RU2574449C2 - Method of removing arsenic (v) compounds from technological solutions - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention can be used in processing of arsenic-containing production and wastes, formed in the process of lewisite destruction by method of alkaline hydrolysis. To remove arsenic (V) compounds from solutions with content of As (V) from 10 g/l in the process of arsenic-containing raw material processing interaction with elementary fine-disperse arsenic is realised by reaction of con-proportioning at medium pH 9-10 and heating solution to 60°C. Elementary fine-disperse arsenic required for carrying out reaction of con-proportioning can be obtained by action of reducing agent on part of solutions to be purified from As (V) compounds.

EFFECT: invention makes it possible to reduce amount of wastes and labour consumption, to reduce amount of applied reducing agent by 2,5 times.

3 cl, 2 ex

Description

The invention relates to the field of technology of the chemical industry and can be used as an integral part of the technology used for processing arsenic-containing products and waste generated during the destruction of lewisite by alkaline hydrolysis.

A method of processing arsenic (V) compounds into commercial products is a key element that determines the feasibility and profitability of technologies for processing arsenic-containing raw materials containing a mixture of arsenic compounds in oxidation states (III) and (V). Such raw materials are products formed as a result of alkaline hydrolysis of lewisite, such as “lewisite reaction mass”, TU 2112-123-04872702-2002 (hereinafter - RM); "Hydrolytic sodium arsenite", TU 2622-159-04872702-2005 (hereinafter - ANG). Methods for isolating arsenic (III) compounds from such raw materials are well known and described in a number of works [1-3]; common features of the methods are the dissolution or leaching of raw materials, concentration of the solution of arsenic salts and the creation of a pH close to neutral in it, while the compounds of arsenic (III) are converted into poorly soluble oxide of arsenic (III). The costs of this process are negligible and the process does not lead to waste generation. The situation is different with the processing of arsenic (V) compounds. When arsenic (III) oxide is isolated, arsenic (V) compounds remain almost completely in solution, which ultimately leads to the need to add a separate process step associated with their elimination. In technologies [1-3], the use of reagent reducing agents for the conversion of As (V) compounds to As (III) or elemental arsenic is proposed. The use of reducing agents leads to the contamination of solutions with by-products, which further leads to the need for the disposal of reaction solutions in the form of production waste. In addition, the proposed reducing agents are characterized by high cost. For these reasons, the main costs of the technological process of hydrometallurgical processing of ANG, RM are associated with the processing of arsenic (V) compounds.

Let us consider in more detail the known methods for removing arsenic (V) compounds from solutions during the processing of arsenic-containing raw materials.

In the method described in [1], it is proposed to carry out the reduction of arsenic (V) compounds into arsenic (III) with hydrochloric acid in a strongly acidic medium at a pH of less than 1:

The disadvantage of this process is the need to work with highly toxic and volatile arsenic trichloride (LD ₅₀ = 48 mg / kg), which is also a lewisite precursor under the Chemical Weapons Convention [5]. Working with such a substance requires the application of the highest standards of industrial and environmental safety and greatly increases the cost of the technical process. Another negative point is the uncontrolled formation of gaseous chlorine.

In the method described in [3], the separation of As (V) compounds from solution in the form of sparingly soluble lithium arsenate is used. Further, lithium arsenate is converted into arsenic acid and reduced to arsenic trichloride by the action of ascorbic acid or hydrazine in the presence of sodium iodide. The disadvantages of this technology are multi-stage process; the use of expensive reagents (according to October 2013: lithium chloride - from 480 rubles / kg, sodium iodide - 1550 rubles / kg, ascorbic acid - 150 rubles / kg); additional pollution of the resulting product As ₂ O ₃ impurities of lithium compounds, iodine, oxidation products of ascorbic acid; production of arsenic trichloride in technology.

The patent [4] describes a method for isolating elemental arsenic from a solution of As (III) compounds, such as sodium arsenite, products of detoxification of lewisite and mustard-lewisite mixtures by reaction with a reducing agent, thiourea dioxide. The development of such an approach to solutions of As (V) compounds is presented in [6]. Compounds As (V) are reduced to elemental arsenic under the action of thiourea dioxide (DTM) in solution in an alkaline medium when heated. The resulting arsenic is cleaned by washing and vacuum drying and is the final product of the process.

