CH208529A - A process for the continuous manufacture of chromic chloride by chlorination in the presence of a reducer of oxidized ores of finely divided chromium, which together contain other metals including iron, calcium and magnesium. - Google Patents

Description

  

  A process for the continuous manufacture of chromic chloride by chlorination in the presence of a reducer of oxidized ores of finely divided chromium, which together contain other metals including iron, calcium and magnesium. This invention relates to a process for the continuous manufacture of chromic chloride by chlorination in the presence of a reducing agent of oxidized chromium ores in the finely divided state, which together contain other metals including iron, calcium and magnesium. ,

   characterized in that the ore in the finely divided state is applied as a thin film coating on larger mobile particles which are themselves chemically inert and which, thus coated, are displaced through a zone of reaction in which these particles and the gaseous chlorinating agent move against the current, the ratio between the quantities of ore and of chlorinating agent introduced into said zone being such as for all times,

   the amount of chlorinating agent is greater than that necessary for all chlorinatable materials to be converted to their chlorides, the reaction being carried out at temperatures at which the chromic chloride separates from the ore by volatilization.



  This invention makes it possible to carry out the satisfactory chlorination of chromite ores, especially those of lower quality than presently acceptable for the production of ferroalloys, in a continuously operated furnace, such chlorination being accompanied by a subli mation of all (or practically all) the elements giving rise to volatile chlorides, chromium in particular.



  It has long been known that the pace or rate of the chemical reaction between solids and gases is accelerated when the degree of fineness of subdivision of the solid is increased, but until now it has not It has been possible to carry out such processes directly in shaft furnaces, because the presence of appreciable quantities of fine material in apparatus of this type tends to obstruct the passage of gases through the charge and makes it necessary to application of such high pressures, as well as of such high relative velocities between gas and solid, that dust difficulties are encountered.

   Hitherto it had been necessary either to resort to the concretion of fine ores, as is often done with ores such as iron, lead and zinc ores, or to endeavor to 'Ensure satisfactory contact by pouring the finely ground ores as a rain through a gas riser. The first of these means gives a permeable charge, but the concretion tends to cause the enlargement of the particles or even, in extreme cases, the formation of partially molten particles, and thus prevents taking advantage of the accelerating action of the particle. fineness of subdivision for chemical or metallurgical purposes. The application of the second means gives rise to a large number of practical difficulties.

   On the front line, the dust problem becomes extremely serious. Moreover, when it comes to carrying out reactions which involve relatively high temperatures and appreciable reaction times, the heat transfer necessary, if any, to satisfy possible endothermic conditions in the reaction becomes difficult and often. impractical in practice. Finally, appreciable contact times can only be ensured by constructing exaggeratedly tall apparatuses or by carefully sorting the introduced ore so that it can be kept exactly suspended on the gas stream.



  The Applicant, having attempted to apply methods known to the technician and described by patents such as, for example, the Adrian patents (see patent issued in the United States under No. <B> 1 </B> 434 -185 of November 7, 1922) to a vertical retort that was continuously worked, found that it is not possible to successfully generate and volatilize the chlorides by observing the conditions known to previous researchers and also specified well in the above patents than in others using the well known reaction of chlorine with oxide ores and reducing agents.

   After an in-depth study of the conditions of volatility of chromium chlorides, the Applicant has discovered how the intended goal of the sublimation of the volatile constituents during chlorination can be achieved in a countercurrent retort working in a manner. keep on going.



  The present process allows the use of shaft furnaces of the usual type while retaining the advantages of a large subdivision, at the same time avoiding the difficulties due to dust and nevertheless maintaining high relative velocities between the gas and the solid. treat. It further has other advantages in that it allows continuous operation of the countercurrent type even when certain constituents of the ore, or its impurities, are temporarily or necessarily converted into the liquid form.



  As chlorinating agent, not only chlorine can be used, but also gaseous substances at the reaction temperature capable of delivering chlorine.



  The following description relates more particularly to the use of free chlorine. The Applicant has discovered that in order to advantageously carry out the process according to the invention, it is desirable to control the chemical conditions defined by a series of complicated chemical mutual relationships existing between the various chromium chlorides and, at the same time, control the physical conditions under which the chlorination takes place.



  In the first attempts made to apply Adrian's conditions, as shown on page 1, line 40, through page 2, line 30 of U.S. Patent No. <B> 1 </B> 434,485 , in a continuous furnace, of the retort type, a chromite ore of a type common to. deposits found in the United States and which gave the following results on analysis:

      
EMI0003.0001
  
    Crz03 <SEP> 51.6
<tb> FeO <SEP> 19.6
<tb> SiO2 <SEP> 5.5
<tb> A103 <SEP> 11.8
<tb> Mgo <SEP> <B> 11.5% </B>
<tb> Ca0 <SEP> <U> 0.8 </U>
<tb> 100.8 When trying to chlorinate with free chlorine and sublimate an ore of this kind in a continuous countercurrent furnace, it is found that no continuous operation is possible with a load of briquettes or simply finely ground because certain liquid chlorides are formed which are not volatile, especially CrCl2, MgCl2 and CaCl2, along with solid CrCl '.

    The Applicant has discovered that the formation of CrCh can be avoided if certain chemical conditions are met, especially if the presence of an excess of at least 5.5% free chlorine in the gas is ensured. exhaust and removal of these gases at temperatures of at least <B> 890 '. </B> She further found that the primary reaction rate of chlorination is then so high at higher temperatures. at <B> 900 ',

  </B> that it would be physically impossible to carry out the application of the required quantities of chlorine and the elimination of the products when a charge of briquettes or finely ground ore in the normal state of the settlement due to gravity since the required gas flow rates would then be so great that exaggerated pressures would be created in the column and the charge would be forced out of the furnace.

