CS270198B1 - A method for removing heavy metals, especially cadium, lead and mercury, from extraction phosphoric acid and liquid multi-component fertilizers based thereon - Google Patents

Abstract

The removal of heavy metals according to the solution consists in the fact that the extraction trihydrogenphosphoric acid, or a liquid multi-component fertilizer based on it, is subjected to a precipitation reaction at a temperature of 15 to 95 °C with a substance that is a source of sulphidic sulfur. The reaction mixture is mixed by circulation without or with the use of a static mixer and/or with pneumatic mixing and/or with the effect of a mechanical stirrer. The precipitation reaction takes place without or in the presence of substances that accelerate the clarification of the system. The excluded sludge is removed by sedimentation and/or blasting and/or flotation.

Description

The invention relates to a process for removing heavy metals, in particular cadmium, lead and mercury, from extractive phosphoric acid and liquid compound fertilizers prepared on the basis of extractive phosphoric acid.

In recent years, among the foreign substances that deteriorate the environment, heavy metals have come to the forefront of the interest of experts, especially cadmium, lead, mercury and chromium. The accumulation of these heavy metals in the soil and their subsequent entry into the food chain through agricultural crops means a serious danger to human health when their occurrence increases. Environmental pollution, especially with cadmium, especially after medical science confirmed its carcinogenic effects, is considered one of the most serious problems in environmental protection in the world.

Heavy metals usually include elements whose specific gravity is greater than 4 g - cm". Many of the heavy metals are necessary for life in low concentrations (copper, zinc, manganese, cobalt and probably also iron in the trivalent form), but in the case of other heavy metals (lead, mercury, nickel and cadmium) their need for life has not yet been proven and, conversely, toxic effects have been attributed to them.

Although the main source of environmental contamination with lead compounds is motor transport, emissions from the production and processing of non-ferrous metals, iron and steel, coal combustion and the production of accumulators and batteries, significant sources of lead pollution are also over-fertilized composts, sewage sludge and phosphate fertilizers. GUNNARSSON, O. /Fertiliser and Agriculture, 1983, 85, 27-42/ states that the lead content in phosphates is in the range of 2-12 ppm. In industrial fertilizers, the content is usually from 2 to 140 ppm Pb.

By Resolution of the CSR Government No. 223/84 on the circulation of foreign substances in the soil, the maximum permissible concentration of lead in the soil was set as 100 mg Pb/kg of dry matter.

The natural mercury content in manure is reported to range from 1.10 to 3.10 ppm. In a relatively small amount, mercury is also introduced into manure by phosphate fertilizers. According to data published by 8KNSSIU, N. /Fertilizers Research 2, 1981, 4, 289 - 302 /triple euperphosphate can contain up to 3.2 ppn Hg. The maximum permissible mercury concentration in manure is prescribed for the CSSR, similar to most other European countries, at 2 ng Hg/kg dry matter of manure.

Cadmium enters the environment either through natural sources or through human intervention in nature. The two natural sources of cadmium are, after the weathering of minerals, sea waves, and airborne dust, its main source is volcanic activity /for example, the professional literature states that 10 to 40 tons of cadmium are dispersed into the air annually by the activity of Etna alone. Sources of cadmium resulting from human activity include mining and processing of ores, chemical production, combustion of fossil fuels, and production and use of industrial fertilizers. The largest source of cadmium contaminating the environment is landfills of various solid waste. The production and use of phosphate industrial fertilizers is currently the fourth source of cadmium polluting the environment. Given the relatively easy penetration of cadmium from waste into the food chain, this source is significant in terms of its negative impact on human health, even though it primarily affects cultivated agricultural waste. For these reasons, great attention is paid worldwide to phosphorus raw materials and phosphorus fertilizers with the aim of eliminating or at least reducing cadmium inputs into the environment from these sources.

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It is known from the professional literature /Cadmium in the Phosphate Industry EPA - Versar, In. August 1979; Cadmium Seminate, The Fertilizer Institute, University of California Davis, October 1978; AUER, C. : Chemical Technology and Economics in Environmental Perspective Cast VI - Cadmium in Phosphate Fertilizer Production, EPA 560/2-77-006, November 1977; WAKEFIELD, Z. T.: Distribution of Cadmium and Selected Heavy Metals in Phosphate Fertilizer Processing Bulletin I - 159, NEDC-TVA, Muscle Shoals, Alabama, October 1980/ that each phosphate contains a certain amount of cadmium. In the case of apatites, which are of volcanic origin, they contain small amounts of cadmium - in the order of ppm. Phosphorites, i.e. phosphates of sedimentary origin, always contain a larger amount of cadmium, which in some types reaches up to hundreds of ppm. Cadmium contents in phosphorites differ not only according to their provenance, but often also vary significantly within a single deposit.

