阅读说明：本技术 含铬污泥制备硫酸铬钾的方法 (Method for preparing chromium potassium sulfate from chromium-containing sludge ) 是由 徐友良 周保学 李金花 于 2021-10-08 设计创作，主要内容包括：一种含铬污泥制备硫酸铬钾的方法,将含铬污泥溶于强酸溶液后过滤得到含有三价铬离子以及铁、铜、镍离子的滤液,向其中缓慢加入NaOH,滤液pH由2.0调至4.0,形成氢氧化铬沉淀；向氢氧化铬沉淀中加入水和浓硫酸并在溶解后加入硫酸钾得到硫酸铬钾溶液；最后经搅拌结晶处理得到紫红色光亮外观、颗粒均匀细小的(KCr(SO-(4))-(2).12H-(2)O)。本发明具有纯度高、结晶颗粒均匀和紫红色光泽外观的特点。(Dissolving chromium-containing sludge in a strong acid solution, filtering to obtain a filtrate containing trivalent chromium ions, iron ions, copper ions and nickel ions, slowly adding NaOH into the filtrate, and adjusting the pH of the filtrate from 2.0 to 4.0 to form chromium hydroxide precipitate; adding water and concentrated sulfuric acid into the chromium hydroxide precipitate, dissolving, and adding potassium sulfate to obtain a chromium potassium sulfate solution; finally, the purple red bright appearance and uniform and fine particles (KCr (SO) are obtained by stirring and crystallizing 4 ) 2 .12H 2 O). The invention has the characteristics of high purity, uniform crystal particles and purple-red luster appearance.)

1. A method for preparing chromium potassium sulfate from chromium-containing sludge is characterized in that the chromium-containing sludge is dissolved in strong acid solution and then filtered to obtain filtrate containing trivalent chromium ions, iron ions, copper ions and nickel ions, NaOH is slowly added into the filtrate, the pH value of the filtrate is adjusted from 2.0 to 4.0, and chromium hydroxide precipitate is formed; adding water and concentrated sulfuric acid into the chromium hydroxide precipitate, dissolving, and adding potassium sulfate to obtain a chromium potassium sulfate solution; finally, the purple red bright appearance and uniform and fine particles (KCr (SO) are obtained by stirring and crystallizing₄)₂.12H₂O)。

2. The method for preparing chromium potassium sulfate from chromium-containing sludge as claimed in claim 1, wherein the slow addition is: the adding amount and speed of NaOH are controlled according to the pH rise rate per hour of 0.25-0.40 delta pH/h, and the adding time of NaOH is 5-8 h.

3. The method for preparing chrome potassium sulfate from chrome-containing sludge as claimed in claim 1, wherein the slow addition is carried out while stirring at a rate of 10 rpm.

4. The method for preparing chromium potassium sulfate from chromium-containing sludge as claimed in claim 1, wherein the ratio of chromium hydroxide precipitate, concentrated sulfuric acid, potassium nitrate and water is 2: 3: 1: 18 to 20.

5. The method for preparing chromium potassium sulfate from chromium-containing sludge as claimed in claim 1, wherein the stirring crystallization treatment is: stirring and standing for intermittent operation, wherein each standing time is 18-24 hours, then stirring is carried out for a short time at a rotating speed of 5-15 r/min, wherein the stirring time of the first four times is 8-12min, a chromium potassium sulfate seed crystal accounting for 0.2% of the total crystallization amount is added before the fifth stirring, then stirring is carried out for 15-20min, and the stirring time after the sixth stirring is 15-20 min; continuously stirring and standing for 6-8 times.