   When the Applicant used ores of lower quality than that specified, in which case the chlorine requirements were lower, they encountered still further difficulties, in that the magnesium chloride which melts at 712 and boils at 1412, forms in a liquid state, which not only binds the fine ore particles together, but also covers them with a film of liquid magnesium chloride. In the case of a briquette load, this magnesium chloride penetrates into the briquette by capillary action. This accumulation of magnesium chloride almost completely stops the chlorination action, and very low extractions result.

   The Applicant has discovered that chlorine is incapable of penetrating thick layers or clumps of liquid magnesium chloride and that, when this state can exist, no chlorination is practically possible.



  The Applicant has discovered that all of these difficulties can be overcome and easy and efficient contact between fine ores and reactive gases in continuous countercurrent shaft furnaces can be achieved by arranging finely ground ore, or mixtures thereof. ore with reagents, in the form of a thin layer or film. This is achieved by depositing the ore on the surface of inert carrier particles, a permeable charge being maintained in the form of a film or semi-permanent coating of clearly controlled or limited thickness.



  In the accompanying drawings, FIG. 1 shows relationships which, as the Applicant has discovered, exist between the average particle size, expressed in millimeters, and the extraction in%, in the case of a chromite ore.



  Fig. 2 shows relationships between the time required for a complete reaction and the thickness of the ore layer.



  Fig. 3 is a schematic view, in partial vertical section, of an apparatus which has been used successfully by the applicant.



  Fig. 4 is a diagram showing the temperature at different points along the height of the retort.



  Fig. 5 is a diagram showing relationships between temperature and chlorine content of the exhaust gas for two different mines.



  Fig. 6 is a diagram showing the relationship between chromium tetrachloride and volatile chromium chloride with excess chlorine.



  In developing the present process, the Applicant has studied the effect of the size of the particles of the ore on the extraction by chlorination and sublimation of the aforementioned chromite ore and obtained the results shown in fig. 1. The average size, shown on the y-axis, is the arithmetic mean of the maximum diameters - measured in millimeters - of the particles retained during the sorting between two sieves of: different numbers of meshes.

   For example, a material designated as having an average size of 1.24 mm is that which passes through a No. 10 sieve (4 meshes per linear cm) but is retained by a No. 20 sieve (8 meshes per linear cm), then that a material which passes through a No. 200 screen (80 meshes per linear cm) is considered to have an average size of 0.05 mm. The percentage of the chlorinable portion of the ore contemplated which has been extracted by the action of chlorine is plotted on the abscissa.



  The curve of FIG. 1 shows that, in order to obtain extractions greater than <B> 90% </B>, it is necessary to use particles smaller than approximately 0.15 mm. The time needed to perform these extractions has not been specified, since, starting with a given amount of ore, the time is determined by the amount of chlorine that must be introduced per unit of time to be just sufficient to satisfy the stoichiometric conditions and the specified excess.

    The fact that this quantity of chlorine is supplied quickly or slowly only slightly modifies the slope of the line in fig. 1 and does not influence this finding of the manderesse that a high extraction can only be obtained if all the ore is crushed to pass at least through a 110 100 sieve and, preferably, finely enough to pass through. through a No. 200 sieve. This may seem a surprising result in comparison with various known products obtained in this industry, but it can be explained by the fact that the chlorination action is counterbalanced by the accumulation of non-vaporized chlorides. , especially magnesium.

   Thus, in the case of large particles, when a certain degree of reaction has taken place, the accumulation of non-volatile chlorides in the capillary ducts of the ore and on the surface stops the chlorination, while in the case of finer particles, with a greater ratio between surface and mass, a greater degree of chlorination takes place before the proportionate quantity of non-volatile chlorides causes the cessation of the action, resulting in a higher extraction.



       Having discovered that high extractions can only be obtained if the minerals have been finely ground, demand has also discovered that if finely ground ore is available on the surface of coarse granular carrier particles, so that When an ore film is exposed to the direct action of the gases, complete chlorination and sublimation can be obtained in a time which is virtually a linear function of the thickness of the film, as shown in fig. 2, on which the thickness in millimeters of the ore layer coating the surface of the support particle has been plotted on the abscissa, and,

   on the ordinate, the time in minutes necessary for the complete chlorination of the rai mine, and a practically achievable control process for the chlorination and the sublimation was thus obtained.



  Capillary conduits at the surface (the coarse grains constituting the carrier particles remove the non-volatile chlorides darkened from the surface of the ore particles due to the film's containment, thus making possible substantially complete chlorination and sublimation of the volatile constituents. The time required to complete the reaction is then a direct function of the thickness of the film.



  To study the chlorination, the aforementioned ore was intimately mixed, after having crushed it to pass through a nc 100 sieve, with 12.25% of its weight of fine carbon (carbon black) and a small amount. of tarry material to act as a binder, and the rri @ Iange was compressed in the form of grains. An attempt has been made to pass these grains continuously through a vertical retort heated to 1100 and to contact these grains with chlorine at various temperatures. Continuous operation of the retort was not possible.

   An incipient reaction began at around 600 and some ferric chloride was distilled off. Around 800. the reaction was vigorous, the gas leaving the retort being mainly composed of CO 2; a certain quantity of iron chloride and a little chromic chloride were generated but, very soon after the introduction of the chlorine stream, it became impossible to keep the load moving, and the back pressure prevailing. in the chlorine supply pipe has become excessive. Having opened the retort, the entire column was found to be obstructed by a mass of tightly textured chromic chloride crystals.

