RU2731269C1 - Processing method of corrosion inhibitor containing compounds of hexavalent chromium and sea water - Google Patents

Abstract

FIELD: chemistry.SUBSTANCE: invention can be used in cleaning solutions containing compounds of hexavalent chromium and sea water, specifically for cleaning spent solution of corrosion inhibitor containing 100–2,000 mg/l of potassium chromate KCrO. Method of processing a corrosion inhibitor containing a hexavalent chromium compound involves acidifying the starting solution to pH 2–3, treating the acidified solution with a reducing agent with reducing Cr(VI) to Cr(III) and obtaining a precipitate of Cr(III) hydroxide, recovering the obtained precipitate. For neutralization of acidified solution and reduction of Cr(VI) to Cr(III) as reducing agent there used is sodium salt NaSOor NaHSO. To the obtained solution is added 10–20 % solution of iron sulphate FeSO⋅7HO or ferric chloride FeCl⋅4HO. Solution is then neutralized with NaOH solution to pH 8–9, and a flocculant is added. Obtained precipitate is separated by filtration and subjected to two-step thermal treatment, which includes drying at 100–120 °C for at least 2 hours and heat treatment at 300–310 °C for 60–70 minutes. Then, aluminothermic reduction of the treated precipitate is performed to obtain ferrochromium and aluminothermic slag.EFFECT: invention increases efficiency of processing solutions containing compounds of hexavalent chromium by improving quality of cleaning and recycling precipitation to obtain commercial products.1 cl, 2 ex, 1 dwg

Description

The invention relates to the purification of chromium-containing solutions and can be used to purify solutions containing hexavalent chromium compounds and seawater, namely, to purify a spent solution of a corrosion inhibitor containing 100-2000 mg / l potassium chromate K ₂ CrO ₄ , with the disposal of solid residue and obtaining marketable products for industrial use.

The proposed method is also suitable for the purification and processing of industrial effluents containing hexavalent chromium compounds (K ₂ Cr ₂ O ₇ , K ₂ CrO ₄ , CrO ₃ , etc.), which, despite the fact that they are poisonous, are widely used in galvanic production, in the chemical and electrochemical processing of metals, as well as in the leather, chemical and radio engineering industries.

At the moment, a wide range of different methods are known for purifying aqueous solutions from hexavalent chromium compounds: reagent, including using ferritization, using sorbents, reagent-free and combining various other methods. However, the overwhelming majority of them are ineffective, unsafe, low-profit, while they do not solve the problem of sufficiently complete neutralization of solutions containing hexavalent chromium compounds, and do not provide for the disposal of the resulting precipitation (sludge).

Disposal of sludge due to the complexity and instability of their composition, as well as taking into account the technical difficulties in separating them in the form of sludge from wastewater and solutions, remains a difficult problem at the moment, especially in terms of disposal with obtaining marketable products for use in industry and construction ... In this regard, out of the whole variety of methods for purifying chromium-containing solutions and inhibitors, only those that allow purifying the solution from Cr (VI) and Cr (III) to MPC are of interest and will be presented, while completely utilizing the resulting precipitate with the return of useful components to industrial production.

A known method of neutralizing aqueous solutions containing hexavalent chromium compounds, including a spent solution of a corrosion inhibitor (RU 2678287, publ. 2019.01.24). The method includes adding an acid solution to the inhibitor, and then hydrazine in the hydrazine hydrate or its salt. The reduction reaction of Cr (VI) proceeds according to the equation:

The hydrazine-containing reductant is introduced with a deficiency (in the amount of 50-70% of the stoichiometry). Further, the reduction of Cr (VI) ions is carried out by adding metallic iron (in the form of chips or powder). After that, reagents for precipitation of Cr (III) hydroxide and iron hydroxide are added to the reaction volume, the solution is neutralized with alkali to a pH of 8-9 and the resulting pulp is allowed to settle for 2-6 hours to ripen the sediment. Using a filter, the precipitate and mother liquor are separated. The resulting solution contains only potassium and sodium chlorides and can be discharged into the general sewer system or directly into the water area. Sludge handling depends on its radionuclide content and may involve either solidification and sending for storage or disposal, or drying and then sending it in a container to a hazardous waste landfill. The disadvantages of this method include the need to use poisonous hydrazine, which does not provide the required efficiency and high speed of the process of reduction of Cr (VI) to Cr (III) in an acidic medium at a high cost. In addition, the residual content of hexavalent chromium in the solution significantly exceeds the value that is ensured when it is reduced with metallic iron, and the reduction with metallic iron requires a large excess of acid, which must be neutralized and disposed of at the end of the process, which entails the need for additional use of reagents and an increase in costs. ...

