CN103570069A - Method for decontaminating potassium sub-molten salt soluble crystal liquid and preparing chromic oxide - Google Patents

Abstract

The invention discloses a method for removing impurities in a potassium-based sub-molten salt crystal solution and preparing chromium oxide, comprising: (1) introducing CO ₂ into the potassium-based sub-molten salt crystal solution to neutralize the hydrogen oxidation in the solution Potassium; then add seed crystals, control the solution temperature at 30-90°C to remove aluminum and silicon; (2) pass CO ₂ into the solution obtained in step (1) for normal temperature pre-carbonization; (3) add the reducing agent to The pre-carbonized feed liquid obtained in step (2) is heated to 120-170°C and kept warm. After the reaction is completed, the solid-liquid is separated to obtain a chromium hydroxide filter cake; (4) the chromium hydroxide obtained in step (3) is The filter cake is dried and calcined to obtain chromium trioxide product. The present invention adopts the pre-acidification method, no reducing agent is added during pre-carbonization; after the pre-carbonized feed liquid is mixed with the reducing agent, CO ₂ is no longer introduced, and the conversion rate of hydrothermal reduction is high by controlling suitable pre-carbonation conditions ; The intermediate product is less, the total recovery rate of chromium is high, and the purity of chromium oxide after burning is high, and there are few impurities.

Description

A method for removing impurities and preparing chromium oxide in a potassium-based sub-molten salt solution

technical field

The invention relates to the field of chromium salt production, in particular to a new method for removing impurities in a crystal solution and preparing chromium oxide after the sub-molten salt reaction of chromium salt production.

Background technique

At present, the production methods of chromium salt are divided into calcium roasting, calcium-free roasting and liquid phase oxidation. Calcium roasting technology will be gradually eliminated due to low conversion rate and large amount of slag discharge. At present, the main methods of chromium salt production in my country are calcium-free roasting and sub-molten salt liquid phase oxidation. The main purpose is to reduce the emission of harmful chromium slag and reduce the pollution of the chromium salt production industry to the environment.

The sub-molten salt liquid phase oxidation method has a high conversion rate, and can convert 99% of chromium into chromate into the leaching solution; at the same time, during the reaction process, some aluminum and silicon compounds react with potassium alkali and enter the leaching solution to become crystal-dissolving solution. main impurities. If the aluminum and silicon impurities in the alkaline solution are not removed, the quality of the product chromium oxide will be greatly affected.

The traditional method of neutralizing aluminum precipitation mainly uses sulfuric acid, phosphoric acid, sodium dichromate, chromium-containing sodium bisulfate and other neutralizing alkaline solutions to cause hydrolysis of sodium aluminate to precipitate aluminum hydroxide precipitation, and further heat preservation and aging to completely remove chromium aluminum in sodium bicarbonate solution. Soviet patent 742381 discloses carbonizing the alkaline solution with CO ₂ (~5%), then neutralizing it with a mixed solution of sodium chromate and sodium dichromate, and finally filtering and removing impurity aluminum after heat preservation and aging. Whether the aluminum slime is easy to filter. US Patent No. 3899568 uses sodium dichromate for neutralization and adds seed crystals, but the time for neutralization and heat preservation is as long as 9 to 12 hours. Chinese patent CN1070892 adopts chromium-containing sodium bisulfate as a neutralizing agent, and adds one of sodium phosphate and phosphoric acid to the bottom liquid containing AlPO ₄ and CrPO ₄ crystal seeds under stirring, so that the aluminum in the solution and the containing Trivalent chromium in chromium sodium bisulfate precipitates in the form of AlPO ₄ -CrPO ₄ , and it is difficult to solve the problem of high chromium strip loss while consuming sulfuric acid.