   It becomes evident that the temperature had been insufficient to volatilize the chromic chloride. The retort was then repaired and the temperature raised to. 900-l000. There was a very strong primary reaction, but the column was blocked in an even shorter time than before. On opening the retort, it was found that it was completely blocked by a mass of chromous chloride. Virtually no chromic chloride had been distilled. Obviously, the difficulty had not been solved by simply raising the temperature to increase the volatility of the chromium chloride.



  An extensive laboratory study of the volatility conditions of chromium chlorides revealed the following facts: 111 Chromium chloride (CrCl @) has been found to have a melting point of 815, at which temperature the pressure of vapor is 0.0007 atm. The pressures at 900, 1000 and <B> 1100 '</B> are respectively <I> 0.004, </I> 0.024 and 0.103 atm, and the normal normal boiling point 1302. At temperatures where chlorine begins to react readily with mixtures of carbon and chromite, there is a low, but definite vapor pressure.



  It has been found that it is impossible to directly determine the vapor pressure of chromic chloride (CrCl3) because this material has an appreciable dissociation pressure at a temperature as low as 700, with chromous chloride and chlorine as primary products. By indirect methods, it was possible to determine the "thermodynamic" vapor pressures of chromic chloride, as follows: at 600, 0.00007, at 700 <B> 0 </B>, 0.002, at 800, 0.039 and at 900, 0.396 atm. The normal subli mation temperature is 947.



  The "thermodynamic vapor pressure" of CrCl 'was determined by measuring the actual volatility at a given temperature and correcting the obtained value for side reactions which cannot be prevented at these temperatures and whose this will be discussed in paragraphs 3 and 4 given below.



  It has been found that the volatility of chromic chloride in a chlorine atmosphere is much greater than in an inert atmosphere such as nitrogen. This has been shown to be due to the formation of a new chromium chloride, which had not yet been described in the technical literature, namely chromium tetrachloride (CrCl '). This material only exists as a gas at elevated temperatures and, on cooling, converts back to chromium trichloride and chlorine.

   It has been found that the equilibrium constant relating to the reaction
EMI0005.0027
  
    2 <SEP> Crel3 <SEP> -f- <SEP> Cl \ <SEP>> <SEP> 2 <SEP> Crcl4 is: 0.001185 to 600, 0.02922 to 700 and 0.364 to 800. Thus, when the total pressure is 1 atm, the partial pressure of chromium tetrachloride in a chlorine atmosphere is respectively 0.033, 0.157 and 0.448 a.tm at these temperatures.



  It was determined that the partial chlorine pressures which would prevent the primary dissociation of chromic chloride, in a condensed CrCl 'containing system, are: 0.00000? at <B> 7 </B> 00, 0.00008 to 800, 0.003 to 900 and 0.043 to <B> 1000 '. </B>



  On the basis of these researches and discoveries, the Applicant has established a way of carrying out the process according to the invention which gives satisfactory results and is based on maintaining the following conditions: a) The arrangement of the chromite mixture and carbon in a vertical countercurrent retort is maintained in a physical state which makes it possible to maintain at all times a determined and regulated excess of chlorine. Briefly, it is envisioned, inter alia, to dispose of the finely divided ore in the form of a thin layer or film usually not exceeding 3 mm in thickness.

   It is easy to obtain such a layer by coating large-mass carrier particles of the fine ore, which is achieved by applying to the particles, by tumbling, rolling or otherwise, a layer of the ore, as will be described below in more detail.



  It will be noted that virtually pure chlorine is admitted to the retort but that, during the reaction, the chlorine is absorbed and carbon dioxide arises according to the well known reactions: 2 Cr @ 03 '-. 3 C + 6 Cl = - 4 CrCl '-j- 3 <B> CO \ </B> or 2 FeO -I- C -f- 3 C12 - 2) FeCf' -I- C02 with analogous reactions for l alumina and magnesia.

   The rate of these reactions depends on the physical conditions and the contact temperature and it is preferable to regulate the chlorine flow according to the temperature so that an excess of chlorine corresponding to <B> 10% </ B> of Cl 'in CO' gas remains after the reaction and condensation of the sublimated chlorides. Due to the high rate of reaction at higher temperatures, in the range of 1000 to 1050, this necessary state is achieved in a continuous countercurrent apparatus set up as described below.

      In this way, the harmful conversion of chromic chloride to chromous chloride by dissociation is completely prevented at all temperatures suitable for the sublimation of chromium chlorides, the stopping of the gas flow and the passage of the ore by the accumulation is prevented. liquid CrCl 'and at the same time increasing the volatility of chromium chlorides, ensuring that appreciable concentrations of chromium tetrachloride are favored by the partial pressure of chlorine in accordance with the results described in paragraph 3 given upper.



  b) Having provided the continuous vertical retort with means for eliminating the gases composed of sublimated chlorides, carbonic anhydride and an excess of chlorine at an intermediate point of the heated zone, the quantity of solids descending is regulated. per unit of time and the application of external or internal heat in such a way that the temperature of the column is maximum at a point situated a little below the aforementioned outlet opening, and in such a way that the latter is not never at a temperature lower than 900. This temperature is slightly higher than that at which the solid chromic chloride is in equilibrium with a partial pressure of about 0.38 atm of gaseous chromic chloride.

   The aforementioned dilution of the gas leaving the retort with CO 2 generated by the reaction limits the maximum concentration of the chromic chloride vapor to an amount close to this value. By maintaining this state as specified, all CrCl 'sublimates as it arises, so that clogging of the column with unvaporized chromium chlorides is prevented and chlorination and continuous sublimation are ensured.



