RU2312820C1 - Method of cleaning arsenic-polluted solvents - Google Patents

Abstract

FIELD: waste water treatment.

SUBSTANCE: invention relates to treatment of waste waters and solvents containing significant amounts of hydrochloric or sulfuric acid and arsenic and can be used in metallurgy, especially in manufacture of nonferrous metals, as well as in chemical industry. Method consists in that arsenic is precipitated by sulfide-containing reagent, e.g. sodium hydrosulfide, said sulfide-containing reagent being added to solution to be treated from below while simultaneously stirring the solution. Specific weight consumption of the reagent is not higher than 1.5 kg*S^-2*h^-1. Hydrodynamic regime of stirring is maintained within the Reynolds number range 600 to 6000. Precipitation of arsenic is effected until its residual concentration in solution not below 0.03 g/dm³. Addition of sulfide-containing reagent is stopped when redox potential value is achieved in the curve break point on diagram depicting consumption of the reagent as function of arsenic concentration in solution.

EFFECT: achieved simplicity and safety of arsenic precipitation excluding emission of toxic gases.

1 dwg, 3 tbl

Description

The invention relates to techniques for treating wastewater and solutions formed in the production of non-ferrous metals and containing significant amounts of hydrochloric or sulfuric acid, arsenic, lead, bismuth, selenium and other toxic substances. The invention can be used in the metallurgical industry, mainly in non-ferrous metallurgy, as well as in the chemical industry.

Arsenic is a common impurity in ores and concentrates of iron, non-ferrous and rare metals and plays a specific role in the processes of their processing. Firstly, it complicates the flow of technological processes and, due to its high toxicity, worsens the quality of marketable products. Secondly, arsenic is one of the few elements for which there is very limited demand. The lack of wide demand leads to the storage and accumulation of various kinds of arsenic-containing waste polluting the environment on the open areas of enterprises. In addition, in the process of metallurgical or chemical production, arsenic partially passes into the wastewater and exhaust gases of plants and pollutes the environment with highly toxic compounds. In connection with the foregoing, in the metallurgical, and often in chemical industries, there is a problem of removing arsenic from technological processes and neutralizing arsenic-containing solid, liquid, and gaseous wastes.

The invention of the present application relates to technical solutions related to the disposal of arsenic-containing industrial wastewater before it is discharged into rivers and water bodies. The invention is based on the selective removal of arsenic from wastewater (solutions) by chemical precipitation of it in the form of sulfides. In this way, arsenic is mainly removed from acidic solutions, hydrogen sulfide is passed through them, or sodium sulfide, sulfides of other alkali or alkaline earth metals, sodium hydrosulfide, etc. are added. Sulfide purification allows to reduce the concentration of arsenic in the cleaned solutions to 0.5-2 mg / l, but to achieve these indicators certain conditions must be met at the stages of deposition and separation of sulfide precipitation. First of all, for a more complete precipitation of sulfide, arsenic is transferred to a trivalent state and the contact quality of the deposited agent is monitored with the solution to be cleaned, for which a certain set of measures is used, such as supplying countercurrent reactants, mixing the solution, observing certain process parameters, such as mixing speed, concentrations and the ratio of the concentrations of the components in certain time ranges of the process, etc.

The patent studies carried out by the applicant have revealed solutions from the world level of technology similar to the claimed one, in which the sulfide purification of arsenic-containing solutions is a fundamental tendency. Consider them and analyze for the effectiveness of cleaning solutions from arsenic and, most importantly, the safety of the cleaning process.

The well-known "Method for the extraction of arsenic (+3) from acidic solutions (see ed. Certificate of the USSR No. 1763372, class C01G 28/00, 1990), including treating them with a sulfide reagent and subsequent separation of the precipitate of arsenic sulfide, characterized in that in order to increase the productivity of the process and the content of arsenic in the sediment when using solutions of wet purification of gases from sulfuric acid production, the initial solution is pre-diluted with water in a volume ratio of 1: (0.6-1), respectively. " As follows from the description of the invention, solutions are treated in this way, "selected in the washing section of the sulfuric acid workshop from the sumps of the wet gas cleaning system" and containing arsenic (+3), sulfuric acid and sulfur dioxide.

Water is poured into the initial solution, then the sulfide reagent is introduced “and the reaction mixture is kept under stirring until the gas formation process is completed”. The pulp with arsenic sediment is drained and sent for sedimentation, and the resulting hydrogen sulfide is returned to the process.