Chinese patent CN101723461 introduces carbon dioxide into the alkaline solution as a neutralizing agent, and uses two stages of carbon to remove aluminum in the solution. The solution is acidified with the chromium salt by-product sodium bisulfate. The method solves the problem of aluminum slime filtration, and is mainly aimed at the sodium chromate alkaline solution obtained by calcium-free roasting, that is, the problem of aluminum removal under the condition of low alkalinity. However, the problem of aluminum removal under the condition of high alkalinity of sub-molten salt still needs to be improved. In addition, the problem of removing silicon impurities in the alkaline solution is not mentioned in the above-mentioned patents. The present invention can easily remove silicon in the solution by controlling appropriate conditions.

About the hydrothermal reduction of sodium chromate solution, there are many patents that have been reported. However, the raw materials of these patent experiments are all aqueous solutions of chromate or dichromate, and after hydrothermal reduction to obtain hydrated chromium oxide, chromium oxide powder is obtained by calcining. Chinese patent CN100999335A reports the use of small molecular substances such as sulfide, formaldehyde, and methanol as reducing agents, _CO2 as acidifying agent, and high-temperature hydrothermal preparation of hydrated chromium oxide. However, the introduction of sulfide will affect the performance of chromium oxide, and formaldehyde and methanol have certain toxicity, so care should be taken to prevent volatilization and explosion protection in production and application. In the Chinese patent application CN1410356A, using sucrose, glucose or fructose as the reducing agent, _CO2 is used for pre-acidification at low temperature, and then hydrothermal reduction at high temperature can be used to prepare hydrated chromium oxide. Chinese patent CN101456588A reported that starch and its derivatives were used as reducing agents without _CO2 acidification to obtain hydrated chromium oxide, and the filter cake was directly calcined without washing to obtain chromium oxide powder. The raw materials used in the hydrothermal reduction of chromate in the above-mentioned patents are aqueous solutions of sodium chromate. However, for the solution containing a large amount of potassium carbonate, potassium bicarbonate and potassium chromate after removing aluminum from the dissolved crystal liquid, the solution is alkaline due to carbonate hydrolysis, which is not conducive to the reduction of hexavalent chromium. At this time, it is difficult to achieve a good conversion rate by directly adding a reducing agent for reduction. However, after adding the reducing agent, a certain amount of CO ₂ is directly introduced, which is easy to generate a large amount of intermediate products. For example (Guangye Wei, 2012, International Journal of Minerals, Metallurgy and Materials), in the presence of a large amount of carbonate, basic sodium chromium carbonate is formed, and this intermediate product can also generate chromium oxide when calcined at high temperature, as shown in the following formula However, a large amount of chromate is formed at the same time, which reduces the overall recovery rate in the redox process.

$\mathrm{<span\; class="notranslate">\; KCr</span>}{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; Cr</span>}{\mathrm{<span\; class="notranslate">\; o</span>}}_{\mathrm{<span\; class="notranslate">\; 4</span>}}<span\; class="notranslate">\; )</span>}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; \&DoubleRightArrow;</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; Cr</span>}{\mathrm{<span\; class="notranslate">\; o</span>}}_{\mathrm{<span\; class="notranslate">\; 4</span>}}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 5</span>}}{\mathrm{<span\; class="notranslate">\; 4</span>}}\mathrm{<span\; class="notranslate">\; C</span>}{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; o</span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 9</span>}}{\mathrm{<span\; class="notranslate">\; 8</span>}}{\mathrm{<span\; class="notranslate">\; o</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}$

Contents of the invention

The main purpose of the present invention is to develop a new method for removing impurities and preparing chromium oxide in an optimized potassium-based sub-molten salt solution crystal solution for the present situation of chromium salt production technology.

Technical scheme of the present invention is as follows:

(1) Pass CO ₂ into the potassium-based sub-molten salt crystal solution to neutralize potassium hydroxide in the solution; then add seed crystals, control the solution temperature at 30-90°C, and remove aluminum and silicon in the solution.

Preferably, the added seed crystal is aluminum hydroxide, and the seed crystal coefficient is 0.1-1.0.

Preferably, after the neutralization is completed, the obtained aluminum slime is subjected to countercurrent washing, and the band loss of sodium chromate and sodium carbonate in the washed and dried aluminum slime is low, which can be applied to alumina production.