6. The method for preparing chromium potassium sulfate from chromium-containing sludge as claimed in any one of claims 1 to 5, which is characterized by comprising the following steps:

step 1) leaching trivalent chromium ions: dissolving chromium-containing sludge in strong acid solution, stirring for reaction, and controlling the dissolved pH value to be 2; filtering to obtain filtrate A containing trivalent chromium ions and impurity ions such as iron, copper, nickel and the like, and separating solid from liquid;

step 2) preparation of chromium hydroxide precipitate and separation of impurity ions: diluting the filtrate A obtained in the step 1) to Cr content³⁺The amount of the NaOH solution is 1-3%, and the NaOH solution is slowly added, specifically: the adding amount and the adding speed of NaOH are controlled according to the pH rise rate per hour of 0.25-0.40 delta pH/h, the adding time of the NaOH is 5-8 h, and stirring is carried out at the speed of 10 r/min, so that the pH rises from 2.0 to the final 4.0, and chromium hydroxide precipitate is formed; washing and filter pressing to obtain chromium hydroxide precipitate B which is easily dissolved in acid;

step 3) preparation of a chromium potassium sulfate solution: using the chromium hydroxide precipitate B obtained in the step 2) as a raw material, and using Cr (OH)₃：H₂SO₄：K₂SO₄：H₂O molar ratio of 2: 3: 1: 18 to 20 in the proportion of Cr (OH)₃Respectively adding water and concentrated H₂SO₄Stirring until the mixture is dissolved, controlling the temperature to be 70-80 ℃, and adding K₂SO₄Stirring and dissolving to obtain a chromium potassium sulfate solution C;

step 4) crystallization of chromium potassium sulfate: standing and cooling the chromium sulfate potassium solution C obtained in the step 3) for 18-24 hours to room temperature; then stirring and standing for intermittent operation, wherein each standing time is 18-24 hours, then stirring for a short time at a rotating speed of 5-15 r/min, wherein the stirring time of the first four times is 8-12min, a chromium potassium sulfate seed crystal with the total crystallization amount of 0.2% is added before the fifth stirring, then stirring is carried out for 15-20min, and the stirring time after the sixth stirring is 15-20 min; continuously stirring, standing for 6-8 times, and mechanically drying to obtain crystal precipitate of purple-red bright-appearance uniform-particle chromium potassium sulfate (KCr (SO)₄)₂.12H₂O)。

Technical Field

The invention relates to a technology in the field of heavy metal recovery, in particular to a method for preparing chromium potassium sulfate from chromium-containing sludge.

Background

The chromium-containing sludge is recycled and prepared into chemical products with wide application and high added value, so that the problem of environmental pollution can be solved, and the recycling level of the chromium-containing sludge can be improved. In the prior art, the chromium-containing sludge and the sulfuric acid solution are mixed and heated to leach soluble trivalent chromium to prepare the basic chromium sulfate, and the prepared basic chromium sulfate has low purity and can only be used for low-grade chrome tanning agents generally due to simple process and incomplete heavy metal separation. The chromium potassium sulfate is a high value-added trivalent chromium salt product, is widely applied to trivalent chromium electroplating, trivalent chromium passivation, mordant, photographic fixing agent, high-grade tanning agent, food additive and oil extraction fracturing fluid crosslinking agent, and is also used in the fields of glass, ceramics and the like. The existing preparation method of chromium potassium sulfate mainly adopts chemical raw material potassium dichromate to prepare by reducing the potassium dichromate with a high-purity reducing agent in the presence of sulfuric acid, but the method needs expensive chemical raw material potassium dichromate; on the other hand, because of the high solubility of the chromium potassium sulfate, a crystallization process under a high concentration condition is generally required for crystallization, but the chromium potassium sulfate is difficult to crystallize under the high concentration condition, and once the chromium potassium sulfate is crystallized, the chromium potassium sulfate is easy to form lumps to form bulk crystals, so that the appearance and the market price of the product are seriously influenced. The chrome potassium sulfate crystal with purple red and bright appearance and uniform and fine particles is a high value-added product welcomed by the market and is a difficult point for preparation. The chromium-containing sludge is adopted as the starting material, so that the impurity content is high, and the purity of the prepared chromium potassium sulfate product can be further influenced.

Disclosure of Invention

Aiming at the defects in the prior art, the invention provides the method for preparing the chromium potassium sulfate from the chromium-containing sludge, which has the characteristics of high purity, uniform crystal particles and purple-red luster appearance.