The resulting solution containing the decomposition products of the reducing agent does not find further use and must be disposed of as production waste.

The objective of the invention is to develop a method for removing arsenic from solutions in which it is in the form of compounds As (V) in significant concentrations (from 10 g / l), with lower economic costs and with better technological parameters than the known methods.

The desired result is achieved as follows: two different reducing agents are used to restore As (V) compounds: the first is one of the known reducing agents that react in an alkaline environment with arsenic (V) compounds to form elemental arsenic, for example, DTM; the second is the finely dispersed elemental arsenic, which is formed when using the first reducing agent. We calculate the amount of reducing agent used in the technology using a new approach: to simplify the calculations, suppose that the solution to be purified is supplied periodically with volumes with an equal content of As (V) compounds in n mol. For generality of calculations, we describe the reaction of As (V) compounds with an arbitrary one-electron reducing agent, which we denote as RH in reduced form and R in oxidized form. The first batch of cleaning solution is treated with RH reducing agent:

5n moles of reducing agent are consumed in the reaction, n moles of elemental arsenic are formed. The second batch of the solution to be purified is treated with elemental arsenic with a reaction of proportionality:

The resulting As (III) compounds, not contaminated with RH oxidation products, are returned to the As (III) compound recovery unit.

моль ранее полученного элементного мышьяка, оставшаяся $$\frac{1}{3}n$$

моль элементного мышьяка используется далее для восстановления еще 0,5n моль As (V). Суммарное количество As (V), переходящее в As (III), приходящееся на одну обработку восстановителем RH, равняется n+n+0,5n=2,5n, тогда как по уравнению реакции (3) видно, что если исключить использование конпропорционирования в технологии и пользоваться только этой реакцией, одна порция восстановителя приводит к выделению мышьяка только из n моль соединений As (V). В итоге, применение нового способа вместо типичного одноэтапного восстановления, рассматривавшегося ранее, позволяет расходовать в 2,5 раза меньшее количество восстановителя RH. Фактически, в новой схеме процесса восстановитель RH используется для восстановления As (V) 2е->As (III) вместо As (V) 5е->As⁰. Важным следствием является то, что если в обрабатываемом растворе содержится, помимо As (V), также и остаточное количество As (III), то эффективность нового способа в сравнении со старым увеличивается еще больше, так как при одноэтапном подходе дополнительное количество RH расходуется на восстановление As (III)->As⁰. Разницу в количестве восстановителя и количестве образующихся отходов можно проиллюстрировать на следующем примере:From reaction equation (4) it is seen that for the reduction of n moles of As (V) to As (III), $$\frac{2}{3}n$$

mole of previously obtained elemental arsenic remaining $$\frac{\mathrm{one}}{3}n$$

a mole of elemental arsenic is further used to recover another 0.5n mol of As (V). The total amount of As (V) passing into As (III) per one treatment with a reducing agent RH is n + n + 0.5n = 2.5n, while it can be seen from reaction equation (3) that if the use of disproportionation in technology and use only this reaction, one portion of the reducing agent leads to the release of arsenic from only n moles of As (V) compounds. As a result, the use of a new method instead of the typical one-step recovery, considered earlier, allows you to spend 2.5 times less amount of RH reducing agent. In fact, in the new process scheme, RH is used to reduce As (V) 2-> As (III) instead of As (V) 5-> As ⁰ . An important consequence is that if, in addition to As (V), the residual amount of As (III) is also contained in the treated solution, the efficiency of the new method in comparison with the old one increases even more, since in a single-stage approach, additional RH is spent on recovery As (III) -> As ⁰ . The difference in the amount of reducing agent and the amount of waste generated can be illustrated by the following example:

To process the solution at the inlet to the purification unit of arsenic (V) compounds of the composition: H ₃ AsO ₄ 1.412 kg, As ₂ O ₃ 0.117 kg, H ₂ O 6.816 kg, NaCl 1.886 kg, DTM - 2.878 kg, NaOH - 2, are used. 15 kg, after washing and isolating arsenic, 6.837 kg of waste remains, containing: Na ₂ SO ₃ 3,354 kg, (NH ₂ ) ₂ CO 1,597 kg, NaCl 1,886 kg.

.For the new method: total $$H_{3}As{O}_{\mathrm{four}}=\frac{m}{M(H_{3}As{O}_{\mathrm{four}})}=\frac{1412}{142}=10\text{mole}$$

.

