at various levels, for the introduction of a calcined material suitable for facilitating the start-up, for the insertion of thermocouples for use. record and regulate the temperature in the bed and in the free space above it, for observation, inspection, etc.



   The apparatus described can be operated as follows:
Pre-calcined aggregates of concentrate are introduced into the apparatus through one or more of the closable openings (not shown) so as to constitute a priming or starting bed.



  Such a bed is necessary to initiate the toasting operation which will follow.



  In other words, the material to be grilled must be mixed in the appliance with material which has already been grilled. Before introducing the crude sulphide concentrates, the start-up bed must be brought to a temperature high enough to allow the roasting operation to maintain itself. To this end, the oil or gas burners 180, 182 (Figure 1) are used to provide the preliminary heat. When the bed has been brought to the temperature necessary to start the combustion of the sulphides, these burners are closed.



   Slurry 16 is formed by mixing the concentrates with an aqueous solution of sodium sulphate, then this slurry (Figure 1) is passed inside tank 12 through feed pipe 10. A sufficient quantity of slurry is introduced into this tank to produce

 <Desc / Clms Page number 12>

 to prevent the continuous discharge of the slurry through the overflow pipe 14 in order to maintain the mass of slurry contained in the tank at a predetermined level 16. The pump 22 is operated at a predetermined constant speed, while the compressor 28 supplies air, under a predetermined pressure, to the slurry supply pipe 24.

   The variable factors which intervene in the operation are regulated so that the slurry admitted by the supply pipe spreads out and is projected downwards in the form of a jet inside the free space 122. As this space is strongly heated, the sprayed slurry in the divided state is subjected to immediate drying which causes agglomeration of the concentrate particles during their passage through said space.



   It may be useful to consider in more detail the facts relating to the slurry 18, from the moment it is admitted through the pipe 24 and begins its journey to the bed 120. As the free and open end of the pipe 24 descends vertically. within the free space 122, at the top of this space, the slurry is projected downwardly and laterally - - at least initially - in the form of a conical stream through the high temperature roasting gases s' raising lita As the porridge spreads, it divides into a multitude of tiny droplets or globules.



  Some of these globules are drops of liquid released from the porridge; others are concentrate particles joined together by liquid retained by or contained in the slurrya This part of the path traveled by the slurry can conveniently be considered to include zone A. Due to the high temperature prevailing in space free, some of the liquid liberated from the slurry and part of the liquid retained in the slurry vaporizes in this zone, but said zone can be considered essentially to be the part of the free space in the coverslip the sprayed slurry is. divided into fine droplets.



   Droplets descend into and through a region which may be designated as B. In this region a large proportion, if not all of the liquid liberated from the remaining slurry evaporates and is discharged out of the free space by pipe 124; and wetted aggregates composed of the concentrate particles form in situ as it travels through the zone. Part of the slurry liquid retained by the aggregates is separated by vaporization.



   The remainder of the liquid released or retained by the slurry evaporates as the wet aggregates enter and pass through what may be called Zone C, located immediately above the fluidized bed. In this zone, the aggregate falls through the rising roasting gases, which have approximately their maximum temperature at this point and are therefore perfectly dried before reaching the bed.



   The above description is of course only a general application of the probable process, but there is obviously no clear demarcation between the zones, which can moreover be arranged overlapping. In all cases, the path followed by the slurry considered as such and by the resulting droplets and aggregates, through the free space, is sufficiently long or extensive to allow ample time for the liquid, both released and retained by the liquid. porridge, to evaporate, and aggregates to become perfectly dry before they get to bed. In other words, there is no possibility for the liquid to reach the bed, either through the slurry or otherwise.



  All the liquid in the porridge vaporizes between the entrance to the oven and the bed; its vapors mix with rising roasting gases; and the mixture of gases and vapors continuously escapes from the upper portion of the head space through pipe 124, cyclones 126 and 128 and the

 <Desc / Clms Page number 13>

 chimney 132, which discharges it into the open air.



   In addition, the liquid in the slurry and the wet or damp aggregates play another very useful function during their short journey, namely the interception, collection and return of a very large proportion of dust or fines. powdery in the oven bed. The, slurry and aggregates tend to behave like a moving filter. The dust particles are carried away by the liquid in the slurry. The entrained dust may in turn coalesce and bind to wet concentrate aggregates, or even convert to aggregates of dust which fall onto the bed. A significant proportion of the dust particles come into contact with the falling wet or damp concentrate aggregates and adhere to them in such a way that they often become an integral part thereof.

   Those dusts which are not returned to the bed create a path to the discharge pipe 124 and to the cyclones 126 and 128, but the dusts which are entrained are perfectly dehydrated on their way to the bed.