The invention is realized by the following technical scheme:

the invention relates to a method for preparing chromium potassium sulfate by chromium-containing sludge, which comprises the steps of dissolving the chromium-containing sludge in a strong acid solution, filtering to obtain a filtrate containing trivalent chromium ions and iron, copper and nickel ions, slowly adding NaOH into the filtrate, and adjusting the pH value of the filtrate from 2.0 to 4.0 to form chromium hydroxide precipitate; adding water and concentrated sulfuric acid into the chromium hydroxide precipitate, dissolving, and adding potassium sulfate to obtain a chromium potassium sulfate solution; finally, the purple red bright appearance and uniform and fine particles (KCr (SO) are obtained by stirring and crystallizing₄)₂.12H₂O)。

The slow addition is as follows: the adding amount and speed of NaOH are controlled according to the pH rise rate per hour of 0.25-0.40 delta pH/h, and the adding time of NaOH is 5-8 h.

The slow addition is preferably carried out while stirring at a rate of 10 revolutions per minute.

The ratio of the chromium hydroxide precipitate to the concentrated sulfuric acid to the potassium nitrate to the water is 2: 3: 1: 18 to 20.

The stirring crystallization treatment is as follows: stirring and standing for intermittent operation, wherein each standing time is 18-24 hours, then stirring is carried out for a short time at a rotating speed of 5-15 r/min, wherein the stirring time of the first four times is 8-12min, a chromium potassium sulfate seed crystal accounting for 0.2% of the total crystallization amount is added before the fifth stirring, then stirring is carried out for 15-20min, and the stirring time after the sixth stirring is 15-20 min; continuously stirring and standing for 6-8 times.

The method specifically comprises the following steps:

step 1) leaching trivalent chromium ions: dissolving chromium-containing sludge in strong acid solution, stirring for reaction, and controlling the dissolved pH value to be 2; filtering to obtain filtrate A containing trivalent chromium ions and impurity ions such as iron, copper, nickel and the like, and separating solid from liquid;

step 2) preparation of chromium hydroxide precipitate and separation of impurity ions: diluting the filtrate A obtained in the step 1) to Cr content³⁺The amount of the NaOH solution is 1-3%, and the NaOH solution is slowly added, specifically: the adding amount and the adding speed of NaOH are controlled according to the pH rise rate per hour of 0.25-0.40 delta pH/h, the adding time of the NaOH is 5-8 h, and stirring is carried out at the speed of 10 r/min, so that the pH rises from 2.0 to the final 4.0, and chromium hydroxide precipitate is formed; washing and filter pressing to obtain chromium hydroxide precipitate B which is easily dissolved in acid;

step 3) preparation of a chromium potassium sulfate solution: using the chromium hydroxide precipitate B obtained in the step 2) as a raw material, and using Cr (OH)₃：H₂SO₄：K₂SO₄：H₂O molar ratio of 2: 3: 1: dissolving at a ratio of 18-20. According to the proportion in Cr (OH)₃Respectively adding water and concentrated H₂SO₄Stirring until the mixture is dissolved, controlling the temperature to be 70-80 ℃, and adding K₂SO₄And stirring and dissolving to obtain a chromium potassium sulfate solution C.

Step 4) crystallization of chromium potassium sulfate: standing and cooling the chromium sulfate potassium solution C obtained in the step 3) for 18-24 hours to room temperature; then stirring and standing for intermittent operation, wherein each standing time is 18-24 hours, then stirring for a short time at a rotating speed of 5-15 r/min, wherein the stirring time of the first four times is 8-12min, a chromium potassium sulfate seed crystal with the total crystallization amount of 0.2% is added before the fifth stirring, then stirring is carried out for 15-20min, and the stirring time after the sixth stirring is 15-20 min; continuously stirring, standing for 6-8 times, and mechanically drying to obtain crystal precipitate of purple-red bright-appearance uniform-particle chromium potassium sulfate (KCr (SO)₄)₂.12H₂O)。

Drawings

FIG. 1 is a schematic view of the preparation process of the present invention.