GUNNARSSON, 0. in the work already cited] reports the following cadmium contents in phosphates of various types and origins:

Type and origin of phosphate:

Cadmium content in phosphate: /mg Cd. kg" ¹ /

Kola apatite /USSR/ Apatite GRANGES /Sweden/ Phosphorite GAFSA /Tunisia/ Phosphorite TAIBA /Senegal/ Phosphorite KHOURIBA /Morocco/

0.2

1.1

BABCHLB, T. and WOLSTEIN, F. in their work /Cadmium Compounds in Mineral Fertilizers. The FERT. Society, Proc. Mo. 226/ state that the cadmium content of phosphoric acid clearly depends on the type of phosphate from which the acid was produced. Similar conclusions were reached ▼ in the work cited earlier] by WAKEFIELD, Z. T. BAECHLE and WLSTEIN, who state the following cadmium contents in extractive H^PO^ /54% PgO^/ origin:

Provenance of the extractive phosphoric acid:

Windmill

Gafsa

Spanish Morocco

Tunisia

USSR

Florida

Cadmium content /mg Cd. kg" ¹ /

0.5 5

From the report of the VÚAnCh /TOMKOVÁ, D. - MAREČEK, j.: Problematika obsahu cadmium v průmyslových hnojivách. VZ-E-1230, Ostí n L. 1988/ it follows that depending on the used extraction phosphoric acid in the period 1986 - 1987 the cadmium content in liquid NP-fertilizer type 8-24-0 fluctuated from 0.8 to 16.1 ppm Cd.

In order to comply with the prescribed maximum dose of 3 g Cd. ha ¹ . year" ¹ it follows for the production of liquid NP-fertilizers that the phosphoric acid intended for their preparation should not contain more than 40 g cadmium per ton of PgO^, * which implies that the acid of commercial concentration /52-54% P ₂ 0j/ should not contain more than about 20 ppm of cadmium.

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As can be seen from the translation published by BAECHLB and WOLSTHN, most commercially supplied acids do not allow compliance with the limit content of cadmium in final phosphate fertilizers.

For the above reasons, considerable attention has been paid in recent years to the study of the possibilities of separating heavy metals, especially cadmium, lead and mercury, from extractive phosphoric acids. Although fertilizer manufacturers are trying to develop a method for removing these heavy metals from phosphoric raw materials by extraction, precipitation or calcination, according to relatively recent data /Kuropěn Chemical News, April 27, 1987/ it is currently not known that any of these procedures are practically used in production practice.

In the interest of solving the problem of precipitation and gradual sedimentation of clay-forming impurities from extracted phosphoric acid and organic fertilizers prepared on its basis, considerable attention has been paid in the recent period to the removal of these clay-forming impurities. In this context, research efforts have focused primarily on methods for removing the following cations: magnesium, aluminum, iron and calcium.

Several separation methods have been studied and partially applied in operational practice for the removal of the above-mentioned sludge-forming primaries, the basis of which is free or variously accelerated sedimentation (flocculation, sedimentation, etc.), isolation of pure acid in the form of a solid phase - its precipitation, removal of sludge-forming impurities by precipitation, purification of the acid with dilute organic solvents (alcohols, ketones, ethers, alkyl or aryl phosphates, amines, organic sulfonic acids), purification processes by liquid extraction, isolation of trihydrogenphosphoric acid in the form of its adduct with boric acid, processes consisting in the use of ion precipitators and many others /TRRBN, J,: Summary of current knowledge on the problem of removal of sludge-forming primaries from technical trihydrogenphosphoric acid. Written paper for the professional postgraduate examination, VÚAgT - Bratislava and the Department of Inorganic Materials Technology ChTF - SVST - Bratislava, January - June 1977/.

In connection with the reduction of arsenic content from thermal trihydrogenphosphoric acid intended for food purposes, for the production of feed additives and the production of pure phosphoric salts - i.e. in connection with the preparation of the so-called dearsenized phosphoric acid, several patents recommend the use of variously modified methods of arsenic precipitation using hydrogen peroxide and sulfides. In the Czech Republic, this type of preparation of thermal dearsenized acid is used in the FOSFA plant - Břeclav, Poštoma /LUCKÝ, J.: Production of desarsenized phosphoric acid in the FOSFA plant. Lecture within the framework of the 3rd National Conference on Phosphorus and its Compounds. Břeclav,'14. - 16.10. 1986.