   All the conditions are thus fulfilled to ensure the formation and maintenance of a layer of aggregates in what may be denoted by zone D, and this prevents, in spite of the presence of moisture, the agglomeration. merger of the aggregates in the form of pieces of exaggerated size which would gradually have the effect of reducing the fluidity of the bed. This is why it is important that the free space is high enough, so that the period of suspension of the slurry and the falling aggregates is long enough to allow the evaporation of all the liquid or moisture they contain. . To this end, it is preferable to exaggerate in the sense of too great a height of the free space compared to the height which would normally be sufficient to effect the desired dehydration.



   As shown in Figure 3, the dried aggregate in the course of the inlet to bed 120 meets the upper surface of that bed in a sharply defined generally circular area, indicated at 186, this being readily seen at through a look when the bed is in a calm state. Said circular zone is formed by projecting the charge for a few seconds through the free space, while the bed is in a state of rest, and allowing the dry aggregates to ignite on the hot surface of the bed. dead. The glow or glow of the freshly deposited aggregates delimits the impact zone of the jet and makes it possible to adjust the means of adjustment of the injection tube, etc., in order to obtain the most favorable conditions as regards concerns the shape and arrangement of the jet.



   Pressurized air is simultaneously passed through the air distribution pipe 70 (figure 2) and the outlet connections 74, 76 and 78 into the compartments 82, 84 and 86 of the wind box or the air distribution box. Air 56. The proportions of air thus introduced into the various compartments can be varied by means of tap meters 92, 94 and 96, if desired, or the volumes of air admitted can be kept substantially equal. The air thus admitted to the compartments escapes therefrom through the numerous air nozzles 54 disposed above the sole plates 48, 50 and 52, in the form of an equal number of fine streams.

   These numerous air currents keep the finely divided aggregates contained in the bed in a turbulent state, which facilitates the mixing of the new aggregates with the old aggregates and ensures intimate air contact with the aggregates. In fact, each of the aggregates is enveloped and supported by air and by the gaseous products of the reactions resulting from roasting, which establishes a favorable environment for the oxidation of the sulphides contained in the particles of concentrate.

 <Desc / Clms Page number 14>

 



   In full operation, calcined aggregates are removed from the bed as quickly as freshly formed aggregates from the sprayed slurry settling on the bed. The calcined products gradually create a path to the discharge pipe 116 and finally fall by gravity into this pipe which ensures their evacuation.



   The roasting operation requires a considerable amount of air and therefore gives rise to a large amount of sulfur gases S02 and SO3. All the water in the slurry is converted to water vapor. The total of air, gases and water vapor represents a very large volume which is continuously withdrawn from free space. Some fines are necessarily produced, although in relatively small amounts, during the roasting operation. Part of the proportion of fines to the particle size of a powder tends to be entrained by the gases. The resulting mixture of gas and fines passes (Figure 1) through pipe 124 into first cyclone 126, where a significant proportion of the heavier fines separates.

   The gases thus partially stripped of the fines which they convey enter the second cyclone 128, in which a large proportion of the lighter fines separate. The gases leave the last cyclone in the series to pass into the chimney 132 which discharges them into the open air.



   It is preferable that the separated fines are returned to the roasting area. To this end, these fines can fall through downpipes 134 and 136 into the feed tube 138, where they are propelled by a screw conveyor 140 which feeds them into the bed 120 of the apparatus.



   The calcined products which are continuously generated play an important role in the chemical reactions which occur during the selective roasting operation considered as a whole. The iron oxide of the products calcined at high temperature of the turbulent bed behaves as a catalyst which effects a rapid conversion of the freshly formed SO2 gas into SO3 gas and thus contributes significantly to the conversion of nickel sulphides (cobalt, and water insoluble copper in the water soluble sulfate form.



   In the event that excessively large pieces are formed by fusion, either during the initial formation of the aggregates above the bed or during their roasting in the bed, these pieces tend to descend to the base of the bed. and, sooner or later, under the action of the very large number of fine air currents, a path is created in and through the upper layer 164 (Figures 1, 4 and 5), from which they pass to the The interior of the lower layer 160 and finally enters the discharge pipes 100, 102 and 104. The taps 106, 108 and 110 of these pipes can be opened from time to time to remove these oversized aggregates.



   Instead of removing calcined aggregate through the discharge pipe 116 (Figure 1), they can be removed through one or more of the discharge pipes 100, 102 and 104. If desired, the calcined products can be re-released. drawn both by the discharge pipe 116 and by one or more of the discharge pipes 100, 102 and 104.



   As is clearly indicated in Figures 1, 4 and 5, the injection openings or conduits 158 of the nozzles 54 are maintained at a good height above the level of the sole plates 48, 50 and 52, for the purpose of to prevent turbulence of the calcined products on or near these plates, whether or not the material is composed of too large aggregates or not. The dead layer 160 replaces the conventional type of heat-insulating refractory (brick or moldable refractory) and has the advantage over it of being easy.

 <Desc / Clms Page number 15>

 to remove sole plates, which facilitates the dismantling of this part of the oven for repair, maintenance, etc.