  In order to understand the conditions a) and b) set out above, additional explanations will be given with reference to the particular conditions which occur when treating an ore of the above composition. Under conditions suitable for the chlorination and sublimation of chromium chlorides, all the oxides of the ore, with the exception of the silica, are chlorinated. The chlorides of iron, chromium and aluminum minium are volatilized, and the chlorides of magnesium and calcium remain as liquids with the solids of the load.

   Assuming that the load on the device is 2 kg, the following stoichiometric conditions apply:
EMI0007.0004
  
    Weight <SEP> in <SEP> g <SEP> moiérulcs Constituents <SEP> Weight <SEP> <B><U>0b</U> </B>
<tb> molecular <SEP> (on <SEP> 2000) <SEP> grams
<tb> Cr20 <SEP> 152,02 <SEP> 5 <B> 1 </B>, 6 <SEP> 1032 <SEP> 6.78
<tb> Fe0 <SEP> 71.84 <SEP> 19.6 <SEP> 392 <SEP> 5.48
<tb> SiO2 <SEP> 60.06 <SEP> 5.5 <SEP> 110 <SEP> 1.82
<tb> A120 <SEP> 101.94 <SEP> 11.8 <SEP> 236 <SEP> 2.32
<tb> <B><I>MgO</I> </B> <SEP> 40.32 <SEP> 11.5 <SEP> 230 <SEP> 5.70
<tb> Ca0 <SEP> 56.08 <SEP> 0.8 <SEP> 16 <SEP> 0.28 The quantity of theoretical carbon months used, equivalent to the carbon of the C02 in gendé, is:

    
EMI0007.0006
  
    For <SEP> <B> the </B>
<tb> constituent <SEP> Quantities <SEP> Totals
<tb> Cr <SEP> 201 <SEP> 1.5 <SEP> X <SEP> 6.78 <SEP> 10.17
<tb> FeO <SEP> 0.5 <SEP> X <SEP> 5.46 <SEP> 2.74
<tb> A1203 <SEP> 1.5 <SEP> X <SEP> 2.32 <SEP> 3.48
<tb> <B> MgO </B> <SEP> 0.5 <SEP> X <SEP> 5.70 <SEP> 2.85
<tb> Cao <SEP> 0.5 <SEP> X <SEP> 0.28 <SEP> <U> 0.14 </U>
<tb> Total: <SEP> 19.38
<tb> equivalent <SEP> to <SEP> 232.3 <SEP> g Similarly, the Cl 'molecules required are:

    
EMI0007.0007
  
    For <SEP> the
<tb> constituent <SEP> Quantities <SEP> Totals
<tb> Cr-0 <SEP> <B> 3 </B> <SEP> X <SEP> 6.78 <SEP> 20.34
<tb> Fe0 <SEP> 1.5 <SEP> X <SEP> 5.48 <SEP> 8.22
<tb> A.120 <SEP> 3 <SEP> X <SEP> 2.32 <SEP> 6.96
<tb> Mg0 <SEP> 1 <SEP> X <SEP> 5.70 <SEP> 5.70
<tb> Ca0 <SEP> 1 <SEP> X <SEP> 0.28 <SEP> 0.28
<tb> Total:

   <SEP> 41.50
<tb> month <SEP> C12 <SEP> or <SEP> 2941 <SEP> g Finally, we will have the volatile chlorides in the proportions of
EMI0007.0009
  
    Constituents <SEP> Mols <SEP> gas
<tb> 2 <SEP> Crcl3 <SEP> 13.56
<tb> 1/2 <SEP> Fe2Ch <SEP> 2.74
<tb> 1 <SEP> A12Cl <SEP> 2.32
<tb> Total <SEP> for <SEP> the <SEP> chlorides <SEP> gas <SEP> 18.62
<tb> C02 <SEP> (see <SEP> plus <SEP> above) <SEP> <U> 19, </U> 38
<tb> Total <SEP> for <SEP> the <SEP> constituents <SEP> gaseous <SEP> 38 It would appear from these figures that if 41.5 months of chlorine were brought into contact with 2000 g of ore from the specified composition, mixed with 232 g of carbon, there should be a complete reaction,

   and complete sublimation of chromium chloride whenever the temperature has reached a point where the vapor pressure of this material is greater than
EMI0007.0010
   or 0.357 atm. In fact, the Applicant has discovered that this is not the case due to side reactions.

   Thus, we first obtain 2 CrCl3 - 2 CrCl = --f- C12 (1) this reaction being followed by: 2 CrClg -f- C12 - 2 CrC14 (2) As reaction (2) uses chlorine, the reaction (1) continues and subsequent reactions continue until a state of equilibrium has been reached.

   When this is complete, a considerable portion of the chromium is found to be in the form of @ CrCl2, which is liquid and relatively non-volatile, and this is why the retort becomes clogged and disabled. .



  The actual chlorine content of the equilibrium mixture is only a few tenths of a percent in the vicinity of 900 0, but this content is not sufficient, for a satisfactory operation, to maintain a slight excess of chlorine by as a primary addition to the above content since adding any excess chlorine initially only results in increasing the relative amounts <CrCl 'compared to CrCl 'and that this excess used to generate CrCl' does not prevent the formation of CrCl2. This situation emerges from FIG. 6, which shows,

   for different temperatures plotted on the abscissa, the relative amounts of CrCl '(ordinate) expressed as a percent of the total amount of volatile chromium chlorides present, when various excess chlorine is added. Each of the curves <I> A, B, C, D, </I> relates to a speci fied amount of Cl = found in the exhaust gas (C02) after the reaction and after cooling and condensation (reaction ( 2) reversed). These curves end at the broken line E which indicates the temperature at which the condensation of CrCl 'begins.