Detailed Description

Example 1

The embodiment specifically comprises the following steps:

step 1) leaching trivalent chromium ions: dissolving chromium-containing sludge in strong acid solution, stirring for reaction, and controlling the dissolved pH value to be 2; filtering to obtain filtrate A containing trivalent chromium ions and impurity ions such as iron, copper, nickel and the like, and separating solid from liquid;

step 2) preparation of chromium hydroxide precipitate and separation of impurity ions: diluting the filtrate A obtained in the step 1) to Cr content³⁺Amount 2%, NaOH solution was slowly added until pH rose from 2.0 to final 4.0 while stirring at 10 rpm, where: the adding amount and the adding speed of NaOH are controlled according to the pH rising rate of 0.3 delta pH/h per hour, the adding time of the NaOH is about 7 hours, and chromium hydroxide precipitate is formed; washing and filter pressing to obtain chromium hydroxide precipitate B which is easily dissolved in acid;

step 3) preparation of a chromium potassium sulfate solution: using the chromium hydroxide precipitate B obtained in the step 2) as a raw material, and using Cr (OH)₃：H₂SO₄：K₂SO₄：H₂O molar ratio of 2: 3: 1: 19 was dissolved. According to the proportion in Cr (OH)₃Respectively adding water and concentrated H₂SO₄Stirring until dissolved, controlling the temperature at 75 ℃, and adding K₂SO₄And stirring to obtain a chromium potassium sulfate solution C.

Step 4) crystallization of chromium potassium sulfate: standing and cooling the chromium potassium sulfate solution C obtained in the step 3) for 22 hours to room temperature; then stirring and standing for intermittent operation, wherein each standing time is 22 hours, then stirring is carried out for a short time at a rotating speed of 10 r/min, wherein the stirring time of the first four times is 10min, a chromium potassium sulfate seed crystal with the total crystallization amount of 0.2 percent is added before the fifth stirring, then stirring is carried out for 18min, and the stirring time after the sixth stirring is 18 min; continuously stirring, standing for 7 times, and mechanically drying to obtain crystal precipitate of chromium potassium sulfate (KCr (SO)₄)₂.12H₂O)。

Recovery of chromium in this example>99% of prepared chromium potassium sulfate (KCr (SO4)₂.12H₂O) crystal is purple red and bright in appearance, uniform in particle and purity>99％。

Comparative example 1

The comparative example specifically included the following steps:

step 1) leaching trivalent chromium ions: dissolving chromium-containing sludge in strong acid solution, stirring for reaction, and controlling the dissolved pH value to be 2; filtering to obtain filtrate A containing trivalent chromium ions and impurity ions such as iron, copper, nickel and the like, and separating solid from liquid;

step 2) preparation of the chromium hydroxide primary precipitate and separation of impurity ions: adding 20% NaOH solution into the filtrate obtained in the step 1) to a pH value of 4, stirring and reacting for 0.5h to form chromium hydroxide precipitate, washing and filter-pressing to obtain chromium hydroxide precipitate B.

Step 3) preparation of a chromium potassium sulfate solution: using the chromium hydroxide precipitate B obtained in the step 2) as a raw material, and using Cr (OH)₃：H₂SO₄：K₂SO₄：H₂O molar ratio of 2: 3: 1: 19 was dissolved. According to the proportion in Cr (OH)₃Respectively adding water and concentrated H₂SO₄Stirring until dissolved, controlling the temperature at 75 ℃, and adding K₂SO₄And stirring to obtain a chromium potassium sulfate solution C.

Step 4) crystallization of chromium potassium sulfate: and 3) continuously standing the chromium potassium sulfate solution C obtained in the step 3) for 5 days to generate a small amount of crystals, generating lump crystals on the 7 th day, mechanically drying the crystals by spin-drying, and crushing the crystals to obtain the powdery chromium potassium sulfate with purple gray stray gloss appearance.