   The irregular lower layer 160 (Figures 1, 4 and 5) should be considered as generally representing the configuration possessed by the part of the general static layer which may be partly composed of too large aggregates, especially if it is It is the discharge pipe 116 which is used for the discharge of the calcined aggregate. The conical depressions that form when large aggregates are removed through tubes 100,102 and
104 fill with regular calcined products which sink from the bed itself and come to a stop. When large aggregates form again, these aggregates sink toward the bottom plates, thus repeating the process of moving the regular calcined products.



   The results just described can be facilitated by appropriate adjustment of a number of variable factors such as, for example, the pressure of the air jets, the section of the openings of the nozzles. injection, their angle of inclination, the direction of said nozzles relative to each other, etc. The nozzles can be rotated in a horizontal plane by rotating the fittings 144, the tubes 146, or both.



   When, as is seldom the case, it is advisable to remove solids which have created a path inside compartments 82, 84 and 86, this material can be dropped from them. -same (figure 4) in the discharge pipes to valve 114.



   As indicated above, a certain control of the temperature prevailing in the free space and, consequently, of the temperature of the bed, can be effected by spraying water, more especially into said free space. To this end (Figure 1), one or more streams of water can be projected, using the inlet connections 196 and 200, into the upper portion of said free space, preferably laterally to the conical stream formed by the slurry sprayed in a divided state by means of the slurry supply pipe 24, so as not to significantly alter the composition of the liquid of the slurry, especially as regards its salt content.



  It is also possible, if desired, to cause the water projected in the divided state to strike the divided jet of slurry at any suitable stage of its descent. In addition, if desired, an additional amount of water can be introduced into the slurry through the middle supply branch 198, thereby diluting the slurry with a refrigerant before it is sprayed in the divided state. in free space. One or more of the expedients listed above can be used. The path traveled by the water introduced through the free space is sufficiently extended to ensure the evaporation of all of this water before it reaches the bed, the latter therefore not being wetted by the water. The water vapor dissolves into the other vapors and gases which escape through pipe 124.

   The specially added water thus directly cools the head space and indirectly cools the bed. It also acts to remove by filtration the dust contained in the rising gases of the device.



   According to a currently preferred practical embodiment, the calcined products resulting from the selective sulphate roasting are subjected to a leaching treatment in a continuous countercurrent settling installation. The addition of acids is neither necessary nor desired for the intended purpose. The solids are collected separately and dried. The liquid or filtrate is also collected separately.



   Of course, the liquid or filtrate contains the water soluble sulphates of nickel, cobalt and copper and can be treated either

 <Desc / Clms Page number 16>

 any of the known manners for the purpose of collecting collectively or separately the quantities of nickel, cobalt or copper which it contains. The sulphate solution can for example be treated with a sodium carbonate solution to precipitate nickel, cobalt or copper in the form of carbonates. The resulting co-precipitates are properly filtered and leave a filtrate rich in sodium sulfate, which is reused as a binder or sulfating agent to treat further amounts of concentrate. Other precipitating agents, such as lime, limestone and caustic soda, can be used.

   Various procedures can be applied to precipitate the metals, separately or in bulk. Sodium precipitating agents are particularly suitable because sodium can be converted to sodium sulfate to ensure an adequate supply of this salt for processing additional amounts of concentrates. As the operation is carried out, at least normally, in a nickel metallurgy plant, the co-precipitate can be roasted with a charge of a nickel-bearing material capable of forming a matte, which in turn is treated with the coating. usual manner in order to collect the nickel, cobalt and copper separately or to collect nickel and cobalt together.



   Leaching or depletion should be carried out carefully and with sufficient water to separate the water soluble sulphates from the material being treated. In this manner the final solid residues will be substantially free of nickel, cobalt and copper sulphates, thus leaving an agglomerated product almost free of sludge and consisting almost entirely of iron oxide. A clear-cut separation is thus obtained between nickel, cobalt and copper on the one hand and iron on the other.



   To determine the effect of sodium sulphate, as well as that of the method applied to add this material, on the extraction of the metal resulting from the sulphate roasting, a series of fluidized bed roasting tests were carried out on pyrrhotite concentrates of the following chemical composition:
 EMI16.1
 eu Ni S Fe Co Insolubles Si02 Ot3O 1.29 3494 5216 0.064 7e6 5 percent
In some of the tests, the pyrrhotite concentrate was roasted without the use of sodium sulfate or any other chemical reagent. In a second group of tests, the pyrrhotite was added with sodium sulfate so as to form a mixture of dry solids; and in a third group of tests, a proportion of sodium sulfate of the same order was added to the pyrrhotite concentrate as an aqueous solution.