However, this method has disadvantages:

- there is no countercurrent sulfide-containing reagent and the initial solution, which reduces the quantity and quality of contact of the reactants with each other, and therefore does not provide complete deposition of arsenic;

- for a volume of 0.5 liters of the initial solution (as in the known test method), the speed characteristics of mixing may be unimportant, however, in an industrial production where sufficiently large volumes of solutions are being purified, a hydrodynamic mixing mode must be calculated that ensures maximum chemical interaction reagents, and hence the completeness of deposition of arsenic.

Closest to the claimed is a method of purification of solutions from arsenic by precipitation with sulfide-containing reagents, for example, sodium sulfide, characterized in that, in order to intensify the purification process and ensure safety, precipitation of arsenic is carried out in the presence of sulfur dioxide in the solution (see Auth. USSR No. 327771, class C01B 27/00, 1970). In this case, the sodium sulfide solution is injected in a thin stream through a tube immersed to the bottom of the reactor with continuous stirring of the solution. In accordance with dependent claim 2, the precipitation is terminated at a sulfur dioxide concentration in the solution of at least 1 g / l. This method is adopted as a prototype.

When implementing the method, the following occurs. The purified sulfuric acid solution is poured into the reactor from above, and the sodium sulfide solution is countercurrent to the initial solution, that is, from the bottom up (due to the immersion of the tube to the bottom of the reactor). The precipitating reagent comes into contact with the solution to be purified, and two reactions proceed simultaneously and simultaneously:

The formation of hydrogen sulfide, the interaction of arsenic ions with sulfide sulfur ions and the formation of a precipitate of arsenic trisulfide. In order to prevent the release of hydrogen sulfide into the atmosphere and to exclude hazardous conditions for human health and the environment, the authors of the known technical solution proposed the deposition of arsenic in the presence of SO ₂ in a solution of sulfur dioxide. The latter should react with hydrogen sulfide, excluding the release of H ₂ S and its release into the atmosphere and causing the formation of a number of “other compounds (thiosulfuric and polythionic acids) precipitating arsenic in the form of sulfide”.

However, a technical contradiction arises: so that toxic hydrogen sulfide is not formed and does not go into the atmosphere, sulfur dioxide, recombining hydrogen sulfide, is used. At the same time, sulfur dioxide itself is toxic, and its maximum permissible concentration (MPC) in the atmosphere, like that of hydrogen sulfide in the atmosphere, cannot exceed 10 mg / m ³ , that is, one toxic gas is removed from the production process by introducing into it gas of the same level of toxicity, which, once in the atmosphere, will poison it.

In the prototype, as in the previous known invention, there are no hydrodynamic mixing parameters, which, with large, industrial volumes of the solution to be purified from arsenic, will not provide high cleaning efficiency of the latter.

In addition, there are organizational difficulties: since not all industrial solutions formed in the metallurgical industry, including the production of non-ferrous metals, contain sulfur dioxide, it is necessary to carry out an additional process of saturating the solution to be purified with sulfur dioxide. To do this, either blowing sulfur dioxide gas into the solution or irrigating the sulfur dioxide gas stream with a solution is carried out. This operation is long, since the rate of dissolution of sulfur dioxide is small and depends on the contact method (blowing or irrigation), on the partial pressure of the gas, the dispersion of gas bubbles, the density of irrigation, the concentration of impurities in the solution and the salt content. During gas injection or irrigation with a solution, a part of the gas is inevitably released into the atmosphere, which creates dangerous working conditions for staff;

- maintaining the given concentrations of sulfur dioxide in the solution (5 g / l at the beginning of the process and 1 g / l at the end of it) require constant monitoring of these values. Currently, there are no continuous monitoring methods for sulfur dioxide concentrations. So, there is no guarantee of achieving in the known method the specified values of the concentration of sulfur dioxide.

The circumstances described above create difficulties in the implementation of the known method, which refutes the claim of the authors of the invention by ed. testimonial. No. 327771 on the simplicity of its implementation, and also do not exclude emissions of toxic sulfur dioxide, which is dangerous for human health and the environment, that is, the process of cleaning solutions from arsenic is not ensured. And to prevent the release of sulfur dioxide into the atmosphere, additional operating costs will be required.

The authors of the proposed method conducted pilot industrial studies of the prototype method at the Sredneuralskiy smelter, the results of which confirmed these shortcomings and its industrial unsuitability.