.Total amount $$A{s}_2{O}_3=\frac{117}{198}=0.591\text{mole}$$

.

The processed solution is divided into two parts, the first part is processed by DTM, the second - by the formed elemental arsenic. We take into account the course of recovery of arsenic (III) compounds:

моль (1,08 кг, что в 2,66 раз меньше, чем при полном восстановлении входного раствора до элементного мышьяка), аналогично находятся количества Na₂SO₃ и (NH₂)₂CO - по 10 моль каждого, что соответствует 1,26 кг Na₂SO₃ и 0,60 кг (NH₂)₂CO. Поскольку объемы 1-й и 2-й частей раствора относятся также как $$\frac{v_1(H_{3}As{O}_4)}{v_2(As{O}_4)}=\frac{\mathrm{3,74}}{\mathrm{6,26}}=\mathrm{0,597}$$

, то можно определить массу хлорида натрия в 1-й части раствора: $$\frac{m_1(NaCl)}{m_2(NaCl)}=\mathrm{0,597}$$

; m₁(NaCl)+m₂(NaCl)=1,886 кг, откуда можно найти, что масса хлорида натрия в 1-й части раствора равна 0,705 кг.Denote by v ₁ (H ₃ AsO ₄ ), v ₂ (H ₃ AsO ₄ ), v ₁ (As ₂ O ₃ ), v ₂ (As ₂ O ₃ ), respectively, the amounts of arsenic acid in the 1st and 2nd parts solution, the amount of oxide of arsenic (III) in the 1st and 2nd parts of the solution. In this case, it is necessary to find the proportion in which the initial solution is divided with the condition that the total amount of arsenic in its 1st part refers to the amount of H ₃ AsO ₄ in the 2nd part as 2 to 3 (in order to provide a stoichiometric ratio elemental arsenic and arsenic (V) for reaction (4)). This condition is satisfied by the values of v ₁ (H ₃ AsO ₄ ) = 3.74 mol, v ₂ (H ₃ AsO ₄ ) = 6.26 mol, v ₁ (As ₂ O ₃ ) = 0.209 mol, v ₂ (As ₂ O ₃ ) = 0.350 mol. The amount of DTM is found as $$\frac{5}{2}\times v_{\mathrm{one}}(H_{3}As{O}_{\mathrm{four}})+\frac{3}{2}\times 2v_{\mathrm{one}}(A{s}_2{O}_3)=10$$

mol (1.08 kg, which is 2.66 times less than when the input solution was completely reduced to elemental arsenic), the quantities of Na ₂ SO ₃ and (NH ₂ ) ₂ CO are similarly found - 10 mol of each, which corresponds to 1, 26 kg Na ₂ SO ₃ and 0.60 kg (NH ₂ ) ₂ CO. Since the volumes of the 1st and 2nd parts of the solution are also related as $$\frac{v_{\mathrm{one}}(H_{3}As{O}_{\mathrm{four}})}{v_2(As{O}_{\mathrm{four}})}=\frac{3.74}{6.26}=0.597$$

, then you can determine the mass of sodium chloride in the 1st part of the solution: $$\frac{m_{\mathrm{one}}(NaCl)}{m_2(NaCl)}=0.597$$

; m ₁ (NaCl) + m ₂ (NaCl) = 1.886 kg, whence it can be found that the mass of sodium chloride in the 1st part of the solution is 0.705 kg.

The total mass of waste in the processing of this solution is equal to the sum of the masses m ₁ (NaCl) (since the second part of the solution is transferred to the processing unit for the separation of arsenic oxide, where sodium chloride is released in the form of commercial products), sodium sulfite, urea and is equal to 2.565 kg , which is 2.66 times less than with a single-stage treatment with a reducing agent. Another advantage of the new method is that elemental arsenic for the reaction of proportionalization can be used in the form of a wet suspension, which eliminates the time-consuming stages of drying and heat quenching [7].