The use of hydrogen peroxide and various sulfides for the removal of arsenic from extractive phosphoric acids has been studied in the past and reported in patent and professional literature by: SOHYA KNNSUKB, CHIRAJAMA KUNICHCKO, ÍÍAKABIN, K. J. and EAGGAMAN, W. H.

SOHYA KENSUKE and co-workers (Tokuyama Soda Co. - Japanese Patent No. 73-06036) propose to remove arsenic from extractive H^PO^ using hydrogen sulfide, zinc sulfide or aluminum sulfide in the presence of ions. In the next step, the acid phase is heated with a cation-active ion exchange resin (Amberlite IR-120) in the presence of an organic solvent. According to the authors of this invention, the extractive phosphoric acid purified by the above method contained less than 1 ppm of arsenic and iron.

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CHIRAJAMA KUNICHIKO and co-workers (Japanese Patent No. 49-37038) recommend precipitation of arsenic from extracted trihydrogenphosphoric acid by treatment with hydrogen sulfide in the presence of activated carbon (0.1 to 2.0% based on the amount of P ₂ 0^ in the purified acid).

By the above method, the arsenic content in the acid can be reduced from the original 1.5.10^% to ^5.106 %.

MAKARIN, K.J. /Nauch. Trudy Mosk. Inzh. Ekonom. Inst. - Chimija i Chim. Preizvod. 1958, No. 10, 157 - 163/ published the results of pilot-scale tests of sulfide precipitation aimed at removing arsenic from hot H^PO^ in a scrubber with a controllable volume.

In connection with the purification of liquid phosphate fertilizers, the research focused primarily on the clarification process stage /e.g. PECK, A. W.: Clarification of Ammonium Phosphate Solutions - U.S. patent δ. 3,697,233; IIPTÁKOVl V. et al.: Method of physical purification of liquid multicomponent fertilizers - Cs. AO δ. 212,023/, monitoring the possibilities of separation of precipitated sludge and removal of organic so-called bituminous substances /STINSON, J. M. et al.: Removal of Carbonaceous Matter From Ammonium Polyphosphate Liquids. U.S. pat. δ. 3,969,483; BURKERT, G. M. - NICKERSON, J.D. Clarifying Ammonium Polyphosphate Fertiliser Solutions. - U.S. pat. 3,630,711; KNARR, W. A. - EDWARDS, T.G.; Decolorizing Ammoniated Superphosphoric Acid Solutions. - U.S. pat. No. 3619 161/.

In connection with the gravimetric determination and separation of cations, analytical chemistry uses the fact that ions with 18 electrons in the outer sphere /Cu*, Zn ²⁺ , As, Ag*, Cd ² *, Sn ¹⁷ , Bb ⁷ , Au*, Hg ² */ and 18 + 2 electrons in the valence sphere /As ¹ , Sn ²⁺ , Sb ¹ \ Ti*, Pb ² *, Bi^*/ provide sulfides that are insoluble or very slightly soluble in acidic media.

From published data //1/ OKÁC, A.: Analytická chemie kvalitavní". Academia - Prague 1966, p. 53. /2/ GOATES J.R.: J. Am. Chem. Soc. 74, 835 /1952/; /REMY, H.: Anorganická chemie, 1. diel, SNTL - Praha 1971, 8. 774/ results from the solubility of cadmium sulfides, of lead and mercury, their solubility product is this:

Sulfide : Solubility! /mol . I ^-1 / Solubility Kb / 1 / ' ' / 1 - / 2,3 / CdS 10’« ΙΟ ²⁹ /7.10“ ²⁷ / PbS 10-« ΙΟ ²⁹ /8.10 ²⁸ / HgS 10 ²⁷ ΙΟ“ ⁵⁴ /3.1Ο ⁵² /

From the definition of the solubility effect as the effect of the corresponding powers of the activities of the reacting ions, it also follows that an effective excess of one of the reacting ions in the reaction medium can reduce the concentration of the other ion in the solution.