   As the pyrrhotite concentrate used in each group of tests was identically the same in all respects, and as the conditions of the roasting were also identical, the average figures given below for nickel extraction for each group clearly highlight the importance of sodium salt on the selective sulphate roasting of pyrrhotite.
 EMI16.2
 
<tb>



  Test <SEP> No <SEP> Addition <SEP> of <SEP> Na2SO4 <SEP>% <SEP> of <SEP> Ni <SEP> extracted
<tb>
<tb>
<tb> 18 <SEP> none <SEP> 15.3
<tb>
<tb>
<tb> 9 <SEP> 10 <SEP>% <SEP> - <SEP> mix <SEP> sec <SEP> 70.8
<tb>
<tb>
<tb> 28 <SEP> 8 <SEP>% - <SEP> mixed <SEP> wet <SEP> 84.9
<tb>
 These figures relate to the proportion of the contained nickel which has been made soluble in water by the roasting of the pyrrhotite concentrate.



   The pyrrhbtite concentrate used in the above test group ---.

 <Desc / Clms Page number 17>

 above had been ground to a degree of fineness such that 60% of the material passed through a 74 micron mesh screen. After having subjected the same concentrate to a new grinding allowing the passage of 95% of the material through the same sieve, the treatment of the concentrate with an aqueous solution of sodium sulphate able to give a mixture containing 8% of the salt ( dry weight) followed by sulphate roasting in a fluidized bed oven, gave a calcined product from which 91% of the nickel content could be extracted by leaching with water.



   The efficiency of the agglomeration action previously described can be evidenced by the particle size distribution of the calcined products resulting from the fluidized bed roasting of a pyrrhotite float concentrate according to the new feeding technique. - tion to which the invention relates. The pyrrhotite concentrate used was subjected to further grinding such that 97% by weight of the dry concentrate passed through the 74 micron mesh sieve, the material therefore containing a high proportion of colloidal, or nearly colloidal particles, usually called "sludge".

   The finely ground pyrrhotite concentrate was mixed with proportions of water such that a slurry was obtained containing about 70% by weight of the dry concentrate and 30% of water, this concentrate being essentially free of water soluble materials. .



  The slurry was discharged into a fluidized bed grilling oven, using a pump, through a tube made of a normal section of 3.16 mm pipe which ran down vertically through the top wall of the oven.



  This slurry was introduced at a rate of approximately 2.25 kg per minute and
 EMI17.1
 was accompanied by compressed air which passed through the same tube and whose approximate flow rate was 112 dm3 per minute. The height of the free space of the furnace, i.e. the vertical distance between the top wall of the furnace and the bed, was approximately 3.5 meters. As the slurry feed tube protruding about 15 cm below the top wall of the oven, the slurry particles fell about 3.35 m.
 EMI17.2
 very before reaching the surface of the fluidized bed. The turbulence or fluidity of the bed was maintained by introducing air to the base of this bed at a flow rate of about 7 dm3 per minute and per dm2 of hearth surface.

   The temperature of the gases in the headspace was about 550OCy while that of the fluidized bed was maintained at about 6750C. The relatively small quantities of dust which escaped with the gases from the furnace passed through two stages of dedusting, of the cyclone type, and the dust thus collected was continuously returned to the fluidized bed by a screw conveyor. The calcined product was continuously withdrawn from the fluidized bed with a rate of removal such that the level of this bed remains substantially unchanged throughout the operation. The table gives the result of sieving tests of the calcined product.



   TABLE I.
 EMI17.3
 
<tb>



  Opening <SEP> of <SEP> mesh <SEP> Weight
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> + <SEP> 3.36 <SEP> mm <SEP> 0.2
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> + <SEP> 2.00 <SEP> 1.1
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> + <SEP> 1.41 <SEP> 3.8
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> + <SEP> 0.65 <SEP> 8.5
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> 0.65 <SEP> 13.9
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> + <SEP> 0.50 <SEP> 16.3
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> + <SEP> 0.30 <SEP> Il, 9
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> + <SEP> 0, <SEP> 23 <SEP> 9, <SEP> 6 <SEP>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> + <SEP> 0.15 <SEP> 9.4
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> - <SEP> 0.15 <SEP> (refusal) <SEP> 9,

  4
<tb>
 

 <Desc / Clms Page number 18>

 
The calcined products for which particle sizes in arbitrary distributions are given in Table I have been found to be ideal for maintaining good bed fluidity and for smooth and continuous operation of the fluidized bed roaster, although that the same sieving tests showed that the proportion of material sufficiently fine to be able to be entrained and conveyed by the gases of the furnace was relatively low.

   If we refer again to the sieving tests of the pyrrhotite feed (97% - mail opening, 74 microns), it is also evident that at least 75% of the feed was converted by agglomeration. in particles greater than 74 microns, while still allowing the formation of aggregates below 74 microns, indicating that slurries of these types would have had a strong tendency to give too large aggregates or pieces if they had been introduced below or directly above the surface of the fluidized bed, in the usual manner.

   On the other hand, if the finely ground pyrrhotite had been filtered, dried and introduced into the roasting apparatus as finely divided solids, the formation of aggregates would have been negligible, and the resulting calcined products, with their high proportions of fine material. , would have been liable to cause an abnormally high dust weight and consequently to cause relatively high dust losses.