Preferably, the volume concentration of the carbon dioxide is 5-55%, which is a mixture of kiln gas from various sources and air, or _CO2 with a purity of 99.99% and air (or nitrogen) obtained in different proportions mixture.

More preferably, the neutralization time of carbon dioxide is 0.5-1 hour.

The crystal solution includes K ₂ CrO ₄ : 90-400 g/L; KOH: 60-100 g/L; K ₂ CO ₃ : 50-200 g/L; Al ₂ O ₃ : 5-10 g/L; SiO ₂ : 0.10～0.30g/L.

Preferably, the pH value of the solution obtained from the reaction is 8-11. More preferably, the pH value of the solution obtained from the reaction is 10-11.

Preferably, the cumulative decomposition time of this step is 0.5-6.0 hours.

(2) Send CO ₂ into the solution obtained in step (1) for normal temperature pre-carbonization.

Preferably, the CO ₂ partial pressure is 0.2-1.5MPa.

Preferably, the pH value after the pre-carbonization is controlled at 9-10.

(3) Add the reducing agent to the pre-carbonized feed liquid obtained in step (2), and the actual amount of the reducing agent is 1.0-1.7 times of the theoretical amount. After the feed liquid and the reducing agent are evenly mixed, the mixed solution is heated to 120-170°C and kept warm. After the reaction, solid-liquid separation was carried out to obtain a chromium hydroxide filter cake.

Preferably, the heat preservation reaction time is 0.5-5h.

(4) Drying and calcining the chromium hydroxide filter cake obtained in step (3) to obtain a dichromium trioxide product.

Preferably, the drying temperature is 100-105° C., and the drying time is 6-24 hours.

Preferably, the calcination temperature is 900-1100° C., and the calcination time is 1-4 hours.

Preferably, the step 4 further includes washing the calcined product with countercurrent water and drying to constant weight.

The present invention proposes a novel method for removing impurities and preparing chromium oxide from an optimized potassium-based sub-molten salt liquid crystal solution, adopting a pre-acidification method, and no reducing agent is added during pre-carbonization; the pre-carbonized feed liquid is mixed with a reducing agent , CO ₂ is no longer introduced, and by controlling the appropriate pre-carbonation conditions, the conversion rate of hydrothermal reduction is high; there are few intermediate products, the total recovery rate of chromium is high, and the purity of chromium oxide after burning is high and there are few impurities.

Compared with the existing chromium salt production technology, the present invention has the following advantages:

(1) The present invention is aimed at the sub-molten salt system, and uses CO ₂ to neutralize potassium hydroxide and precipitated aluminum. The source of CO ₂ is abundant and the price is low. At the same time, CO ₂ in the industrial kiln gas can be used (concentration of CO ₂ is about 30-40%) to save costs; after carbonization, the aluminum and silicon in the alkaline solution are reduced to below 1ppm; the precipitated aluminum sludge particles are coarse , low chromium band loss, easy to filter and wash, and can be used for alumina production after drying.

(2) The present invention is aimed at the sub-molten salt system. After the crystallization solution is neutralized, the solution contains a large amount of potassium carbonate, potassium bicarbonate and potassium chromate. The present invention adopts a pre-carbonization-hydrothermal reduction method to obtain hydrated chromium oxide. The reduction conversion rate is high, reaching 97-99%; the total recovery rate of chromium is high, reaching 90%-95%; the chromium oxide can be obtained with high purity (above 99%) after calcination. And the slurry is easy to filter and wash. Hydrated chromium oxide is dehydrated and calcined to prepare chromium oxide, which is clean and pollution-free.

(3) The present invention has simple process, short process, strong operability, low cost, low requirements on equipment materials, and easy realization of industrialized production.

(4) The process of the present invention does not produce waste water. The final filtrate and the alkaline washing liquid produced during the production process can be mixed with chromite ore through evaporation and crystallization, and then enter the roasting process, so as to realize the recycling of alkali liquid.

