CN102976409B - Process for recycling sodium dichromate in chlorate production - Google Patents

Abstract

The technology for recycling sodium dichromate in chlorate production belongs to the field of membrane separation technology, and a technology for recycling sodium dichromate in chlorate production by combining nanofiltration technology and crystallization separation technology is proposed. The devices in the process include a raw material box, a pump, a pressure gauge, a nanofiltration device, a cooling crystallization device, etc.; the specific process flow is: firstly, by opening the pump and adjusting the valve, the inlet pressure is kept between 0.5-1.5MPa, and the chlorine The mother liquor after the crystallization of sodium phosphate passes through the nanofiltration device stably, so that the sodium chloride in the mother liquor is permeated, and the sodium sulfate and sodium dichromate are intercepted and concentrated. Then freeze the concentrated solution obtained by membrane filtration to -5 to -10°C to crystallize and separate sodium sulfate from the concentrated solution, thereby removing sodium sulfate, and the remaining solution can be directly obtained in the production of electrolytic sodium chloride solution recycle and re-use. Thereby realize the recycling of sodium dichromate while realizing the removal of sodium sulfate. After the test, the chromium ion content of the salt mud is less than 10ppm and can be discharged up to the standard.

Description

Recycling process of sodium dichromate in chlorate production

technical field

The invention belongs to the field of novel membrane separation technology, and proposes a process for realizing the recovery and utilization of sodium dichromate in chlorate production by combining nanofiltration technology and crystallization separation technology.

Background technique

When electrolyzing saturated sodium chloride solution, in order to reduce the reduction loss of electrolytic products or intermediate products at the cathode, it is necessary to add a certain amount of sodium dichromate to the electrolysis mother liquor as an additive to prevent the reduction of the cathode. During the production process, the hexavalent chromate ions in the circulating mother liquor, when the electrolytic cell is overhauled, are discharged from the flushed sewage and the sludge from the brine section, and penetrate into the ground, causing pollution. In order to save water resources, most enterprises return the mother liquor in the crystallization process to the salt tank after treatment. Sulfate is the main impurity ion in brine, and the continuous accumulation of sulfate will increase the side reaction of the electrolysis process and reduce the current efficiency. For long-term, high current rate operation of the electrolyzer, it is necessary to keep the sulfate content low. At present, the commonly used method is to use chemical precipitation to remove sulfate, but it will produce a large amount of chromium-containing sludge.

Domestic chlorate enterprises with an annual production capacity of about 10,000 tons emit about 3.4 tons of hexavalent chromium salt (calculated as solid sodium dichromate) every year. According to the current national annual output of 400,000 tons of chlorate, more than tens of thousands of tons of chromium-containing waste residue will be produced every year.

At present, the treated salt mud in chlorate production is generally discharged to a specific chromium salt mud storage. However, due to the large amount of chromium salt mud discharged every year, simple storage cannot solve practical problems. Inner Mongolia Lantai Company proposed a plan to burn cement with raw materials, but the substances contained in the chromium salt mud are relatively complicated, and the hexavalent chromium in the salt mud cannot be completely eliminated, which still has great toxicity and endangers the environment. None of the salt production enterprises can achieve complete harmless treatment. Moreover, my country's chromium salt mud treatment technology is still immature. Some experts also proposed the method of water washing-detoxification-comprehensive treatment, but the results are not very satisfactory at present.

For this reason, we have invented a process for recycling sodium dichromate in chlorate production. By combining nanofiltration technology and crystallization separation technology, the mother liquor after crystallization is separated by nanofiltration membrane device, and the filtrate can be It directly enters the production recycling, and the concentrated solution can recycle the sodium dichromate through crystallization and separation, and discharge the sodium sulfate mud whose chromium content reaches the standard.

Contents of the invention

The invention provides a technical method for recycling sodium dichromate in chlorate production in combination with nanofiltration technology and crystallization separation technology, so as to solve the pollution problem of chromium-containing sludge in chlorate production and realize Recycling of sodium dichromate.

The invention combines nanofiltration technology and crystallization separation technology for the process of recycling sodium dichromate in chlorate production, and the devices in the process include raw material boxes, pumps, pressure gauges, nanofiltration devices, cooling crystallization devices and the like.

The specific process flow is as follows: the mother liquor after crystallization of sodium chlorate is pumped into the nanofiltration device, so that the sodium chloride solution in the mother liquor is permeated, and sodium sulfate and sodium dichromate are intercepted and concentrated. The concentrated solution is removed by crystallization in the freezing crystallization device, and the remaining solution is returned to the motor system for reuse. Thereby realize the recycling of sodium dichromate, solve the problem of recycling of sodium dichromate in chlorate production.

The devices in the process include a raw material box, a pump, a pressure gauge, a nanofiltration device, and a cooling crystallization device;

The specific process flow is as follows: firstly, by opening the pump and adjusting the valve, the inlet pressure difference is kept between 0.5-1.5MPa, and the mother liquor after sodium chlorate crystallization is stably passed through the nanofiltration device, so that the sodium chloride in the mother liquor is permeated , Sodium sulfate and sodium dichromate are intercepted and concentrated. Then freeze the concentrated solution obtained by membrane filtration to -5 to -10°C to crystallize and separate sodium sulfate from the concentrated solution, thereby removing sodium sulfate, and the remaining solution can be directly obtained in the production of electrolytic sodium chloride solution recycle and re-use. Thereby realize the recycling of sodium dichromate while realizing the removal of sodium sulfate. In the experiment, the change of the content of each substance in the solution was measured by ion chromatography.