  Above and below the temperature at which the normal vapor pressure of CrCf is suitable to ensure complete volatilization of chromium chloride when diluted with the amounts of CO2 generated by the reaction, two different sets of conditions s 'apply the minimum excess of chlorine sufficient to prevent the formation of CrCl2. These groups have been shown in FIG. 5.

   In this figure, in which the temperatures are plotted on the abscissa, and the chlorine content of the flowing gas, on the ordinate, curve A indicates the quantity of <B> C12 </B> present in the C02 which escapes when the formation of CrCl2 in the presence of solid CrCl 'is just prevented and the curve B la. quantity of C12 present when the quantity of CrCl 'generated is insufficient to kill the gas phase at temperatures above the normal saturation temperature.

   It can be seen that these curves intersect at a temperature just below the normal saturation point for a partial pressure of 0.357 atm and that the locus of the peak corresponds to 5.5% Cl 'in the exhaust gas. Below these curves, chromium chloride arises.



       Since, in a continuous countercurrent operation, the chlorine and the mixtures of ore and carbon must necessarily be in contact with. a series of different temperatures, at the working temperature or close to this temperature, it is clear that an excess of chlorine corresponding to at least 5.5% must be used. Confirmation of the amount which should be used in excess is very easy to obtain by an analysis of the exhaust gas after the volatile chlorides have been condensed,

   and it has been found that it is not advisable to adopt a value too close to the critical point in practice. It has been found that a safe working limit in the CO2 of the exhaust gas is between 10 and 11%, which leaves a safety margin to account for the inevitable fluctuations that occur under working conditions and in variations in the ore processed. In addition, it is evident that although the critical point is largely determined by the chemical mutual relationships between chromium chlorides, this point also depends to some extent on the composition of the ore being processed.



  Thus, to bring out the conditions under which the greatest excess chlorine would be necessary, which would correspond to the most severe working conditions, the upper curves A 'and B' have been drawn in fig. 5 to indicate the limits that occur when chlorinating pure Cr20 'rather than chromite ore. Note that the curves A and A ', which show the conditions limiting the formation of CrCl2 when solid CrCl' is present,

   differ only slightly in the case of ore chlorination compared to that of pure Cr20 '. In this case, the partial pressure of CrCl 'in the gas is determined by the vapor pressures of this material, and the variation results from the weak dilution effect of the volatile chlorides and their CO2 equivalent acting on the concentration of the other constituents. , with the exception of CrCl '.



  The curves <I> B </I> and <I> B ', </I> which relate to temperatures above the dew points of CrCl', differ to a more appreciable extent, because, in this case, the concentrations are influenced by stoichiometric relationships.

   However, it is clear that, whatever the conditions of the chlorination of chromite ores, one could not obtain Cr el concentrations greater than those corresponding to the concentrations equivalent to the action of Cl 'on pure Cr2O3, so that the peak, or 10.7%, can be considered as the upper limit under all working conditions and for all minerals.

   As this point must be determined indirectly and is susceptible to a slight error, the value between 10 and <B> 11%, </B> which coincides with the experimental limit found in the actual operation of a retort. continuous chlorination, can be considered as the upper working limit in practice. Thus, while, from the foregoing, it would seem that one can reduce the excess of chlorine by treating an ore poor in Cr203, it would result in this way of proceeding that one would approach a state where the risk of retort stoppage is increased.

   This can be explained as follows: the iron contained in chromite is a little easier to chlorinate than chromium, and it is possible to have in a column of moving ore various relations between Crz03 and FeO which correspond to the degree to which chlorination took place, this state of affairs being equivalent to an increase in the CrzO 'content of the ore.

   Thus, the only absolutely safe working conditions are those which correspond to pure Cr2O3, given that all real working conditions must necessarily be less severe than in the case of pure material and a Any change of the ore to Cr2O3 as a result of selective chlorination at some intermediate point of the chlorination column cannot require more severe conditions than those relating to Cr203 alone.



  Referring now to condition b), it emerges from fig. 6 that the temperature at which condensation takes place is slightly reduced when excess chlorine is used.

   It has been found that if care is taken that the temperature of the gas leaving the retort does not drop below 900-920, that is, it is 10-15 above the dew point true when 10 to <B> Il% </B> of chlorine are kept in the residual gas after the condensation of the chlorides if the ore treated is that specified, a continuous process is possible, with a safety margin for the practice needs. There is obviously no exact critical upper limit temperature when using the desired excess of chlorine.

   While the vapor of chromic chloride in equilibrium with solid crystals would require approximately 0.01 atm of chlorine to prevent dissociation at the normal point of sublimation (947), dilution of the gas phase with CO2 from the reaction prevents saturation at temperatures above 900-920, resulting in a continuously increasing decrease in excess Cl 'at temperatures above this point.

   However, the curves of FIG. 6 show that the amount of CrCl4 decreases with temperature and that, to ensure the benefits of the volatility of this material, it is good not to use temperatures above 1050-1100. These temperatures are also not recommended because of the increased reactivity with the silica contained in the ores and with the walls of the furnace.



  If we refer to the stoichiometric quantities indicated above, it is clear that <B> 10% </B> of C12 in the CO 2 of the exhaust gas corresponds to 2.15 mols of C12 for 19, 38 MONTHS of C02, so the total chlorine requirement is 41.5 -f- 2.15 = 43.65 mols, or 1.053 times the theoretical value.

   This gives the theoretical basis for adjusting the amount of chlorine introduced, but it is good to periodically check the condition required by an exhaust gas analysis, especially if the composition of the ore changes to a marked extent.