(5) No waste residue is generated in the present invention. The aluminum salt product obtained in the process can be used in the production of alumina; the product chromium oxide can be used in metallurgy, pigments, etc. Therefore, the use of this process is not only environmentally friendly and pollution-free, but also has significant economic benefits, and is an effective way for chromium salt enterprises to produce.

Description of drawings

Fig. 1, the process flow chart of the present invention prepares chromium oxide.

Detailed ways

The following are preferred embodiments of the present invention, and these embodiments are only used to explain the application rather than limit it. Referring to Fig. 1 for the flow chart of the present invention, detailed descriptions are also provided in the following embodiments.

Example 1.

The potassium-based sub-molten salt solution crystal solution from a chromium salt factory was selected

The alkaline liquid obtained by dissolving the crystal liquid of the potassium-based sub-molten salt is pumped into the liquid storage tank, and the temperature is controlled at 60°C. Introduce CO ₂ (high-purity CO ₂ mixed with air, volume fraction 55%) to neutralize potassium hydroxide for 0.4h; then add aluminum hydroxide seed crystals with a seed coefficient of 0.5 to neutralize and remove aluminum. The cumulative decomposition time is 2h. The removal rate of carbon and aluminum is 99.9%, and the removal rate of silicon is 98.5%. The aluminum hydroxide filter cake was washed 4 times in countercurrent at 80°C for 30 minutes. After carbon separation, due to the loss of some water-soluble chromium by the filter cake, K ₂ CO ₃ in the filtrate decreased slightly (90g/L), K ₂ CO ₃ : 150g/L; KHCO ₃ : 62g/L. The filtrate is pre-carbonized, the pressure is 0.8MPa, and the time is 0.6h. After pre-carbonization, the carbon dioxide is evacuated, and then potato starch is added, wherein, according to the stoichiometric ratio, the excess coefficient of the reducing agent is 1.8. The mixed solution was heated to 180°C and kept for 3.5h. After the reaction is finished, chromium hydroxide slurry is obtained. The reaction conversion rate was 94.5%. Chromium hydroxide filter cake, countercurrent washing 4 times, temperature 80 ℃, time 30min. The washed and dried chromium hydroxide powder is sent into a rotary kiln or a roasting furnace, and the calcining temperature is 900°C, and it is kept for 3 hours. The product was countercurrently washed 4 times at a temperature of 80°C for 30 minutes to obtain chromium oxide with a purity of 99.3%. The total recovery rate of chromium is 90%.

Example 2.

Select sodium chromate alkaline solution from a chromium salt factory

The alkaline liquid obtained by dissolving the crystal liquid of the potassium-based sub-molten salt is pumped into the liquid storage tank, and the temperature is controlled at 70°C. Introduce CO ₂ (high-purity CO ₂ mixed with air, volume fraction 36%) to neutralize potassium hydroxide for 1.0 h; then add aluminum hydroxide seed crystals with a seed coefficient of 1.0 to neutralize and remove aluminum. The cumulative decomposition time is 3h. The removal rate of carbon and aluminum is 99.5%, and the removal rate of silicon is 98.5%. The aluminum hydroxide filter cake was washed 4 times in countercurrent at a temperature of 90°C for 15 minutes. After carbon separation, due to the loss of some water-soluble chromium by the filter cake, K ₂ CO ₃ in the filtrate decreased slightly (190g/L), K ₂ CO ₃ : 190g/L; KHCO ₃ : 86g/L. The filtrate was pre-carbonized at a pressure of 0.6MPa for 1.0h. After pre-carbonization, the carbon dioxide is evacuated, and then cornstarch is added, wherein, according to the stoichiometric ratio, the excess coefficient of the reducing agent is 1.6. The mixed solution was heated to 160°C and kept for 3.5h. After the reaction is finished, chromium hydroxide slurry is obtained. The reaction conversion rate was 98.1%. Chromium hydroxide filter cake, countercurrent washed 4 times at 90°C for 15 minutes. The washed and dried chromium hydroxide powder is sent into a rotary kiln or a roasting furnace, and the calcining temperature is 1100°C, and it is kept for 2 hours. The product was countercurrently washed 4 times at a temperature of 90°C for 15 minutes to obtain chromium oxide with a purity of 99.5%. The total recovery rate of chromium is 93%.