Estimation of the total amount of waste in the processing of ANG

In a method using a one-step recovery of solutions purified from As (V) using DTM, the total amount of waste is calculated as m _out = m _s (w ₁ + w _{out (V)} ),

where w ₁ is the percentage of raw bentonite and polymeric organic substances,

m _with - the mass of raw materials, which accounts for one completed cycle of processing of raw materials;

w _{otkh (V)} - the mass of waste generated during the processing of solutions of arsenic (V) using DTM. If, however, to use the new method for the removal of arsenic (V) compounds, the total amount of waste generated can be estimated as follows:

w _{otx (V)} = w _{otx (V)} / 2.66 - the changed amount of waste generated in the unit for the removal of arsenic compounds (V);

m _exx = m _s (w ₁ + w ′ _{exx (V)} ).

For raw materials with an As (V) content of 4.3 wt.%

m _out = m _s (w ₁ + w _{out (V)} ) = 50 * (0.04 + 6.837 / 50 * 2.66) = 4.57 kg, which is 9.1 wt.%.

For raw materials with an As (V) content of 14.5 wt.% M _ref = m _s (w ₁ + w _{ref (V)} ) = 15 * (0.04 + 6.837 / 15 * 2.66) = 3.17 kg, which is 21.1 wt.%.

The technological process is as follows:

- the first batch of the purified solution containing As (V) compounds is fed to the treatment of thiourea dioxide in the reactor P1 by heating to 60-70 ° C and stirring for 1-1.5 hours; the resulting suspension is fed to the filter, finely divided elemental arsenic is washed with water on the filter; the filtrate is combined with wash water and sent for disposal;

- the second batch of the purified solution is fed into the reactor P2, where it is mixed with a portion of elemental arsenic obtained earlier. The reaction is carried out at a temperature of 30-50 ° C for 30 minutes, while an alkali solution is also fed into the reactor P2 to maintain the pH at a level of 9-10. After completion of the reaction, the pH of the solution is adjusted to 7-8 by the addition of hydrochloric acid, the resulting precipitate of arsenic oxide after filtration and washing is the final product; the filtrate and washings are combined, the resulting solution is used to leach or dissolve a new portion of arsenic-containing raw materials.

Example 1

10 g _of grade ₂ Na ₂ HAsO ₄ grade xc were introduced into a round bottom flask, dissolved in 100 ml of water. To the solution was added 40 ml of a buffer solution with pH = 6.3 and 7.14 g of wet elemental finely dispersed arsenic with a water content of 65%, obtained in advance by reducing sodium arsenite with sodium borohydride in solution when heated to 50 ° C. The round bottom flask was heated to 60 ° C with stirring. The precipitate in the solution quickly brightens due to the dissolution of elemental arsenic and the formation of sparingly soluble arsenic oxide in a slightly acidic medium. After 15 minutes, the total volume of the solution was brought to 200 ml with distilled water, a sample was taken and the content of As ^{5+ was} determined in it by the method of reverse iodometric titration. According to the titration result, the residual As ⁵⁺ content in the solution was determined; it amounted to 0.0037 mol, which confirms the restoration of the main amount of As (V) to As (III).

Example 2

, произведя расчет, аналогичный рассмотренному выше. В первую часть раствора массой 411,4 г добавили 116 г ДТМ, 85,7 г сухого NaOH, добавили 1 л дистиллированной воды, нагрели до 60°C и оставили перемешиваться на 30 мин. Образовавшийся осадок элементного мышьяка отфильтровали на фильтре Шотта, промыли двумя порциями по 100 мл воды, во влажном виде сняли с фильтра и внесли в химический стакан на 2 л. Туда же залили вторую часть обрабатываемого раствора массой 688,6 г, смесь нагрели до 60°C, и при перемешивании добавляли раствор NaOH для поддержания pH на уровне 9-10. После завершения реакции и полного растворения осадка к раствору добавили соляную кислоту до pH 7. Выпал белый кристаллический осадок оксида мышьяка, который отфильтровали и высушили. Полученная масса технического продукта - 125 г, содержание основного вещества - 94%.We used an arsenic-containing solution with a total mass of 1100 g with a content of H ₃ AsO ₄ 151.8 g, As ₂ O ₃ 12.6 g, NaCl 202.8 g to verify the complete cycle of operations of the method. The solution was divided into 2 parts, in the ratio $$\frac{3.74}{6.26}$$

by making a calculation similar to that considered above. 116 g of DTM, 85.7 g of dry NaOH were added to the first part of the solution, weighing 411.4 g, 1 l of distilled water was added, heated to 60 ° C and allowed to mix for 30 minutes. The precipitate of elemental arsenic was filtered on a Schott filter, washed with two portions of 100 ml of water, removed from the filter in the wet state and placed into a 2 L beaker. The second part of the treated solution weighing 688.6 g was poured there, the mixture was heated to 60 ° C, and a NaOH solution was added with stirring to maintain the pH at 9-10. After completion of the reaction and complete dissolution of the precipitate, hydrochloric acid was added to the solution to pH 7. A white crystalline precipitate of arsenic oxide precipitated, which was filtered and dried. The resulting mass of technical product is 125 g, the content of the main substance is 94%.