   During the course of the chemical reactions which take place between the aggregates and the gases in the fluidized bed, each individual aggregate behaves physically as a separate unit, even though it would be composed of extremely fine particles of concentrate interstitially united by sludge. ; however, since, chemically, the fine particles of the aggregates are porously bonded to each other, they are capable of reacting as very fine discrete particles. In this way, the reacting system has all the advantages which one would normally expect from finely divided solids in their chemical reaction with the fluidized gases, without having the disadvantages which such a reaction usually entails, such as, for example, example, the high rates of dust loss.



   The application of the present invention to the sulphate selective roasting of nickel-containing pyrrhotite is logical in that the finely divided nickel-containing pyrrhotite concentrate can be treated with the proper proportion of sodium sulphate and an amount of water. sufficient to form a slurry containing, for example, 70% solids and capable of being pumped into the fluidized bed roasting furnace in accordance with the principle of the present invention.

   In one such operation, a slurry was prepared by mixing a concentrate of nickel-containing pyrrhotite, about 97% of which passed through a 74 micron mesh sieve and which contained high proportions of slurry, with a solution. aqueous sodium sulfate which was present in such proportions that the slurry contained about 70% pyrrhotite, 27% water and 3% sodium sulfate, all by weight. As in the previously described case, the slurry was introduced into the fluidized bed roasting oven through a feed tube made from a section of a normal 3.16mm pipe passing through the center of the top wall of the oven.

   The flow rate of the slurry stream was maintained between 1.8 and 2.25 kg per minute and atomizing compressed air was introduced with the slurry at a rate of 113 liters per minute. As before, the slurry particles sprayed in the divided state were subjected to a drop of about 3.35 meters, i.e. the vertical distance between the end of the feed tube and the surface of the bed. fluidized. Air was introduced to the base of the bed at a rate of about 7.3 liters per minute per dm2 of hearth area, which maintained the bed in a turbulent fluid state. The

 <Desc / Clms Page number 19>

 temperatures of the gases in the headspace and in the fluidized bed were 550 and 675 C.

   In this operation, the proportion of solids entrained by the oven gases in the form of dust was even smaller than that obtained in the operation previously described, in which no sodium sulfate was used. sodium.



  The collected dust was continuously returned to the fluidized bed and the calcined products were removed at a continuous rate compatible with maintaining a stable level of the bed. The calcined products resulting from this treatment gave the following results in the sieving test:
TABLE II.
 EMI19.1
 
<tb>



  Opening <SEP> of <SEP> mesh <SEP> Weight, <SEP>% <SEP>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> @ <SEP> 3.36 <SEP> mm <SEP> 0
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> + <SEP> 2.00 <SEP> 5.2
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> + <SEP> 1.41 <SEP> 7.8
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> + <SEP> 0.84 <SEP> 15.3
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> + <SEP> 0.65 <SEP> 19.7
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> + <SEP> 0.50 <SEP> 17.7
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> + <SEP> 0.30 <SEP> 10.2
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> + <SEP> 0.23 <SEP> 6
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> + <SEP> 0.15 <SEP> 5 '
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> - <SEP> 0.15 <SEP> (refusal) <SEP> 12,

  5
<tb>
 
A comparison of the arbitrary particle size distribution shown in Table II with that of Table I shows the extent to which the presence of sodium sulphate increases the agglomeration action of the sludge fraction contained in it. load, relative to the fluidized bed roasting oven. As in the previous case, the material of which the bed was composed, and which is represented by the sieving tests of Table II, was particularly suitable for the treatment in a fluidized bed, both from a mechanical point of view and. from the metallurgical point of view. The calcined products shown in Table II were leached with water in a countercurrent settling plant, and the following metallurgical results were obtained:
TABLE III.
 EMI19.2
 
<tb>



  Weight <SEP> Or. <SEP>% <SEP> Ni. '% <SEP> Co, <SEP>%
<tb>
<tb> Calcined <SEP> products <SEP> 1000 <SEP> g <SEP> '0.25 <SEP> 1.29 <SEP> 0.05
<tb> residue <SEP> after <SEP> leaching <SEP> 750 <SEP> g <SEP> 0.02 <SEP> 0.14 <SEP> none
<tb> extraction, <SEP>% <SEP> 94 <SEP> 92 <SEP> Ca <SEP> 100
<tb>
 
Comparison of the nickel extraction figure resulting from the roasting of a pyrrhotite containing about 4% sodium sulphate with the figures given previously indicates that the proportion of sodium sulphate can be considerably reduced compared to the previous figures of About 8%, without this causing an additional loss of nickel in the leaching residue.

   In other words, the treatment with an aqueous solution of sodium sulphate to which the pyrrhotite is subjected has the effect of obtaining a slurry for use as a filler for the roasting apparatus, in accordance with In principle, the effectiveness of the salt doubles by allowing very intimate contact between the salt and the pyrrhotite particles of which the individual aggregates are composed.