There is a known method of neutralizing aqueous solutions containing hexavalent chromium compounds (RU 2433961, publ. 2011.11.20), which is based on the reduction of the above-mentioned compounds with solutions of hydrazine or hydroxylamine. The reduction reactions take place in the pH range from acidic to slightly alkaline without the additional introduction of acid or alkali to change the pH. The disadvantages of the known method, as described above, include the high cost of hydrazine and its dangerous properties. The quality of separation of chromium hydroxide from the solution is not high enough due to the very rapid contamination of the pores of the filter material with small particles of hydroxide. Disposal of the resulting sediment and neutralization of solutions containing seawater are not provided in a known manner.

There is a known method of neutralizing acidic wastewater from hexavalent chromium compounds (RU 2042646, publ. 1995.08.27), which consists in filtering waste water on a material made by cementation of copper on metallized aluminum glass fibers, at a filtration volume rate of 1-10 column volumes / h, at pH <2 and the ratio of the mass of aluminized fibers to copper (1-500: 1). The known method provides the reduction of Cr (VI) to the trivalent state and allows water to be purified from Cr (VI) to MPC values, while the Cr (III) content may exceed MPC. As a result, the resulting solution turns out to be unsuitable for draining, especially in the water area. The known method does not provide for a sufficiently complete purification and utilization of chromium-containing wastewater.

There is a known method for purifying wastewater from galvanic industries from hexavalent chromium compounds (RU 2550890, publ. 2015.05.20), providing for contacting the effluent with a dispersed reagent containing iron, which is used as crushed iron or steel shavings, while simultaneously exposed to a controlled magnetic field created by an electromagnet with subsequent neutralization of the reacted mixture with alkali and obtaining chromium hydroxide Cr (OH) ₃ . The disadvantages of this method include its laboriousness associated with preliminary preparation, including crushing of sufficiently hard iron or steel shavings, and the need to use special equipment to create a controlled magnetic field. In addition, due to the formation of a loose layer of Fe (OH) ₃ , the volume of the resulting sediment to be preserved or buried increases.

The closest to the claimed method is the method of disposal of spent chromium plating electrolytes containing hexavalent chromium compounds (RU 2557608, publ. 2015.07.27), including neutralizing the electrolyte with a 10-30% aqueous solution of sodium sulfite Na ₂ SO ₃ at the rate of 3.63 ÷ 3 , 64 mg per 1 mg of Cr ⁶⁺ at a pH of 2.5 ÷ 3.0, followed by the precipitation of Cr ³⁺ hydroxide by alkalinization with 10% NaOH solution to pH 8.0 ÷ 9.5. The resulting precipitate of chromium hydroxide is washed, then heat treatment is carried out in two stages: dried at a temperature of 200 ÷ 220 ° C for 1 + 2 hours, then calcined at a temperature of 900 ÷ 1100 ° C for at least 1 hour and subjected to metallothermal reduction to metal chrome.

The disadvantages of the known method include the difficulty of separating the finely dispersed precipitate of trivalent chromium hydroxide from the solution, which significantly reduces its efficiency, while the aluminothermal reduction of Cr (III) oxide requires the introduction of so-called "heating" additives into the reaction mixture, for example, iron oxides Fe ₂ O ₃ , or heating the reaction mixture to 500-600 ° C, which is due to the high melting point of the metal being reduced and the resulting oxide and leads to an increase in the cost of the method. Neutralization of solutions containing sea water in a known manner is not provided.

The objective of the invention is to create an efficient and highly profitable method of disposal of solutions containing hexavalent chromium compounds and seawater, providing for the disposal of the formed sediment (sludge) to obtain marketable products suitable for secondary use in industry.

The technical result of the method is to increase the efficiency of processing solutions containing hexavalent chromium compounds and seawater, with an increase in the quality of purification and an increase in the profitability of the method due to the disposal of the resulting precipitation to obtain marketable products.