Instead of sodium sulfide, sodium hydrosulfide was used as a sulfide-containing reagent. This reagent is cheaper than sodium sulfide, more widespread and accessible, and in chemical activity when interacting with As (+3) is not inferior to sodium sulfide.

The specialists were faced with the task of creating such a technical solution, the implementation of which would allow the reaction to obtain arsenic trisulfide to occur, but at the same time to eliminate the formation of hydrogen sulfide, that is, to clear the solution from arsenic safely and without additional costs.

The specified problem is solved in the proposed invention for examination. It, like the prototype, is based on the precipitation of arsenic from a solution by a sulfide-containing reagent and includes the supply of sulfide-containing reagent into the solution to be purified from the bottom up with simultaneous stirring.

The inventive method differs from the prototype in that it supplies a sulfide-containing reagent with a specific mass flow rate of not more than 1.5 kg S ^2- per hour per 1 kg of As ³⁺ in the solution to be cleaned, the hydrodynamic mixing mode is maintained in the range of Reynolds numbers 600-6000, and deposition arsenic is produced to a residual concentration in the solution of not less than 0.03 g / dm ³ and the sulfide-containing reagent is stopped feeding when the value of the redox potential of the solution (Red OX potential) is reached at the inflection point of the curve in the diagram, which reflects the consumption rate of sulfide-containing reagent on the concentration of arsenic in the solution.

The claimed invention meets all the criteria of patentability.

It has novelty, since the applicant has not identified technical solutions from the prior art that are characterized by the same set of features, as evidenced by the above analysis of known methods.

The inventive method is industrially applicable. And the method as a whole, and individual actions and operations of the method are feasible and reproducible, nothing in the proposed technical solution does not contradict and does not interfere with the use of the invention in industrial production with the achievement of the expected technical result.

The technical solution proposed for patent examination has an inventive step, since it does not follow explicitly from the prior art for a specialist. When conducting patent research, no solutions were identified that have the same set of essential features as the claimed solution, and no solutions have been identified that have signs that match the distinctive features of the proposed method.

The following description of the proposed method and its implementation, as well as the resulting benefits, is confirmation of this.

Example. To implement (test) the proposed method for purification by precipitation of arsenic with a sulfide-containing reagent, the following were used:

- as a purified solution - arsenic sulfate solution with a concentration ratio of sulfuric acid and arsenic equal to 2 or more;

- as a sulfide-containing agent - sodium hydrosulfide NaHS, previously diluted with water to a concentration of sulfide sulfur of 80-120 g / DM ³ .

Arsenic sulfate solution was poured from above into a sealed reactor equipped with a stirrer located near the bottom, for example a paddle, communications for supplying solutions, draining the pulp and control and automation devices. Sodium hydrosulfide was fed through a nozzle located in the lower part of the reactor, which, leaving the outlet of the nozzle, moved in a spiral from the bottom up. In this case, the sulfide-containing reagent was supplied with a specific mass flow rate of not more than 1.5 kg S ^2- per hour per 1 kg of As ³⁺ in the solution to be purified (or 1.5 g of sulfide sulfur per hour per 1 g of arsenic in the solution). It was experimentally established that the hydrogen-free mode of deposition of arsenic ions in the form of its trisulfide is maintained at a given supply of a sulfide-containing reagent. This is more than twice the stoichiometric flow rate equal to 0.64 g per 1 g of arsenic, determined from the ratio of molecular weights.

Simultaneously with the supply of sodium hydrosulfide, a stirrer was switched on to mix the initial solution with sodium hydrosulfide solution. With stirring, the hydrodynamic regime was maintained in the range of Reynolds numbers 600-6000. Sodium hydrosulfide made a complex movement in the layer of the initial solution, namely, the simultaneous movement up and radially from the center of the reactor to the periphery. Such a movement of a sulfide-containing reagent in a layer of an arsenic sulfate solution resulted in a high contact frequency of S ^2– with As ³⁺ , while maintaining the hydrodynamic regime in the claimed range of Reynolds numbers ensured a uniform distribution of sulfide sulfur in the solution volume, which allowed it to react only with arsenic ions .