  The manner in which these principles may be applied to the continuous countercurrent chlorination of chromite ores will be demonstrated by the following discussion of a particular example: Given a vertical shaft furnace of given dimensions and the usual construction, being able to be a simple vertical cylindrical retort with a gas outlet orifice placed near or below the center of the heated zone, one first determines the size of the large carrier particles which will move from there. in a regular and uninterrupted fashion. through the retort under the action of gravity.

   The carrier particles must be large enough to allow the gases rising in the retort to pass easily through the finasse of Ft particles fine enough to descend with a regular movement, as mentioned above. Retorts 5 to 10 cm in diameter usually require particles of 4.7 to 8 mm (the average size; but particles of 9.4 to 12.7 mm can be used in retorts of 15 to 20 cm and, in the case of even larger apparatus, carrier particles of 12.7 to 19 mm of average size can be used Carrier particles of these dimensions have a relatively large mass compared to ore.



  The material of which the carrier particles are made can be any refractory inert to the chemical reagents used in the process. The Applicant has used quartz, mullite, coke, carbon and even, in certain cases, the ore particles themselves, these last three refractories falling within the class of materials, inert due to the size of the particles. and the action illustrated in FIG. 1.



  Having determined the correct particle size, the resistance offered to the countercurrent flow of gas through the horn when such particles descend through it under the normal action of gravity is measured. By this means, we determine that we can. passing a marked maximum of chlorine (in the particular example considered here) under an appropriate pressure.

   In general, it is preferable to work with pressure drops (gauge) not exceeding 50 to 65 cm of water through the retort, because if high pressures or speeds are used, the fine particles of ore likely to be accidentally detached from the film (or supported particles) by movement of the column tend to be expelled from the retort before they can be chlorinated, and even higher pressures may also tend to cause to an exaggerated extent the formation of channels of lesser resistance, so that the gases cease to come fully into contact with the films covering the carrier particles.



  Of course, the pressures and flow rates measured will depend on the nature, as well as the size, of the carrier particles as well as, to some extent, on the construction of the furnace or retort. So, for example, in a simple vertical cylindrical retort <B> 5.7 </B> cm in diameter and with a heated zone about 50 cm high, the applicant has found that the upper limit of the desirable flow gas is between 6 and 8 liters per minute, when using quartz carrier particles having an average thickness of 6.3 mm. This is equivalent, from a stoichiometric point of view, to 11-14.8 g per minute of ore of the above composition with an excess of cor rect chlorine.

   It is evident that the maximum capacity of a given retort will be determined by the rate at which chlorine can be supplied, provided, however, that it can be ensured that this quantity of ore per unit of time is not greater than the limit corresponding to the rate of the reaction.



  To prepare the layer for the carrier particles, the first step is to mix and thoroughly grind the chromite ore with the finely divided carbon. Although the stoichiometric proportions of carbon relative to the ore in question correspond, as explained previously, to 11.6% of the weight of the ore, the Applicant has found that it is preferable to use an excess corresponding to the application of 14.5 to 16% of the weight of the chromite,

      in order to ensure a rapid reaction rate under the conditions which prevail when the major part of the chromite has been chlorinated. As stated previously, it is preferable that the size of the ore particles, after grinding, is less than that corresponding to the No. 100 sieve and, preferably, that it is close to that corresponding to the No. 200 sieve.



  The finely ground mixture of chromite and carbon is then added to the large carrier particles, having 6.3 mm in this example, at a rate of 100 to 200 g of said mixture per 1000 g of particles and tumbling, rolling or rolling. agitates or otherwise processes the mixture, until the fine particles of carbon-ore adhere to the large carrier particles as a thin, uniform film. This action is promoted by adding, before the stirring action, small amounts of a volatile liquid wetting agent, such as carbon tetrachloride, mineral spirits or even water, for example.

   Surface tension and capillary forces then tend to spread the finely ground mixture of ore and carbon very uniformly over the carrier particles. In the case of carrier particles made of quartz or other materials which have been used previously, there are usually small adherent amounts of residual magnesium chloride and, due to the hygroscopic properties of this material, it tends to get wet. slightly and causes adhesion of finely ground ore.

   Even in a completely dry state, the cohesive forces are still sufficient to cause the adhesion between the fine particles and the large particles to a sufficient degree to form the desired thin layer on the large particles. The addition of the wetting agent is therefore in no way essential and satisfactory results have been obtained without resorting to it, but it is noted that the presence of this agent facilitates the obtaining of a uniform film and increases the adhesion properties of the film after it has been formed, particularly when small amounts of tarry or wax material are present.



  The bulk density of the ground mixture of chromite and carbon is close to 1.15, so that the thickness of the film, when 100 g of said mixture are dispersed on 1000 g of 6.3 mm quartz carrier particles, is close to 0.25 mm, this thickness being about 0.5 mm with the upper limit of 200 g of fines per kilogram of quartz.

   Although it is possible to use thicker layers of up to 3mm in thickness, it has generally been found that it is not recommended that the supine quartz particles carry more fines than the corresponding amount. amounting to the amount of magnesium chloride which can normally be removed from the ore by capillary action and surface wetting of quartz particles.

   It is preferable that this factor be determined empirically for each ore and each type of support, since the thickness of the film which is suitable for the envisioned needs obviously depends on the percentage of impurities in the ore, as well as on the nature of the surface of the carrier particles.



  In the example given, with a combined content of CaO and MgO of 12.3%, the upper limit is close to 125 g of mixture of ore and carbon per kilo of quartz in pieces of 6.3 mm. When carrier particles made of a porous material such as coke are used, the upper limit is raised to a value in the region of 400 g per kilogram of coke, the difference being attributable in part to the lower bulk density of the coke. coke particles and in part to the increased porosity of the latter.