The specified technical result is achieved by the method of processing a corrosion inhibitor containing hexavalent chromium compounds, including acidifying the initial solution to pH 2-3, treating the acidified solution with a sulfite reducing agent with the reduction of Cr (VI) to Cr (III) and obtaining a Cr (III) hydroxide precipitate, isolation the resulting precipitate, preparation and conduct of a metallothermal reaction, in which, unlike the known one, sodium salt Na ₂ SO ₃ or NaHSO ₃ is used as a reducing agent in a calculated amount per 10 to neutralize the acidified solution and reduce Cr (VI) to Cr (III) -40% less than the following from the stoichiometry of the neutralization equation, a 10-20% solution of ferrous sulfate FeSO ₄ ⋅7H ₂ O or iron chloride FeCl ₂ 4H ₂ O is added to the resulting solution in a calculated amount that ensures the recovery of the remaining unreduced Cr (VI) , after which the mentioned solution is neutralized with an alkali solution of NaOH to pH 8-9, a flocculant is added, and filtering the resulting precipitate and subjecting it to a two-stage heat treatment, which includes drying at 100-120 ° C for at least 2 hours and heat treatment at 300-310 ° C for 60-70 minutes, after which the aluminothermal reduction of the treated precipitate is carried out to obtain ferrochrome and aluminothermic slag. The method is carried out as follows.

The chromium-containing solution is placed in a reactor with a stirring device at a rotational speed of 600-900 rpm and a sulfuric acid solution is added to it with constant stirring until a pH <3.0 (2-3) is reached. Next, Cr (VI) is reduced to Cr (III) by adding a reducing agent selected from the following series: NaHSO ₃ , Na ₂ SO ₃ , Na ₂ S ₂ O ₅ , Na ₂ S ₂ O ₄ , in an amount 10-40% less than the calculated , which follows by stoichiometry from the corresponding neutralization equation:

At the second stage of processing, also with stirring, the resulting solution is treated with a 10-20% solution of ferrous salt, mainly ferrous sulfate FeSO ₄ ⋅7H ₂ O or tetrahydrous iron chloride FeCl ₂ ⋅ 4H ₂ O, containing the calculated amount of iron salt required for the reduction of Cr (VI) remaining unreduced after treatment with a reducing agent.

At the next stage of processing, the reaction mixture is neutralized with NaOH solution to pH 8-9 and a flocculant is added. After stirring, the formed precipitate is separated by filtration, and the resulting clarified solution, which is subjected to periodic monitoring for the content of Cr (VI) and Cr (III), at a level of the latter below the MPC, can be drained directly into the water area.

When processed by the proposed method, the purified solution does not contain foreign compounds foreign to sea water, which could be considered as polluting. Its qualitative composition is almost identical to that of seawater.

The filtered precipitate is a mixture of hydrated oxides of trivalent chromium and trivalent iron, which after heat treatment provides sufficient energy to carry out the aluminothermic reaction without the use of additional heating and additional "heating" additives.

The heat treatment of the filtered precipitate in two stages and the preparation of the reaction mass for aluminothermy is carried out in a known manner described in patent RU 2674206, publ. 2018.12.05. The precipitate is dried at 100-120 ° C for at least 2 hours, and then subjected to heat treatment at 300-310 ° C for 60-70 minutes. The treated sludge is mixed with aluminum grains in a mass ratio (3.0-3.2): 1

The prepared reaction mass is loaded into a crucible, which is placed in an apparatus for metallothermal recovery of sludge, described in patent RU 2419659, publ. 2011.05.27, and initiate an aluminothermic reaction. After the end of the reaction, the formed cake is removed, the bead of the metal alloy is separated and sent for recycling as scrap metal.

Aluminothermic slag, which is corundum in chemical composition, is crushed and used as an abrasive, for example, in sandblasting machines.

Examples of specific implementation of the method

A cavitation mixer with a rotational speed of 900 rpm was used to stir the reaction mixture.

For effective sedimentation of the resulting suspension to obtain a sediment, the cationic flocculant Praestol 650 TR was added in the form of an aqueous solution with a concentration of 1 g / L.

The determination of the content of Cr (VI) and Cr (III) in purified seawater was carried out by atomic absorption (AAS) using a Thermo Scientific SOLAAR M Series spectrometer (Thermo, United States).

The separated precipitate was dried in a BinderFD53 forced convection oven.

For heat treatment, a Nabertherm L9 / 13 / B180 muffle furnace was used.

The elemental composition of metal and slag samples was determined by the energy-dispersive X-ray fluorescence method using a Shimadzu EDX 800 HS instrument.

The phase identification of the sample of the obtained aluminothermal slag was carried out using X-ray phase analysis (XRD) on a D8 Advance multipurpose X-ray diffractometer (Bruker AXS, Germany).