As tests have shown, with a Reynolds number below 600, uniform distribution and mixing of sodium sulfide in the entire solution volume is not ensured, local supersaturation is formed, where the specific mass consumption of sulfide sulfur exceeds a predetermined norm of 1.5 kg S ^2- per hour per 1 kg of arsenic (3 +) in the solution to be purified, and excess sulfur reacts with sulfuric acid, which leads to undesirable release of hydrogen sulfide. And when the Reynolds number is more than 6000, the solution foams and the sulfide-containing reagent is removed to the surface of the solution, which again leads to the release of hydrogen sulfide. At Reynolds numbers in the declared interval, hydrogen sulfide is not released, since neither supersaturation sites nor sulfide-containing reagent emissions are formed on the solution surface, which means that there is no process for the formation of hydrogen sulfide.

Arsenic was precipitated to a residual concentration of arsenic in solution up to ~ 0.03 g / dm ³ (and not lower). Since the rate of the chemical reaction of arsenic trisulfide formation depends on the concentration of arsenic in the solution, its effect on the release of hydrogen sulfide was studied. This made it possible to determine the critical concentration of arsenic in the solution, at which, despite maintaining other parameters at optimal intervals, the inevitable release of hydrogen sulfide occurs. At the same time, the process of supplying a given amount of sulfide-containing reagent and the end of the reaction of the interaction of sulfide sulfur ions and arsenic ions were monitored by a graphical line in the diagram that reflects the changes in the redox potential (Red OX potential) and the concentration of arsenic in solution from the consumption of sulfide sulfur. As soon as there was a sharp change in the value of the indicated potential in the direction of decreasing it, the sulfide-containing reagent was stopped. On the graphical curve of the line drawn by the recorder, this moment is reflected by the inflection point (see drawing). Tests of the proposed method proved that it was at the time of the jump in the redox potential that the concentration of arsenic in the solution was 0.03 g / dm ³ , and at values below this value, hydrogen sulfide was released, which can be explained by the insufficiency in the solution of arsenic to interact with sulfur sodium in the declared amount. The presence of an excess of sodium sulfide in the solution will cause the interaction of sodium sulfide with sulfuric acid with the release of toxic hydrogen sulfide.

The results of laboratory experiments and pilot studies are shown in tables 1, 2 and 3.

At the end of the process, a pulp with a precipitate of arsenic trisulfide was obtained. The pulp was defended and filtered. The resulting precipitate was washed and analyzed. Sludge was assigned to the second hazard class and, in order to transfer it to a state that is safe for the environment, it is processed according to the known technology by drying, pressing into briquettes and applying a layer of bitumen on the surface of the briquettes to obtain a sludge of the fourth hazard class, which can be stored in earth trenches with clay insulation.

At the stage of sulfide purification, for example, when the initial concentration of arsenic in the solution is 4000 mg / dm ³ and after purification equal to 30 mg / dm ³ , the degree of purification from arsenic is 99.25%.

According to the results of a chemical analysis, the arsenic content in the clarified solution after sulfide treatment is 30 mg / dm ³ , and it can be directly used in the recycling water supply system of the workshop (plant), for example, for washing wet electrostatic precipitators, spray and mist eliminators, or subjected to purification with known by methods, for example, by neutralization to a pH of 8.5 with milk of lime to obtain a purified neutral solution and gypsum-containing precipitate.

Thus, the implementation of the method in accordance with the claims ensured the deposition of arsenic in the form of sulfide - the least toxic form for people and the atmosphere. The claimed technology is safe for service personnel, since its implementation excludes the release of toxic sulfur dioxide and hydrogen sulfide.

All these advantages provide a safe process of deposition of arsenic when cleaning solutions containing it, and do not require additional costs to exclude emissions of toxic gases into the atmosphere.

Claims (1)

A method of purifying solutions from arsenic by precipitation with a sulfide-containing reagent, including supplying a sulfide-containing reagent into the solution to be purified from the bottom up with simultaneous stirring, characterized in that the sulfide-containing reagent is supplied with a specific mass flow rate of not more than 1.5 kg (S ^2- ) per hour per 1 kg (as ³⁺⁾ in the cleaning solution, the hydrodynamic mixing mode is maintained in the range of Reynolds numbers 600-6000, the deposition to produce a residual arsenic concentration in the solution is not lower than 0.03 g / dm ^3, and feeding sulfidsoderzh present reagent is stopped when the value of the redox potential of the solution at the inflection point in the graph curve reflecting the dependence of the flow of reagent from the arsenic sulfide-concentration in the solution.