Method for improving cobalt yield in production process of cobalt hydroxide

Abstract

The invention discloses a method for improving cobalt yield in the production process of cobalt hydroxide, which comprises the following steps of: performing online video monitoring on the washing condition of each filter press, and controlling a washing water outlet pipeline in a system to be automatically switched into a corresponding mother liquor storage tank when running and mixing in the washing process are monitored in the video monitoring; and the wastewater in the mother liquor tank passes through the corresponding product filter press again, so that the running and mixing slag is subjected to productization recovery, and the direct recovery rate and recovery rate of metal cobalt in production are improved. Meanwhile, a hot pure water pipeline and a hot compressed air pipeline are additionally arranged on the filter press, washing is carried out before discharging of the filter press, and meanwhile, the materials are blow-dried by heating the compressed air pipeline, so that the recycled materials meet the raw material index requirements of cobalt oxide, the turnover, pulpifying washing, discharging and turning work of the materials is reduced, and the production efficiency is improved. According to the method, the amount of labor and the labor intensity of workers are reduced, meanwhile, the content of cobalt metal discharged into a group wastewater master station is controlled within 1 mg / L, more importantly, the direct recovery rate of the cobalt metal is increased to 99.78%, and the recovery rate is increased to 99.92%.

Description

Technical Field

[0001] The invention relates to the technical field of cobalt hydroxide production, and in particular to a method for improving the cobalt yield in the cobalt hydroxide production process. Background Art

[0002] When the nitric acid production line produces cobalt hydroxide of different particle sizes, when using a filter press for washing, the filter cloth is often damaged, compacted, and filtered, and the filter plate is deformed or cracked, resulting in mixing of the mother liquor and the washing water. The mixed materials enter the wastewater storage tank with the wastewater, and then enter the wastewater filter press, and are intercepted and recovered by the wastewater filter press. The recovered materials are re-added to the synthesis kettle for slurrying and filter pressing washing. During the slurrying and washing process, there are tasks such as hanging, transportation, slurrying and washing, and disassembly of the filter press after washing, which increases the labor intensity of the employees on the job, is time-consuming and labor-intensive, and the direct recovery rate and recovery rate of cobalt metal are low. In order to improve the direct recovery rate and recovery rate of cobalt metal in the production process, according to the characteristics of cobalt nitrate synthesis, the washing process of synthesized cobalt nitrate of different specifications is optimized and designed, so that different products can be mixed during washing, and the mixed slag can be recovered as the same product to make it a product. Summary of the invention

[0003] The present invention aims to provide a method for improving the cobalt yield in the production process of cobalt hydroxide, so as to solve the problems of mixing of wash water and mother liquor caused by damage, compaction, and perfiltration of filter cloth of filter press during the existing cobalt hydroxide production, as well as deformation or cracks of filter plates, and the problem of excessive cobalt content in wash water being discharged.

[0004] To achieve the above object, the present invention adopts the following technical solutions: A method for improving the cobalt yield in a cobalt hydroxide production process, the method is equipped with a cobalt hydroxide production system, the system comprises a synthesis kettle, a homogenizing tank, a product filter press, a wastewater filter press, an ion exchange column and an industrial control host computer arranged in sequence, the liquid outlet end of the synthesis kettle is connected to the liquid inlet end of the homogenizing tank, the liquid outlet end of the homogenizing tank is connected to the liquid inlet end of the product filter press, the liquid outlet end of the product filter press is connected to a section of transparent glass tube, an electromagnetic three-way valve is installed at the end of the transparent glass tube, and the electromagnetic three-way valve is respectively connected to a mother liquid storage tank and a wash water storage tank, the liquid outlet end of the mother liquid storage tank is connected to the liquid inlet end of the product filter press, the liquid outlet end of the wash water storage tank is connected to the liquid inlet end of the wastewater filter press, the liquid outlet end of the wastewater filter press is connected to the wastewater storage tank, the liquid outlet end of the wastewater storage tank is connected to the liquid inlet end of the ion exchange column, and the liquid outlet end of the ion exchange column is connected to an external discharge pool; A monitoring camera is installed directly above the transparent glass tube, and the monitoring camera is connected to the electromagnetic three-way valve through an industrial control host computer; The product filter press and the wastewater filter press are both connected with a hot pure water pipeline and a hot compressed air pipeline; The cobalt hydroxide production and cobalt recovery process is as follows: 1) After the cobalt hydroxide slurry is synthesized and aged in the reactor, it enters the homogenization tank for uniform mixing and is then pumped to the product filter press. The solid-liquid separation of the slurry is achieved by the product filter press. The cobalt hydroxide wet material containing chemical impurities is retained in the filter chamber of the filter press. The mother liquor of the filter press enters the mother liquor storage tank and returns to the liquid inlet end of the product filter press for secondary filtration; 2) Open the hot pure water pipeline valve and use hot pure water to wash the cobalt hydroxide wet material in the product filter press for multiple times in all directions until the color of the washing water no longer changes; during the washing process, the washing water with a darker color at the beginning is discharged into the mother liquor storage tank, and the washing water with a lighter color at the later stage enters the washing water storage tank and is pumped to the wastewater filter press for subsequent treatment; The control principle of the electromagnetic three-way valve during drainage is as follows: if the monitoring camera observes that the wash water color becomes darker through the transparent glass tube, it will automatically switch to the mother liquid storage tank through the industrial control host computer, otherwise it will enter the wash water storage tank; 3) After multiple washings, the wet cobalt hydroxide material is still retained in the filter press chamber of the filter press. At this time, the hot compressed air pipeline valve is opened, and the wet cobalt hydroxide material is dried with hot compressed air and then unloaded for recovery; 4) The wash water in the wash water storage tank is pumped to the wastewater filter press for filtration and washing. Since the metal content in the wash water is relatively low, the filtrate and wash water produced by the wastewater filter press are temporarily stored in the wastewater storage tank. The filter residue is dried by hot compressed air and then discharged for recovery; 5) The wastewater temporarily stored in the wastewater storage tank is pumped into the ion exchange column, and the cobalt in the wastewater is further removed by ion exchange before being discharged into the external discharge pool; the cobalt content in the wastewater entering the external discharge pool is less than 1 mg / L.