Information sources

[1] Patent for the invention of the Russian Federation No. 2359725, IPC A62D 3/36, A62D 3/37, A62D 101/40. A method of processing the reaction masses formed during the detoxification of lewisite / Demakhin A.G., Oliskevich V.V., Ramazanov K.R., Mishin V.N., Shveikin V.A. - announced on February 13, 2008, application No. 2008104843/15. Bull. No. 18, 06/27/2009.

[2] Patent for the invention of the Russian Federation No. 2359915, IPC C01G 28/00, A62D 3/00, C02F 11/08. A method of recovering arsenic (V) compounds contained in alkaline detoxification products of lewisite into arsenic (III) compounds / Demakhin A.G., Oliskevich V.V., Eliseev D.A., Mishin V.N., Shveikin V.A. . - declared on February 13, 2008, application No. 2008104842/15. Bull. No. 18, 06/27/2009.

[3] Patent for the invention of the Russian Federation No. 2389526, IPC A62D 3/36, A62D 3/37, A62D 101/02. A method of processing products of alkaline hydrolysis of lewisite into commercial products / Demakhin A.G., Nikiforov A.Yu., Oliskevich V.V., Mishin V.N., Shveikin V.A. - announced on December 10, 2008, application No. 2008148750/15. Bull. No14, 05/20/2010.

[4] RF patent No. 2371391, IPC A62D 3/30, C01G 28/00. A method of producing elemental arsenic from aqueous and aqueous-organic solutions of arsenic compounds / Sheluchenko V.V., Utkin A.Yu., Korolkov M.V., Pavlichenko V.F., Makarochkina S.M. - announced on 05/15/2008, application No. 2008119099/15. Bull. No. 30, 10.27.2009.

[5] URL: http://www.opcw.org/en/konvencija-o-khimicheskom-oruzhii/prilozhenie-po-khimikatam/v-spiski-khimikatov/ of 08/08/2013

[6] Rastegaev O.Yu., Malishevsky A.O., Tolokonnikova TP, Chentsov A.M., Chupis V.N. Methods for the determination of arsenic in technical products and wastes for environmental control purposes at CWD facilities // 3rd All-Russian Conference with International Participation “Chemical Disarmament-2009. "CHEMDET-2009": Sat scientific labor. - Izhevsk: IPM Ural Branch of the Russian Academy of Sciences, 2009. - P.214-218.

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Claims (3)

1. The method of removing compounds of arsenic (V) from solutions with an As (V) content of 10 g / l during processing of arsenic-containing raw materials, comprising reacting with elemental finely dispersed arsenic by a proportional reaction at a pH of 9 to 10 and heating the solution to 60 ° C . 2. The method according to p. 1, characterized in that the elemental finely dispersed arsenic required for the reaction of the proportionalization is produced by the action of a reducing agent on part of the solutions to be purified from As (V) compounds. 3. The method according to p. 2, characterized in that thiourea dioxide is used as a reducing agent and the general process scheme has the following form:
- a portion of the purified solution containing As (V) compounds is treated with thiourea dioxide;
- the resulting fine elemental arsenic is washed with water on the filter;
- the filtrate is combined with wash water and sent for disposal;
- a new portion of the solution to be purified is fed into the reactor, where it is mixed with the calculated amount of elemental arsenic obtained earlier, after which, when heated to 60 ° C and while maintaining the pH of the solution at the level of 9-10, the reaction of proportionality is carried out;
- after completion of the reaction, after 30 minutes the pH of the solution is adjusted to 7-8 by the addition of hydrochloric acid, the resulting suspension is fed to the filter, where the arsenic (III) oxide precipitate, which is the final product, is washed and separated;
- the filtrate and washings formed during the filtration of arsenic (III) oxide are combined, the resulting solution is used to leach or dissolve arsenic-containing raw materials.