        Referring again to fig. 2, we see. that the film thickness of 0.25 to <B> 0.5 </B> mm corresponds to reaction times of 50 seconds to 1.5 minutes for complete chlorination.

   The carrier particles containing the thin layer of fine ore are then introduced into the retort, in countercurrent with respect to an ascending stream of chlorine gas, with a flow rate such as the residence time of the carrier particles in the reaction zone (which is the part of the retort below the gas exhaust port and held at 900 and extending downward to the point where the load leaves the heated area and cools rapidly) is a little more greater than the reaction times shown in fig. 2.

   Thus, in the particular example of a 57mm retort shown above, one can conveniently and effectively place the gas exhaust port above the lower end of the heated area ( 50 cm high) and a quarter of the height of this zone, so that the reaction zone has a height of 12.5 cm. If the quartz carrier particles are introduced at a rate of 4 to 5 kg per hour, the travel speed will be approximately 5 cm per minute and the residence time in the reaction zone will be slightly more than 2 minutes, which is just above theoretical requirement (0.8 to 1.5 mi nute) based on fi--. 2.

   In this way it is possible to effect the contact between the ore and the chlorine in a continuous and countercurrent manner, with a substantially complete distillation of the volatile chlorides and with the regulated chemical conditions which are necessary, such as as we have seen previously, to avoid the formation of lower non-volatile chromium chlorides and by volatilizing large parts of the chromium in the form of chromium tetrachloride.



  The carrier particles exiting the bottom or retort outlet contain the unchlorinated residue of the ore, which is primarily composed of silica, and the non-volatile chlorides of lime and magnesia. These are easily removed by washing with water, which also removes the residue of fine silica ore, and the carrier particles are then superficially dried for reuse after re-coating them with fine ore. In some cases, agitation of the particles on a sieve fine enough to retain the quartz support but large enough to pass the residue particles is sufficient to ensure good separation without the need for washing and drying.



  The part of the furnace which is located above the gas exhaust port is mainly a preheater. Although the main role of the ore film coating of the carrier particles is to arrange the physical factors in such a way that the reduced chemical conditions can be satisfied and to remove fine particles of the ore, for example. capillary action, liquid non-volatile chlorides which would ordinarily cause chlorination to stop and low extraction rates if they were allowed to accumulate in a load of briquettes or a thick layer of ore,

   a certain number of secondary advantages are also achieved which are of great importance for the achievement of a satisfactory continuous process, namely: a) The maintenance of a high relative velocity between the ore particles and the gas, resulting in high diffusion rates of chlorine going to the ore and volatilized chlorides and <B> CO '</B> separating from the ore; b) the application of means to maintain the thermal equilibrium of the system during the chlorination and the sublimation by the use of the sensible heat stored in the carrier particles.



  The importance of the first of these advantages is apparent from the following table of the relative densities and diffusion rates of the various gases and vapors in question:
EMI0013.0001
  
    Gas <SEP> or <SEP> vapors <SEP> Density <SEP> <SEP> rate of <SEP> diffusion
<tb> relative <SEP> gases <SEP>
<tb> Air <SEP> 1 <SEP> 1
<tb> Chlorine <SEP> 2.4 <SEP> 0.5
<tb> Carbon dioxide <SEP> <SEP> 1.5 <SEP> 0.8
<tb> Vapor <SEP> of <SEP> ferric chloride <SEP> <SEP> 11.2 <SEP> 0.8
<tb> Vapor <SEP> of <SEP> trichloride <SEP> of <SEP> chromium <SEP> 5.5 <SEP> 0.43
<tb> Steam <SEP> of <SEP> tetrachloride <SEP> of <SEP> chromium <SEP> 6.5 <SEP> 0,

  39 The high densities and low relative diffusion rates of many of the vapors and gases involved in the process cause great difficulty in supplying fresh reacting chlorine to the ore particle and, especially, in removing the heavy vapor generated by the ore. chlorination, and this difficulty can only be overcome by ensuring a high relative velocity of the gas with respect to the ore particles and the carrier particles.

   If the fine ore were contained only in the interstices which separate larger grains, it would result in such speeds that the fine particles would be expelled from the retort before being chlorinated and would give rise to serious trouble due to the retort. dust, besides that the extraction would be poor.

   In a briquette charge, the heavy vapors can only be removed from the interior channels and capillary ducts of the briquette with extreme slowness, resulting in insufficient chlorine and excess chloride vapors, accompanied by the inevitable formation of lower liquid non-volatile chlorides, causing rapid clogging of pores and cessation of chlorination. Thus, it is only by arranging the ore in the form of a thin layer in contact with which the gases can move at high relative speeds that it is possible to work continuously, in counter-current. and with a high extraction rate of volatile chlorides.



  Regarding the thermal requirements, it is observed that the chlorination of mixtures of chromite and carbon ore is exothermic if solid or liquid chlorides are formed, and that it is endothermic if the volatile chlorides originate in the form of vapors, since these chlorides have relatively high heat of vaporization.

   Thus, unless an adequate supply of readily conductive heat is provided during the chlorination, there will be a great tendency for the formation of solid chromic chloride during the early stages of the countercurrent action. This will jam the load and interrupt the flow of gas and solids.

   It is evident that the present process allows most of a retort to be used to preheat the carrier particles and store sensible heat therein which is freely transferred to the ore film by a direct contact transmission during the period. final chlorination below the gas exhaust port.



  In this regard, it should be noted that addiop. a small amount of chlorine at the ore inlet end of the retort, together with the ore, helps to ensure that the adhesive character of the film is maintained during the preheating period, since the weak The amounts of non-volatile chlorides thus generated tend to bind the film of fine ore to the carrier particles.