Example 1

Took 500 ml of a corrosion inhibitor solution containing 100 mg / l Cr (VI) in seawater, acidified with 0.2 ml of a sulfuric acid solution with a density of 1.26 g / cm ³ . The primary reduction of Cr (VI) was carried out by adding 1.2 ml (40% less than the calculated) 10% sodium sulfite Na ₂ SO ₃ solution with stirring for 30 minutes. Additional reduction of Cr (VI) was carried out with continued stirring by adding 3.5 ml of a 10% solution of ferrous sulfate FeSO ₄ ⋅7H ₂ O. Alkalization was carried out with a 10% NaOH solution to pH 8.2. After the formation of a greenish-brown suspension, 0.5 ml of an aqueous solution of flocculant was added. After 10 minutes, the precipitate that formed was separated and the chromium content in the purified water was determined. The content of Cr (VI) and Cr (III) in the purified water was <0.1 mg / L (MAC for Cr (VI) is 0.1 mg / L, for Cr (III) - 0.6 mg / L).

Example 2

Took 500 ml of a corrosion inhibitor solution containing 2000 mg / l Cr (VI) and acidified 6.7 ml of a sulfuric acid solution with a density of 1.26 g / cm ³ . The primary reduction was carried out by adding 13.5 ml (10% less than the calculated) 20% sodium bisulfite NaHSO ₃ solution for 30 minutes. Additional reduction of Cr (VI) was carried out with continued stirring by adding 8.0 ml of a 20% solution of tetrahydrous iron chloride FeCl ₂ ⋅ 4H ₂ O. Alkalization was carried out with 10% NaOH solution to pH 8.3. For effective sedimentation of the resulting suspension, 0.5 ml of a cationic flocculant was added. After 10 minutes, the formed precipitate was separated and the chromium content in the purified water was determined. The content of Cr (VI) and Cr (III) in the purified water was <0.1 mg / liter.

The precipitate obtained in two examples was combined, dried at 110 ° C for 2 hours, and then subjected to heat treatment at 300 ° C for 1 hour. The composition for aluminothermy, obtained by mixing the prepared precipitate with aluminum grains in a mass ratio (3.0-3.2): 1, was placed in a graphite crucible and subjected to aluminothermic reduction. The resulting cake was crushed to obtain a metal bead and aluminothermic slag.

The chromium content in the metal bead was 26.24%, the rest was reduced iron and impurities; the chromium content in the aluminothermic slag is 1.42%.

The phase identification of the sample of the obtained aluminothermic slag was carried out using X-ray phase analysis (XRD). As evidenced by the diffractogram given in the graphical appendix, chromium in the aluminothermic slag is in the form of aluminum-chromium oxide (Al _0.9 Cr _0.1 ) ₂ O ₃ in a sintered encapsulated form; it is incapable of leaching under conditions of soil acidity and does not pose a threat to the natural environment.

The presence of a magnesium compound in the aluminothermal slag is explained by the use of magnesium powder to initiate the aluminothermic reaction. The presence of silicon compounds is associated with the use of sand to create a mode for removing gaseous reaction products. In addition, some of the magnesium could be in the waste products from the sea water.

Claims (1)

A method for processing a corrosion inhibitor containing hexavalent chromium compounds, including acidifying the initial solution to pH 2-3, treating the acidified solution with a reducing agent with the reduction of Cr (VI) to Cr (III) and obtaining a precipitate of Cr (III) hydroxide, isolating the resulting precipitate and carrying out metallothermic reaction, characterized in that, to neutralize the acidified solution and reduce Cr (VI) to Cr (III), the sodium salt Na ₂ SO ₃ or NaHSO ₃ is used as a reducing agent in a calculated amount 10-40% less than the following from the stoichiometry of the neutralization equation, to the resulting solution is added with a 10-20% solution of ferrous sulfate FeSO ₄ ⋅7H ₂ O or iron chloride FeCl ₂ ⋅ 4H ₂ O in a calculated amount ensuring the recovery of the remaining unreduced Cr (VI), after which the said solution is neutralized with an alkali solution of NaOH to pH 8-9, add a flocculant, separate the resulting precipitate by filtration and subject it to a two-stage heat treatment, which includes drying at 100-120 ° C for at least 2 hours and heat treatment at 300-310 ° C for 60-70 minutes, after which the aluminothermal reduction of the treated sludge is carried out to obtain ferrochrome and aluminothermic slag.

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