GOATES /J. Am.Chem. Soc. 74, 835, 1952/ published values of illusory affinities of sulfides:

Sulfide Sulfide Affinity:

/kcal. mol ¹ /

33.6

22.15

10.22

CdS PbS HgS

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It has been found that for the removal of heavy metals, especially cadmium, lead and mercury from extractive trihydrogenphosphoric acid and liquid multicomponent fertilizers prepared on its basis, a method according to the solution based on the use of precipitation chemical reactions associated with the formation of sulphides can be used. The method of removing heavy metals according to the solution consists in that the extractive trihydrogenphosphoric acid, or a liquid multicomponent fertilizer prepared on its basis, is subjected to a precipitation chemical reaction with a substance that is a source of sulfur sulfide at a temperature of 15 to 95 °C, preferably at a temperature of 30 to 60 °C, so that for each part by weight of phosphorus ⁱⁿ the reaction mixture, expressed in the form of P ₂ 0-, there are 1.02.10 ⁵ to 1*87.10 ³ parts by weight, preferably 1.45.10* to 1.15.10$ parts by weight of sulfur sulfide, while the reaction mixture is mixed in a circulation manner, without or with the use of a static mixer and/or is mixed pneumatically and/or by the effect of a mechanical mixer. For the intensity of mechanical mixing, which is characterized by the value of the Reynolds criterion Re^, the following relation applies:

Re _u nd /D - d/ . P . ,

T- · ----—h· £ 1.2 - io ³ where Re _H is the Reynolds criterion for mixing,

K is the efficiency dependent on the velocity distribution in the annular cross-section between the vessel wall and the stirrer, n is the frequency of rotation of the stirrer /s ¹ /,

D is the diameter of the vessel /a/ d is the diameter of the stirrer /·/ is the density of the reaction mixture in the reaction medium /kg.m~3/ is the dynamic viscosity coefficient of the reaction mixture in the reaction medium /kg.m ¹ . s ¹ /.

For the power input of the mechanical mixer N per unit volume of the stirred reaction mixture V, it is valid that: 2.5 - 1θ!< $ < 2.8 . 10 ³ /kg . _B 1 . s ³ /.

The precipitation reaction takes place without or in the presence of substances accelerating the clarification of the solution. The precipitated sludge is removed by sedimentation and/or filtration and/or separation and/or flotation.

As a substance which is a source of sulphide sulphur, one of the sulphur compounds with three electropositive elements is used. Examples of compounds of this type include the following: hydrogen sulphide* sulphides and hydrosulphides of alkali metals* alkaline earth metals* elements of metallic character with higher valency, polysulphides, polysulphic acids* polysulphates and salts. I

The following examples illustrate and clarify, but do not limit, the subject matter of the invention.

- and

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Example 1

In connection with the verification of the removal of heavy metals from liquid nitrogen-phosphorus fertilizer of the monophosphate type /8-24-0/ by the method according to the invention, a quarter-scale apparatus consisting of a circulating precipitation reactor and a continuously operating lamellar settler was used. The circulating reactor consisted of a collection tank connected by a glass pipe to a polypropylene centrifugal pump. The suction branch of the circulating reactor - in front of the centrifugal pump - was fed with a partial liquid fertilizer and also with an aqueous sodium sulfide solution.

The discharge branch of the circulation reactor opened below the level of the reaction mixture retention in the collection tank. The reaction mixture flowed through the overflow of the collection tank into the lower-placed lamellar settler, in which the sludge fraction was continuously separated.

An average of: was continuously dosed into the suction branch of the precipitation reactor using a peristaltic-hose dosing pump:

8.33 g. with ¹ NP - solution type 8-24-0 and 4.17.10 g. with approximately 13% aqueous solution of sodium sulfide /Na ₂ S/.

The purified NP-solution of type 8-24-0 was prepared by continuously neutralizing the extractive H^PO^ with concentrated ammonia in a tubular reactor. The extractive trihydrogenphosphoric acid of the so-called black type, used for the preparation of the liquid NP-fertilizer, contained:

53.91 wt. % of total P ₂ 0^

0.14% AI

0.30% of

0.82% MgO

0.01% Ca ppm/mg. kg” ¹ / Cd

0.3 ppm/mg. kg" ¹ / Pb and 620 ppm / mg. kg ^-1 / Cr.

The approximately 13% sodium sulfide solution used was prepared by dissolving 40 parts by weight of Na ₂ S. 9 H ₂ 0 in 60 parts by weight of water at room temperature.

Every minute, approximately 0.5 kg of clear NP-solution type 8-24-0 continuously flowed from the upper part of the lamella settler, in which the content of the monitored heavy metals was reduced by an average of 7 to 12 times /cadmium, lead/, dirty gray sludge was occasionally discharged from the lower part of the lamella settler.