The recovery rate of chromium in the comparative example is 97%, the purity of the chromium potassium sulfate is more than 94%, the chromium potassium sulfate is crystallized into lump crystals (the size is more than 10cm), the crystals are mechanically crushed into powdery chromium potassium sulfate solid particles before use, the appearance is purple gray and stray gloss, and the market acceptance degree is low.

Comparative example 2

The comparative example specifically included the following steps:

step 1) leaching trivalent chromium ions: dissolving chromium-containing sludge in strong acid solution, stirring for reaction, and controlling the dissolved pH value to be 2; filtering to obtain filtrate A containing trivalent chromium ions and impurity ions such as iron, copper, nickel and the like, and separating solid from liquid;

step 2) preparation of chromium hydroxide precipitate and separation of impurity ions: diluting the filtrate A obtained in the step 1) to Cr content³⁺Amount 2%, NaOH solution was slowly added until pH rose from 2.0 to final 4.0 while stirring at 10 rpm, where: the adding amount and the adding speed of NaOH are controlled according to the pH rising rate of 0.3 delta pH/h per hour, the adding time of the NaOH is about 7 hours, and chromium hydroxide precipitate is formed; washing and filter pressing to obtain chromium hydroxide precipitate B which is easily dissolved in acid;

step 3) preparation of a chromium potassium sulfate solution: using the chromium hydroxide precipitate B obtained in the step 2) as a raw material, and using Cr (OH)₃：H₂SO₄：K₂SO₄The molar ratio is 2: 3: 1, wherein Cr (OH)₃First dissolved in dilute H₂SO₄Preparing a chromium sulfate solution and then adding solid K₂SO₄Preparing a chromium potassium sulfate solution (the solubility of chromium potassium sulfate is 24.39g/100g water at 25 ℃), and concentrating the chromium potassium sulfate solution at high temperature to 213-231 g/100g water to obtain a chromium potassium sulfate solution C.

Step 4) crystallization of chromium potassium sulfate: and 3) continuously standing the chromium potassium sulfate solution C obtained in the step 3) for 5 days to generate a small amount of crystals, generating lump crystals on the 7 th day, mechanically drying the crystals by spin-drying, and crushing the crystals to obtain the powdery chromium potassium sulfate with purple gray stray gloss appearance.

The recovery rate of chromium in the comparative example is 99 percent, the purity of the chromium potassium sulfate is 99 percent, the chromium potassium sulfate is crystallized into lump crystals (the size is more than 10cm), the crystals are mechanically crushed into powdery chromium potassium sulfate solid particles before use, the appearance is purple gray and stray gloss, and the market acceptance degree is low; compared with example 1, the energy consumption for evaporating the concentrated water is at least 9 times.

Comparative example 3

The comparative example specifically included the following steps:

step 1) leaching trivalent chromium ions: dissolving chromium-containing sludge in strong acid solution, stirring for reaction, and controlling the dissolved pH value to be 2; filtering to obtain filtrate A containing trivalent chromium ions and impurity ions such as iron, copper, nickel and the like, and separating solid from liquid;

step 2) preparation of chromium hydroxide precipitate and separation of impurity ions: diluting the filtrate A obtained in the step 1) to Cr content³⁺Amount 2%, NaOH solution was slowly added until pH rose from 2.0 to final 4.0 while stirring at 10 rpm, where: the adding amount and the adding speed of NaOH are controlled according to the pH rising rate of 0.3 delta pH/h per hour, the adding time of the NaOH is about 7 hours, and chromium hydroxide precipitate is formed; washing and filter pressing to obtain chromium hydroxide precipitate B which is easily dissolved in acid;

step 3) preparation of a chromium potassium sulfate solution: using the chromium hydroxide precipitate B obtained in the step 2) as a raw material, and using Cr (OH)₃：H₂SO₄：K₂SO₄：H₂O molar ratio of 2: 3: 1: 19 was dissolved. According to the proportion in Cr (OH)₃Respectively adding water and concentrated H₂SO₄Stirring until dissolved, controlling the temperature at 75 ℃, and adding K₂SO₄And stirring to obtain a chromium potassium sulfate solution C.