   The sieving tests of the leached residue gave the following results:

 <Desc / Clms Page number 20>

 TABLE IV.
 EMI20.1
 
<tb>



  Opening <SEP> of <SEP> mesh <SEP> Weight
<tb>
<tb>
<tb> + <SEP> 2.0 <SEP> mm <SEP> 0.4
<tb>
<tb> + <SEP> 1.41 <SEP> 0.4
<tb>
<tb> + <SEP> 0.84 <SEP> l, <SEP> 2 <SEP>
<tb>
<tb> + <SEP> - <SEP> 0.65 <SEP> 5 '
<tb>
<tb> + <SEP> 0.50 <SEP> Il, 1
<tb>
<tb> + <SEP> 0.30 <SEP> 15.2
<tb>
<tb> + <SEP> 0.23 <SEP> 14, <SEP> 8 <SEP>
<tb>
<tb> + <SEP> 0.15 <SEP> 11.1
<tb>
<tb> + <SEP> 0.1 <SEP> Il, <SEP> 5 <SEP>
<tb>
<tb> + <SEP> 0; 074 <SEP> 4.5
<tb>
<tb> - <SEP> 0.074 <SEP> (refusal) <SEP> 24.8
<tb>
<tb> 100
<tb>
 
The figures given above with regard to the distribution of the aggregate sizes of the leached residue show that this distribution is clearly advantageous, if one takes into account the leaching, washing and filtration operations which constitute a necessary part of all hydrometallic operations.

   While the initial charge of the roaster was in a state of subdivision so fine that 97% could pass through a screen with a 74 migron mesh size, the proportion of the final product having this fineness was less than 25. %. Thus, the advantage of a relatively dust-free operation in the fluidized bed roasting oven is obtained, by the present invention, in all stages of the processes for treating roasted products (leaching, washing and filtration). The above-described steps being a simple example, since calcined products are obtained which are easy to handle because they are composed of relatively large particles.



   Further advantages of the new technique of feeding sulphide slurries to a fluidized bed roasting oven over conventional methods by spraying them in a divided state downwardly through the oven from the furnace. oven top may be best understood by comparing how a single droplet of slurry descends through the hot gases contained in the oven chamber. When the droplet encounters the gases at the top of the chamber, it is rapidly brought to a temperature above the boiling point of the water or solution it contains. As vapor is therefore emitted from the droplet as it falls, the water vapor content of the furnace gases is greater in the upper regions of the chamber than at other levels of the free spar of the furnace.

   Therefore, the water vapor content of furnace gases such as; oxygen, SO2 and SO3, are higher at the bed surface than at the point where the slurry enters the furnace chamber. It can thus be seen that the gases which are the most effective, from the chemical point of view, in the roasting reactions which take place in the fluidized bed have not been diluted by water vapor if the elimination of the l The water contained in the slurry is carried out before it has reached the bed instead of only afterwards.



   The observations given above with regard to the variable diluting effect of water vapor, which is a product of the dehydration of the slurry aggregate as it descends through the headspace of the furnace, may be extended to the thermal dilution which results from the latent heat of vaporization of the water in the slurry and the heat capacity of the steam. It may be useful to consider, by way of explanation: (a) feeding a fresh porridge aggregate directly to the bed of

 <Desc / Clms Page number 21>

 fluidized roasting; (b) the fall of such aggregate through the free space gases before this aggregate meets the bed.

   In the first case, the water contained is vaporized at the temperature of the bed, the vapor thus formed borrowing a certain quantity of heat from the bed as it rises with the other gaseous products from the roasting oven. In the second case, the heat necessary to volatilize the water contained is borrowed from the gases after the latter have left the fluidized bed. In other words, the water in the slurry has lowered the temperature of the gases, while that of the bed remains unaffected.



   It is therefore obvious that the supply of a fluidized bed roasting furnace with sulphide slurries, carried out in accordance with the present invention, has the consequence of improving the heat balance, this advantage being added to those previously described. Such an improved heat balance is an important factor in certain processes which aim to achieve the selective sulphate roasting of ores or concentrates in fluidized bed roasting ovens, in that the minimum feed rate compatible with the temperature that the bed must have corresponds to the maximum average residence time of the solid particles in the fluidized bed.

   In other words, since, under normal working conditions, the rate of removal of calcined products from the fluidized bed is the same as the rate of introduction of the load, the time of stay or of average treatment of the solids subjected to the scorch varied to the reverse of the feed rate of feed, so that any means of reducing this feed rate increases the degree to which the reactions are pushed by increasing the time processing.

   It follows that, in the autogenous fluidized bed sulphide roasting processes to which the present invention relates, improvements in the heat balance brought about by the practice of the invention resulting in residence times. longer solids and, therefore, obtaining the calcined products which have been subjected to a further reaction.



   The water vapor resulting from the drying of a droplet of slurry also increases the total volume of the gases in the furnace and, at any given temperature, effects a corresponding increase in the space velocity of the rising gases. As the dust entrainment rate is a function of the linear space velocity of the furnace gases, it is important for the success of the operation that this space velocity be as low as is practically possible. The direct addition of the slurries to the fluidized bed obviously increases the space velocity and hence the dust load carried by the gases rising through the bed, whereas when the dehydration of the slurry takes place in the area above the bed, the space velocity of the gases inside the bed remains unaffected.