Example 2

For the purpose of laboratory studies of a continuous method of removing heavy metal cations - cadmium, lead and mercury from extractive trihydrogenphosphoric acid, an all-glass apparatus was assembled consisting of two cascade-mounted glass stirred reactors, the total volume of each of which was 1500 cm^/working volume determined by the height of the side overflow of the reaction mixture was 1200 cm^/.

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Both reactors had a diameter of 120 nm, while the diameter of the double-blade glass stirrer, coaxially installed in each of the reactors, was 55 nm. The stirrers were driven by an electric motor, with the set rotation frequency being 400 ^rpm . Both reactors were placed in a water temperature bath, the temperature of which fluctuated between 57 and 65 °C.

The first, well-placed - with members of the reactor cascade was equipped with a backwater cooler, through which the reactor was aerated. This laid reactor was connected by means of a side overflow of the reaction vessel to the second, which was also equipped with a backwater cooler, serving for condensation of vapors and aeration of the reactor.

An average of 8.33 10 ² gs ¹ of black type extraction 3^4, specified by the chemical composition given in Example 1, and 8.33 10 ⁴ gs ¹ of a 13% aqueous solution of sodium sulfide were dosed into the first member of the reactor cascade using peristaltic (tube) dosing pumps from inclined tanks placed on scales.

An average of 8.42 x 10 g/l of ^a 1% aqueous solution of the flocculant Liagnofloc was dosed into the second settled reactor in a similar manner.

Under the stated conditions, at a steady state and the first to the second member of the reactor cascade, 302.88 g of acid reaction solution was overflowing per hour.

The average residence time of the reaction mixture in the first and second reactors was about 4 hours. The Reynolds criterion for settling was equal to:

Re _u nd /Dd/ . 0 6.67.0.055. /0.12-0.055/.1648.7 —- - ----TT"---l· · ------------- ²¹ 2312.6

K' 0.017

The acid flowing down the overflow from the second settled reactor was concentrated in a glass beaker with a volume of 5000 au.

After the continuous run of the precipitation experiment was completed, the acid reaction slurry was allowed to settle freely to the second bottom. Then, a clear layer of acid was separated above the layer of gray-brown sludge settled at the bottom.

Chemical analysis of the acid obtained in this way using the atomic absorption spectroscopy method revealed that it contained about 10 times lower levels of cadmium and lead and practically contained no mercury compared to the extraction acid dosed into the first member of the reactor cascade.

Claims (2)

8 EN 270 198 B1 PRBDMBT EXAMPLE 1. SpBsob Removal! heavy metals, pre-milled cadmium, lead and mercury, from extractive trihydrogenphosphoric acid and liquid multicomponent fertilizers based on the use of precipitating chemical reactions associated with the formation of sulphide, characterized in that the extraction of hydrogen phosphate or liquid component fertilizer based thereon at a temperature of 15 ° C to 95 ° C, preferably 30 ° C to 60 ° C, is subjected to a chemical reaction with a substance which is a sulfuric sulfur reservoir such that for each part by weight of phosphorus in the reaction mixture, expressed as P 2 O 5, "1.02 - 10 - 5 to 1.87 - 10" 3 parts by weight, preferably 1.45 - 10 "- 1.15. 10 3 3 parts by weight of sulphide sulphide, the reaction mixture being milled by circulation and using a static mixer and / or pneumatically and / or by a mechanical stirrer, the intensity of the mechanical stirrer being characterized by the Reynolds criterionReu for which Relation: Re "d. / D - d / 1.2. 10J where Re. n D D%%% is the Reynolds criterion for scaling, the coefficient is dependent on the distribution of velocities in the annular cross-section between the vessel wall and the milling agent, the rotational frequency m / m / s is the diameter of the vessel (m /, is the diameter m / m) m /, is the reaction medium density in the reaction medium (kg) and ** 3 /, is the dynamic viscosity coefficient of the reaction mixture in the reaction medium (kg · m · 3 · s 1 1) and for the mechanical mixing of N per unit volume of the mixed reaction mixture Applies: 2.5 - ΙΟ1 γ γ <2.8, 103 / kg. m -1 -1, where the precipitation reaction takes place, or in the presence of clarifier systems and cured the sludge is separated by sedimentation and / or filtration and / or by shooting and / or flotation. 2. Process according to claim 1, characterized in that one of the sulfur-containing compounds with multiple electropositive elements is used as the source of the sulfide sulfur.