Step 4) crystallization of chromium potassium sulfate: standing and cooling the chromium potassium sulfate solution C obtained in the step 3) for 22 hours to room temperature; then stirring and standing are carried out intermittently, each standing time is 22 hours, then stirring is carried out for a short time, the rotating speed is 10 revolutions per minute, and the stirring time is 10 minutes; continuously stirring and standing for 10 times, and no crystallization of chromium potassium sulfate occurs.

No crystallization of chromium potassium sulfate occurred in this comparative example.

Comparative example 4

The comparative example specifically included the following steps:

step 1) leaching trivalent chromium ions: dissolving chromium-containing sludge in strong acid solution, stirring for reaction, and controlling the dissolved pH value to be 2; filtering to obtain filtrate A containing trivalent chromium ions and impurity ions such as iron, copper, nickel and the like, and separating solid from liquid;

step 2) preparation of chromium hydroxide precipitate and separation of impurity ions: diluting the filtrate A obtained in the step 1) to Cr content³⁺Amount 2%, NaOH solution was slowly added until pH rose from 2.0 to final 4.0 while stirring at 10 rpm, where: the adding amount and the adding speed of NaOH are controlled according to the pH rising rate of 0.3 delta pH/h per hour, the adding time of the NaOH is about 7 hours, and chromium hydroxide precipitate is formed; washing and filter pressing to obtain chromium hydroxide precipitate B which is easily dissolved in acid;

step 3) preparation of a chromium potassium sulfate solution: using the chromium hydroxide precipitate B obtained in the step 2) as a raw material, and using Cr (OH)₃：H₂SO₄：K₂SO₄：H₂O molar ratio of 2: 3: 1: 19 was dissolved. According to the proportion in Cr (OH)₃Respectively adding water and concentrated H₂SO₄Stirring until dissolved, controlling the temperature at 75 ℃, and adding K₂SO₄And stirring to obtain a chromium potassium sulfate solution C.

Step 4) crystallization of chromium potassium sulfate: standing and cooling the chromium sulfate potassium solution C obtained in the step 3) to room temperature; then stirring and standing are carried out intermittently, each standing time is 11 hours, then stirring is carried out for 11 hours, the rotating speed is 10 r/min, stirring and standing are carried out continuously for 12 times, and chromium potassium sulfate crystals do not appear.

No crystallization of chromium potassium sulfate occurred in this comparative example.

Example 2

The embodiment specifically comprises the following steps:

step 1) leaching trivalent chromium ions: dissolving chromium-containing sludge in strong acid solution, stirring for reaction, and controlling the dissolved pH value to be 2; filtering to obtain filtrate A containing trivalent chromium ions and impurity ions such as iron, copper, nickel and the like, and separating solid from liquid;

step 2) preparation of chromium hydroxide precipitate and separation of impurity ions: diluting the filtrate A obtained in the step 1) to Cr content³⁺Amount 1%, NaOH solution was added slowly, NaOH solution was added, pH was raised from 2.0 to final 4.0 while stirring speed 10 rpm, where: the adding amount and the adding speed of NaOH are controlled according to the pH rising rate of 0.25 delta pH/h per hour, the adding time of NaOH is 8h, and chromium hydroxide precipitate is formed; washing and filter pressing to obtain chromium hydroxide precipitate B which is easily dissolved in acid;

step 3) preparation of a chromium potassium sulfate solution: using the chromium hydroxide precipitate B obtained in the step 2) as a raw material, and using Cr (OH)₃：H₂SO₄：K₂SO₄：H₂O molar ratio of 2: 3: 1: 18, was dissolved. According to the proportion in Cr (OH)₃Respectively adding water and concentrated H₂SO₄Stirring until dissolved, controlling the temperature at 80 ℃, and adding K₂SO₄And stirring to obtain a chromium potassium sulfate solution C.