The mother liquor after the crystallization of the sodium chlorate is a solution containing 113 g/L of NaCl, 29 g/L of Na ₂ SO ₄ and 3.35 g/L of Na ₂ Cr ₂ O ₇ .

The nanofiltration membrane used in the above-mentioned nanofiltration membrane device is a roll-type nanofiltration membrane; the model of the nanofiltration membrane used in the nanofiltration membrane device is HDNF90-4040.

In the present invention, the nanofiltration technology and the crystallization separation technology are combined and applied to the recycling of sodium dichromate in the production of chlorate, the sodium chloride is recycled through the nanofiltration device and the mother liquor is concentrated, and then the sodium sulfate is removed through low-temperature crystallization, and the Sodium dichromate recycling. The method can solve the problem of removing the harmful impurity sulfate in chlorate production without causing dichromate pollution, realize the recycling of sodium dichromate in sodium chlorate production, and reduce the environmental impact caused by the discharge of chromium-containing salt mud pollution, so that production into a virtuous circle.

The process of the recycling of sodium dichromate in the chlorate production of the present invention is further described in detail below by the examples.

Description of drawings

Fig. 1 is the schematic flow sheet of the technology that combines nanofiltration technology and crystallization separation technology to be used for the recycling of sodium dichromate in the production of chlorate.

1—raw material box; 2—pump; 3—pressure gauge; 4—nanofiltration device; 5—cooling crystallization device

Detailed ways

The following examples further illustrate the present invention's process for recycling sodium dichromate in chlorate production in combination with nanofiltration technology and crystallization separation technology.

Example 1

Connect each device according to the flowchart shown in Figure 1, namely:

The nanofiltration membrane of the nanofiltration membrane device adopts a roll-type membrane, and the effective membrane area of the membrane is 2.6m ² .

Add 40 L of a solution containing 113 g/L of NaCl, 29 g/L of Na ₂ SO ₄ and 3.35 g/L of Na ₂ Cr ₂ O ₇ into the raw material box.

The operation method is: pump the solution containing 113g/L NaCl, _29g /L _Na2SO4 , and _3.35g /L _Na2Cr2O7 in the _raw material box into the nanofiltration device, and observe the inlet pressure gauge , adjust the pump and valve to keep the inlet pressure between 0.5-1.5MPa, when the solution flow remains basically unchanged, stop the membrane filtration. Then freeze the concentrated solution obtained by membrane filtration to -5 to -10°C to crystallize and separate sodium sulfate from the concentrated solution. During operation, the waste liquid exchanges heat with the cooling water through the heat exchanger to maintain the temperature of the feed liquid at 25°C. The changes in the content of each substance in the solution were measured by ion chromatography.

Nanofiltration membrane separation principle: when the waste liquid passes through the nanofiltration membrane, due to the charge effect and sieving effect of the nanofiltration membrane, monovalent Cl ^— and Na ⁺ can pass through the membrane, while divalent SO ₄ ^2- , Cr ₂ O ₇ ^2- Intercepted.

The principle of crystallization and separation: the concentration of sodium sulfate in the concentrated mother liquor after membrane treatment is as high as 60-80g/L, and sodium dichromate is 10-20g/L. Refer to the solubility data table of sodium dichromate and sodium sulfate in water (Table 1) , According to our engineering experience, freeze the concentrate to -5 to -10 ℃ to separate sodium sulfate from the concentrate. The solution after sodium sulfate crystallization contains sodium dichromate and can be directly recycled in the production of electrolytic sodium chloride solution.

Table 1: Solubility data table of sodium dichromate and sodium sulfate in water

After concentration and filtration, samples were taken from the concentrate tank and the permeate tank for analysis. After crystallization and separation, sediment samples were taken to determine the chromium ion content of salt mud. In the experiment, CIC-100 ion chromatograph was used to measure the content of each component in each solution. See Table 2 for detailed data.

Table 2: Experimental data of sodium dichromate recycling scheme in chlorate production

Claims (3)

1. the technique of the recycling of sodium dichromate 99 in chlorate production, is characterized in that:

In described technique, device comprises material box, pump, tensimeter, nanofiltration device, equipment of crystallisation by cooling;

Concrete technology flow process is: first by opening pump, regulated valve, makes intake pressure difference remain between 0.5-1.5MPa, the mother liquor after Sodium Chlorate crystallization is stably passed through nanofiltration device, make sodium-chlor in mother liquor through, sodium sulfate, sodium dichromate 99 are retained down concentrated; Again the concentrated solution obtained through nanofiltration device is chilled to-5 to-10 DEG C by sodium sulfate Crystallization Separation from concentrated solution, thus is removed by sodium sulfate, remaining solution can directly be recycled in the production of electrolytic chlorination sodium solution; Thus while realizing the removing of sodium sulfate, realize the recycling of sodium dichromate 99.

2. the technique of the recycling of sodium dichromate 99 in chlorate production as claimed in claim 1, is characterized in that: the mother liquor after described Sodium Chlorate crystallization for being 113g/L containing NaCl, Na ₂sO ₄for 29g/L, Na ₂cr ₂o ₇for the solution of 3.35g/L.

3. the technique of the recycling of sodium dichromate 99 in chlorate production as claimed in claim 1, is characterized in that: described nanofiltration device nanofiltration membrane used is rolling nanofiltration membrane.