  Fig. 3 is a schematic representation of an apparatus which has been used with success by the applicant. In this apparatus, there is provided a vertical tank 6 made of an inert material such as silica, and the applicant has successfully used the material called "Vitreosil". At the upper end of the tank, an ore inlet? allows the introduction of the charge in the form of the aforementioned carrier particles coated with the mixture of ore and coal. A heater, indicated at 8 in the form of an electrical resistance, is placed around part of the intermediate tank of its. height.

   The upper part of the tank 6 is extended by a metal extension 32. The joint between the tank and the extension is protected by a cooling water jacket 9 which surrounds it. About <B> 10% </B> of the chlorine is introduced through an intake pipe 11 placed at the top of the tank, the rest being introduced through an intake pipe 12 adjacent to the lower end of the equipment . A thermocouple tube 14 descends inside the tank to a point below the conduit 16 through which the volatilized materials escape. The conduit 16 opens above the lower end of the heating zone, in order to surer the elimination of the volatilized chlorides.

    Usually this duct is placed about a quarter of the height of the zone above its lower end, as this has given good results. At the base of the tank is provided another refrigerating section 17 serving to protect the seal between the tank 6 and the metal base structure 33 and to cool the materials which pass through this seal to be evacuated by conveyors 18 arranged in a housing 19 located at the lower part of the. tank.



  The volatilized products pass through the exhaust port. 16 in a condenser 21. This condenser is also made of silica (usually vitreosil). The volatilized chlorides are condensed in the condenser 21 and a scraper 22 allows the volatilized chlorides thus condensed on the internal wall of the condenser to be detached from it to fall to the bottom of the equipment on a plate 23 from which they can be removed. using another scraper 24 which brings them into a duct 26. The material collected in the duct 26 is drawn by the scraper 24 into a collector 27.

    Non-condensed matter, fine dust, etc. pass into a container 28 in which the gas is filtered through a filter bag 29, usually made of asbestos. A shaker 31 supports the upper end of the bag and the latter is occasionally shaken so that the collected dust falls into the collector <B> 27. </B>



  In fig. 4, the temperature scale has been shown opposite the height of the retort, the temperatures expressed in hundreds of degrees being shown on the abscissa. These are the actual temperatures prevailing in all parts of the equipment working under constant conditions.



  The present process is not limited to the application of elemental chlorine as a chlorinating agent. Thus, instead of dispersing mixtures of ore and carbon on the surface of carrier particles, it may in some cases be preferable to use only ore as the substance to form the film and to provide the reducing agents. in a gaseous form. For example, the vapors of carbon tetrachloride or other volatile chlorinated hydrocarbons can be substituted for chlorine, since these hydrocarbons provide active chlorine. It is also possible to use other compounds which give active chlorine.

   In some cases, it is possible to introduce hydrocarbon gases and chlorine either in the form of mixtures or in alternating positions of the column of the. retort. When hydrogen-containing materials are used in this way, the effluent gases contain hydrochloric acid and water vapor, but the physical principles involved are the same as in the process first described and advantages also important are achieved in operation.

 

Claims (1)

CLAIM Process for the continuous manufacture of chromic chloride by chlorination, in the presence of a reducing agent, of oxidized chromium ores in the finely divided state, which together contain other metals including iron, calcium and magnesium, characterized by the fact that the ore in the finely divided state is applied in the form of a thin film coating on larger mobile particles which are themselves chemically inert and which, thus coated, are moved through a reaction zone in which these particles and the gaseous chlorinating agent move in countercurrent, the ratio between the quantities of ore and chlorinating agent introduced into said zone being such that at all times the quantity of chlorinating agent be greater than that necessary for all chlorinable materials to be converted into their chlorides, the reaction being carried out at temperatures at which the chromic chloride separates from the ore by volatilization. SUB-CLAIMS 1 Process according to claim, characterized in that the excess of the chlorinating agent is chosen such that all the chlorinatable constituents of the ore, with the exception of that containing chromium, are converted. in their highest chlorides, whereas the chromium separates from the ore mainly as tri chloride and the exhaust gas has a chlorine content of about 107o. 2 Process according to claim, characterized in that a temperature of approximately 900 is maintained during the chlorination. 3 Process according to claim, characterized in that the formation of CrCl 'is avoided by ensuring the presence of an excess of at least 5.5% of free chlorine in the exhaust gas and the elimination of these gas at temperatures of at least <B> 890 '. </B> 4 Process according to claim, characterized in that the size of the carrier particles is sufficient to substantially ensure flow by gravity, and that the thickness of the film layer is suitable for ensuring the perfect penetration of the ore by the gas in a vertical retort. 5 A method according to claim, characterized in that one uses particles of the ore whose size corresponds to the sieves nos 100 to 200. 6 Method according to claim, characterized in that the finely divided ore is mixed with fine particles of carbon as reducing agent before coating. 7 Method according to claim, characterized in that the thickness of the ore layer on the carrier particles does not exceed 3 mm. 8 A method according to claim, characterized in that chlorine is used as a chlorinating agent. 9 Process according to claim, characterized in that a compound giving active chlorine is used as chlorinating agent. 10 A method according to claim and sub-claim 9, characterized in that the chlorinating agent uses aliphatic or aromatic hydrocarbons substituted with chlorine. 11 The method of claim and sub-claim 1, characterized by the fact that the ore is supplied in the form of a vertically descending stream in a heated column having a side exhaust port and that the flow rates of ore and chlorinating agent are regulated so as to maintain a zone chlorination near said orifice.