   In addition, the increase which the mass of gas contained in the headspace of the furnace is liable to undergo as a result of the production of water vapor derived from the slurry is not accompanied by a corresponding increase in volume. me and the space velocity of the gases, because of the opposite effect of the contraction which results from the cooling effect of the slurry on the gases.

   Thus, to the previously described advantages of the invention which relate to the effective elimination of dust as such, and which result from the fact that said dust adheres to the wet particles of the slurry, there is added the reduced ease afforded to the dust to leave the chamber. fluidized bed by the effect of gas entrainment
Further advantages of the invention over conventional methods of slurry feed to fluidized bed roasting furnaces will be evident to those skilled in the art taking into account the wide variety of material. sludges and tacky substances which may be present in sulphide slurries treated in accordance with the present invention.

   Through

 <Desc / Clms Page number 22>

 example, in copper roasting-leaching processes, in which it is necessary to carry out the sulphate-selective roasting of sulphide-based slurries in fluidized bed type furnaces, the presence of carbon oxide-based minerals. copper in the slurry has hitherto been a disadvantage due to the tendency of these minerals to undergo excessive pulverization in the grinding process, which gives rise to sludge, which itself causes the formation of coarse aggregates having the effect of reducing the fluidity of the screen bed. Such slurry oxide-based sulphide slurries can be easily and successfully processed using a fluidized bed roaster according to the present invention.



   Likewise, it may happen that such a slurry does not contain all the sulfur which is necessary, in practice, to form sulphates with all of the copper present, even though the thermal requirements would be satisfied, for which In this case, it might be desirable to compensate for the insufficient sulfur content by adding to the slurry a spent electrolyte which contains sulfuric acid and iron sulphate. In this case again, the addition of such solutions would be a hazardous undertaking if the mixture were to be introduced into the fluidized bed according to conventional methods, but it would be very advantageous in an operation according to the invention.



   Another, and perhaps more striking, means of applying the principles of the invention relates to the fluidized bed roasting of ores or concentrates which originally contain large proportions of sludge. For example, certain pyritic float concentrates containing lead, zinc and copper were mixed with about 0.75% sodium and subjected to selective sulphate roasting in a fluidized bed roasting oven.

   As the concentrates contained negligible amounts of sludge and only a relatively small amount of water soluble material had been added, only a very small degree of agglomeration occurred. grains between them, and the dust entrainment losses were relatively high. In addition, the fluidization characteristics of the bed thus obtained were poor. In this case, it has been found that it is really advantageous to add to the concentrate, as a sludge-forming material, a suitable clay substance, such as bentonite, in order to effect the union or agglomeration of the compounds. grains, in accordance with the present invention.



   The deliberate addition of sludge not only significantly decreases the proportion of bed material which is discharged as dust, but it improves metallurgical efficiency to a marked degree, as can be seen by comparing the figures in the table below, resulting from two experiments in which a pyritic concentrate of lead, zinc and copper was subjected to a fluidized bed roasting treatment, the calcined products being exhausted with the aid of a dilute sulfuric acid. The conditions of the two tests being identical, unless otherwise indicated.
 EMI22.1
 
<tb>



  Test <SEP> Bentonite, <SEP> Na2SO4, <SEP>% <SEP> Percentage <SEP> of extract
<tb>
<tb>
<tb>
<tb>
<tb> No <SEP>% <SEP> ¯¯¯¯¯¯¯ <SEP> Zn <SEP> Cu <SEP> Fe
<tb>
<tb>
<tb>
<tb> 13 <SEP> 0 <SEP> 0.75 <SEP> 72 <SEP> 73.3 <SEP> 13.4
<tb>
<tb>
<tb>
<tb> 14 <SEP> 2 <SEP> 0.75 <SEP> Si, 2 <SEP> 93, <SEP> 2 <SEP> 4
<tb>
 
The above figures show that the addition of 2% bentonite not only improved the extraction of the desired metals, but also resulted in a cleaner separation between zinc and copper on the one hand, and iron from the concentrate on the other hand.



   Not only can the invention be usefully applied to the field

 <Desc / Clms Page number 23>

 sulphate-selective roasting, but it is also advantageously applicable to processes in which it is sought to obtain simple oxidation. gold, partly in a directly pulverizable form and partly in a refractory or "blocked" form. In such a case, it is desirable to extract the directly pulverizable gold by subjecting the concentrate to extremely crushing. fine, followed by amalgamation or cyanide wash.

   The residue of such a treatment is then roasted in order to release the "blocked" fraction of the gold to allow its extraction by a second cyanide washing treatment, and this is in its application to the bed roasting. The present process is advantageous in fluidifying such materials containing finely divided sludge.