Step 4) crystallization of chromium potassium sulfate: standing and cooling the chromium potassium sulfate solution C obtained in the step 3) for 18 hours to room temperature; then stirring and standing for intermittent operation, wherein each standing time is 18 hours, then stirring is carried out for a short time at the rotating speed of 5 r/min, wherein the stirring time of the first four times is 8min, chromium potassium sulfate seed crystals with the total crystallization amount of 0.2 percent are added before the fifth stirring, then stirring is carried out for 15min, and the stirring time after the sixth stirring is 15 min; continuously stirring, standing for 6 times, and mechanically drying to obtain crystal precipitate of chromium potassium sulfate (KCr (SO)₄)₂.12H₂O)。

Recovery of chromium in this example>99% of prepared chromium potassium sulfate (KCr (SO4)₂.12H₂O) crystal is purple red and bright in appearance, uniform in particle and purity>99％。

Example 3

The embodiment specifically comprises the following steps:

the invention relates to a method for preparing chromium potassium sulfate from chromium-containing sludge, which comprises the following steps:

step 1) leaching trivalent chromium ions: dissolving chromium-containing sludge in strong acid solution, stirring for reaction, and controlling the dissolved pH value to be 2; filtering to obtain filtrate A containing trivalent chromium ions and impurity ions such as iron, copper, nickel and the like, and separating solid from liquid;

step 2) preparation of chromium hydroxide precipitate and separation of impurity ions: diluting the filtrate A obtained in the step 1) to Cr content³⁺Amount 3%, NaOH solution was added slowly, NaOH solution was added, pH was raised from 2.0 to final 4.0 while stirring speed 10 rpm, where: the adding amount and the adding speed of NaOH are controlled according to the pH rising rate of 0.40 delta pH/h per hour, the adding time of NaOH is 5h, and chromium hydroxide precipitate is formed; washing and filter pressing to obtain chromium hydroxide precipitate B which is easily dissolved in acid;

step 3) preparation of a chromium potassium sulfate solution: using the chromium hydroxide precipitate B obtained in the step 2) as a raw material, and using Cr (OH)₃：H₂SO₄：K₂SO₄：H₂O molar ratio of 2: 3: 1: 20 was dissolved. According to the proportion in Cr (OH)₃Respectively adding water and concentrated H₂SO₄Stirring until dissolved, controlling the temperature at 70 ℃, and adding K₂SO₄And stirring to obtain a chromium potassium sulfate solution C.

Step 4) crystallization of chromium potassium sulfate: standing and cooling the chromium potassium sulfate solution C obtained in the step 3) for 24 hours to room temperature; then stirring and standing for intermittent operation, wherein each standing time is 24 hours, then stirring is carried out for a short time at a rotating speed of 15 r/min, wherein the stirring time of the first four times is 12min, a chromium potassium sulfate seed crystal with the total crystallization amount of 0.2 percent is added before the fifth stirring, then stirring is carried out for 20min, and the stirring time after the sixth stirring is 20 min; continuously stirring, standing for 8 times, and mechanically drying to obtain crystal precipitate of chromium potassium sulfate (KCr (SO)₄)₂.12H₂O)。

Recovery of chromium in this example>99% of prepared chromium potassium sulfate (KCr (SO4)₂.12H₂O) crystal is purple red and bright in appearance, uniform in particle and purity>99％。