Example 3.

Select sodium chromate alkaline solution from a chromium salt factory

The alkaline liquid obtained by dissolving the crystal liquid of the potassium-based sub-molten salt is pumped into the liquid storage tank, and the temperature is controlled at 60°C. Introduce CO ₂ (high-purity CO ₂ mixed with air, volume fraction 20%) to neutralize potassium hydroxide for 0.5h; then add aluminum hydroxide seed crystals with a seed coefficient of 0.6 to neutralize and remove aluminum. The cumulative decomposition time is 2h. The removal rate of carbon and aluminum is 99.1%, and the removal rate of silicon is 98.0%. The aluminum hydroxide filter cake was washed 4 times in countercurrent at 80°C for 20 minutes. After carbon separation, due to the loss of some water-soluble chromium by the filter cake, K ₂ CO ₃ in the filtrate decreased slightly (280g/L), K ₂ CO ₃ : 80g/L; KHCO ₃ : 29g/L. The filtrate was pre-carbonized at a pressure of 0.4 MPa for 1.5 hours. After pre-carbonization, the carbon dioxide is evacuated, and then potato starch is added, wherein, according to the stoichiometric ratio, the excess coefficient of the reducing agent is 1.4. The mixed solution was heated to 200°C and kept for 3.5h. After the reaction is finished, chromium hydroxide slurry is obtained. The reaction conversion rate was 99.3%. Chromium hydroxide filter cake, countercurrent washing 4 times, temperature 80 ℃, time 20min. The washed and dried chromium hydroxide powder is sent into a rotary kiln or a roasting furnace, and the calcining temperature is 1000°C, and it is kept for 2.5 hours. The product was countercurrently washed 4 times at a temperature of 80°C for 20 minutes to obtain pigment-grade chromium oxide with a purity of 99.3%. The overall conversion of chromium was 95%.

Claims (10)

1. A method for removing impurities and preparing chromium oxide in a potassium-based sub-molten salt solution, comprising: (1) Pass CO ₂ into the potassium-based sub-molten salt crystal solution to neutralize potassium hydroxide in the solution; then add seed crystals, control the solution temperature to 30-90°C, and remove aluminum and silicon in the solution; (2) feed CO into the solution obtained in step (1) Carry out _normal temperature pre-carbonization; (3) Add the reducing agent to the pre-carbonized feed liquid obtained in step (2), the amount of the reducing agent is 1.0-1.7 times of the theoretical amount, after the pre-carbonized feed liquid and the reducing agent are mixed evenly, heat to 120-170°C, And heat preservation, after reaction finishes, solid-liquid separation obtains chromium hydroxide filter cake; (4) The chromium hydroxide filter cake obtained in step (3) is dried and calcined to obtain the dichromium trioxide product. 2. The method according to claim 1, characterized in that the step (1) further comprises countercurrent washing the aluminum sludge obtained by removing aluminum and silicon in the solution. 3. The method according to claim 1, wherein the seed crystal is aluminum hydroxide, and the seed crystal coefficient is 0.1-1.0. 4. The method according to claim 1, characterized in that the volume concentration of carbon dioxide in step (1) is 5-55%. 5. The method according to claim 1, characterized in that the pH of the solution obtained in step (1) is 8-11. 6. The method according to claim 1, characterized in that the partial pressure of CO ₂ in step (2) is 0.2-1.5 MPa. 7. The method according to claim 1, characterized in that, the pH value after the normal temperature pre-carbonization is 9-10. 8 . The method according to claim 1 , wherein the drying temperature in step (4) is 100-105° C., and the drying time is 6-24 hours. 9. The method according to claim 1, characterized in that the calcination temperature in step (4) is 900-1100° C., and the calcination time is 1-4 hours. 10 . The method according to claim 1 , characterized in that, the step (4) further comprises washing dichromium trioxide with countercurrent water and drying to constant weight. 11 .

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