   The two examples above, among others previously described,
 EMI23.1
 seem to represent and illustrate quite typically there, diteri3iÏédî-enombreu-


    

Claims (1)

its other useful applications of the invention which will naturally occur to those skilled in the art. In the present description, the terms "mud" and "mud particles" are understood to mean particles of sulphide or oxide minerals, especially those of a clayey nature, the particles of which are extremely fine and have colloidal dimensions, or nearly colloidal, as well as materials generally referred to as "gelatinous hydrate precipitates", such as those produced by the addition of alkalis to solutions which contain salts of metals such as iron or aluminum The term "water-soluble material" is understood to mean any acidic or inorganic alkali salt which is soluble in water, as well as any water soluble mineral material which would be found to be advantageously additive to a sulfide slurry in the practice of this invention. SUMMARY. Process for roasting metal sulphide concentrates by the fluidized bed method, characterized by the following points, separately or in combinations: 1) It consists in preparing a slurry by mixing these concentrates with an aqueous liquid, in projecting the slurry in the divided state and over a sufficient distance into and through the free and hot space above the fluidized bed, so as to produce a large number of very small wetted aggregates which are each composed of several particles of concentrate bound, at least in part, by the liquid of the slurry, to be vaporized com- pletely within the free space, to the 'using the hot gases rising from the fluidized bed, the excess aqueous liquid which is released from the slurry as a result of the formation of the wet aggregates, in order to prevent the liquid thus liberated from reaching the fluidized bed and wetting it, to pass along a long path extending inside the hot free space the freshly formed aggregates, so that they have sufficient time to separate from their moisture, to evaporate the moisture from the aggregates as they follow their extended journey through the free space in contact with the hot gases to effect the drying of said aggregates and to prevent aggregates wet to reach and moisten the fluidized bed, to allow the resulting mixture of vapors and gases to escape into the open air from the upper portion of the head space, to knock down aggregates dried on the fluidized bed for roasting, to maintain the aggregates in the fluidized bed in a continuously dry state in order to prevent them from agglomerating in the form of too large pieces which would have the effect of reducing the fluidity of the bed, in autogenously roasting the aggregates dry in the fluidized bed, to maintain <Desc / Clms Page number 24> aggregates generally in the same size and shape during roasting to ensure bed uniformity, and to remove the resulting calcined aggregate from the fluidized bed. 2) The aqueous liquid used to prepare the porridge is water. 3) The water or other aqueous liquid used for preparing the slurry contains a salt, preferably an alkali metal salt such as, and preferably, sodium sulfate. 4) Compressed air is incorporated uniformly into the slurry before it enters the free space, in order to facilitate the projection in the divided state of this slurry. 5) The spraying of the slurry in the divided state is carried out entirely inside the free space in order to prevent the contact of said slurry with the upper wall and the side walls of the roasting chamber and, consequently, to avoid the formation of sulphide growths on said walls. 6) The prolonged course of the aggregates within the free space is obtained by projecting the slurry in the divided state through a relatively high chamber. 7) The extended run is obtained by projecting the slurry in the divided state and up and down from the upper central portion of a free space of relatively large height. 8) The temperature of the fluidized bed is regulated, at least in part, by adjusting the temperature prevailing in the free space. 9) The temperature prevailing in the free space is regulated by adjusting the quantity of aqueous liquid contained in the sprayed slurry in the divided state in the free space. 10) To adjust the temperature prevailing in the free space, in the divided state, regulated quantities of water are sprayed, independently and separately from that contained in the slurry, into said free space, the injected water being completely vaporized in said headspace so that it cannot reach and wet the fluidized bed, and the resulting water vapor being mixed with said vapors and gases escaping from the headspace. 11) The dust particles entrained by the hot gases rising from the fluidized bed are brought into contact with the sprayed slurry in the divided state and the freshly formed aggregates, and a substantial amount of dust particles adhere to the descending aggregates. inside the free space up to the bed, instead of escaping from said free space with the vapors and gases. 12) The concentrate particles are small enough to prevent "through-burn". 13) The concentrate particles are ground so that the majority of said particles pass through a 74 micron mesh size sieve to prevent "through-scorch". 14) Sludge is present in the slurry, at least in part, as a specially added clay material or bentonite. 15) The aggregates formed are of various sizes, but are small enough that the majority of said aggregates pass through a sieve with a mesh size of 4.75 mm. 16) The aggregates are small enough to pass, in the form <Desc / Clms Page number 25> of classified portions, through a series of sieves the mesh opening of which varies from approximately 4.75 mm to approximately 74 microns, so as to obtain a series of arbitrary sizes suitable for ensuring good fluidity of the bed. 1.7) Concentrates of metal sulphides for roasting include the following: sulphides of the nickel-bearing pyrrhotite type and other nickel-bearing sulphides; copper sulphides, including mixtures of the sulphide-oxide type; auriferous iron sulphides, including those containing arsenic; at- . highly iron sulphides, including those containing lead, zinc and copper; etc ..