Compared with the prior art, the invention provides that KCr (SO) is passed under the condition of higher temperature₄)₂.12H₂Dissolving KCr (SO) in O with crystal water₄)₂Formation of KCr (SO)₄)₂.12H₂Highly concentrated supersaturated solutions of O and using the crystallization procedure of the present invention, uniform and ordered KCr (SO) is formed₄)₂.12H₂O crystallization of the particles. Specifically, the invention is based on KCr (SO4)₂.12H₂Composition of O, Cr (OH) produced from chromium-containing sludge₃Using Cr (OH) as raw material₃：H₂SO₄：K₂SO₄：H₂The molar ratio of O raw material is 2: 3: 1: 18 to 20 at a high temperature of 70 to 80 ℃ to form KCr (SO4)₂.12H₂The O solution is only equivalent to 12-13H reserved in the reaction system₂O molecule, i.e. BiKCr (SO4)₂.12H₂O crystal 12 crystals H₂O is 0 to 1H more₂O molecules, thereby forming a supersaturated solution (corresponding to 213-231 g/100g of water dissolved) at room temperature; then, by controlling the crystallization procedure, the chromium potassium sulfate with high purity, uniform crystal particles and purple-red luster appearance is obtained; the invention does not need high-temperature concentration, thereby having low energy consumption and avoiding the problems that the prior art needs to prepare solution (with the highest concentration of 24.39g/100g water at 25 ℃) at room temperature and then concentrate and dehydrate energy consumption procedures, and the crystal process can not obtain purplish red luster appearance and uniform crystal particles; the invention also avoids the environmental protection problem of preparing the chromium potassium sulfate by reducing toxic hexavalent chromium.

Aiming at the problems that the chromium potassium sulfate is difficult to crystallize and is easy to form lumps once crystallized, the invention designs a crystallization process of supersaturated chromium potassium sulfate solution, crystal nucleation for an ultra-long time, seed crystal culture, seed crystal growth and polycrystal control. Firstly, cooling and standing for 18-24 hours at room temperature for a super-long time to uniformly disperse supersaturated solution so as to be beneficial to forming crystal nuclei, and then stirring for 8-12 minutes at a low speed for a super-short time so as to be convenient for forming polycrystal and prevent the formation of single crystal; standing for a long time, temporarily stirring for a short time till the fifth time, introducing seed crystals to promote the growth of the seed crystals, and increasing the stirring time to 15-20min to prevent the occurrence of uneven crystallization and large crystals; finally, stirring for about 7-10 days, and standing to form purplish red bright-appearance chromium potassium sulfate crystal precipitate with uniform particles.

In the process of preparing the chromium hydroxide primary precipitate from the chromium-containing sludge, the method adopts lower heavy metal ion concentration, controls the pH of the precipitate to be within the range of 2-4 all the time, further controls the addition amount and speed of NaOH to control the increase change of the pH according to the concentration of metal ions at each stage and the reaction process, realizes the effective control of the alkali concentration in the whole process, avoids the possible local severe change of the pH, has wide pH change range in the prior art, particularly severe pH change at the later stage of reaction, and has the alkali concentration and Cr³⁺The concentration is not matched, the problems of mutual aggregation, cross linking and aging among particles are caused, the pH value of the reaction system in the reaction process is small in fluctuation all the time, the reaction is slow and mild, the crystallization time is long, the obtained precipitate is fine in granularity, uniform in dispersion, good in crystallinity, uniform and consistent, high in purity and easy to dissolve in weak acid, and a good foundation is laid for the subsequent preparation of chromium potassium sulfate. Although the existing trivalent chromium hydroxide precipitation process also adopts an alkali precipitation process, the prior art adopts a method of directly adding alkali precipitation into a trivalent chromium salt solution to prepare chromium hydroxide, the alkali concentration is difficult to uniformly control in the reaction process, large-particle chromium hydroxide precipitates which are difficult to dissolve in weak acid (such as acetic acid) are formed by mutual aggregation, cross-linking and aging among particles, and Cr is formed³⁺With Fe³⁺、Ni²⁺、Cu²⁺Coprecipitation of the hydroxide of (1); despite the use of low Cr concentration³⁺Reacting with low-concentration alkali solution or mixing alkali solution with Cr³⁺Simultaneously adding into water is beneficial to reducing the cross-linking and aggregation of chromium hydroxide precipitate, but the Cr with too low concentration³⁺And alkali liquor consumes a large amount of water, and the problems of cross-linking and aggregation caused by uneven concentration distribution in the reaction process or the later stage of the reaction still cannot be solved; and the ammonia (amine) precipitator is adopted, so that the problem of ammonia nitrogen sewage discharge is caused.

The foregoing embodiments may be modified in many different ways by those skilled in the art without departing from the spirit and scope of the invention, which is defined by the appended claims and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.