[0005] Compared with the prior art, the present invention has the following advantages: Through online video monitoring of the washing conditions of each filter press, when the video monitoring detects that the washing process is mixed, the washing water outlet pipe in the control system is automatically switched to the corresponding mother liquid storage tank, and the waste water in the mother liquid tank passes through the corresponding product filter press again, so that the mixed slag can be recycled as a product, and the direct recovery rate and recovery rate of metal cobalt in production can be improved. At the same time, hot pure water pipes and hot compressed air pipes are installed on the filter press to wash before unloading the filter press. At the same time, the material is dried by heating the compressed air pipe, so that the recovered material meets the raw material index requirements of cobalt oxide, reducing the turnover of materials, slurry washing and unloading and transfer work, reducing the labor and labor intensity of employees, and at the same time controlling the cobalt metal content discharged into the group's wastewater terminal to within 1mg / L, and more importantly, improving the direct recovery rate and recovery rate of cobalt metal. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 The present invention is a schematic diagram of the structure of the device used in the method.

[0007] In the figure, 1-synthesis kettle, 2-homogenization tank, 3-product filter press, 4-transparent glass tube, 5-surveillance camera, 6-wash water storage tank, 7-mother liquor storage tank, 8-1# wastewater filter press, 9-1# wastewater storage tank, 10-2# wastewater filter press, 11-2# wastewater storage tank, 12-ion exchange column, 13-external discharge pool. DETAILED DESCRIPTION

[0008] The method of the present invention is further explained below with reference to the accompanying drawings and specific embodiments.

[0009] like Figure 1 As shown, the cobalt hydroxide production system of the method of the present invention comprises a synthesis kettle 1, a homogenizing tank 2, a product filter press 3, a 1# wastewater filter press 8, a 2# wastewater filter press 10, an ion exchange column 12 and an industrial control host computer which are arranged in sequence, the liquid outlet end of the synthesis kettle 1 is connected to the liquid inlet end of the homogenizing tank 2, the liquid outlet end of the homogenizing tank 2 is connected to the liquid inlet end of the product filter press 3, the liquid outlet end of the product filter press 3 is connected to a section of transparent glass tube 4, an electromagnetic three-way valve is installed at the end of the transparent glass tube 4, and the electromagnetic three-way valve is respectively connected to a mother liquid storage tank 7 and a washing liquid storage tank 8. The liquid outlet ends of the water storage tank 6 and the mother liquid storage tank 7 are connected to the liquid inlet end of the product filter press 3, the liquid outlet end of the wash water storage tank 6 is connected to the liquid inlet end of the 1# wastewater filter press 8, the liquid outlet end of the 1# wastewater filter press is connected to the 1# wastewater storage tank 9, the liquid outlet end of the 1# wastewater storage tank 9 is connected to the liquid inlet end of the 2# wastewater filter press 10, the liquid outlet end of the 2# wastewater filter press 10 is connected to the 2# wastewater storage tank 11, the liquid outlet end of the 2# wastewater storage tank 11 is connected to the liquid inlet end of the ion exchange column 12, and the liquid outlet end of the ion exchange column 12 is connected to the external discharge pool 13.

[0010] In the above device, a monitoring camera 5 is installed just above the transparent glass tube 4, and the monitoring camera 5 is connected to the electromagnetic three-way valve through the industrial control host computer. Since the color difference between the cobalt-rich mother liquor and the cobalt-poor washing water is obvious, the opening of the electromagnetic three-way valve can be controlled by the principle of image color recognition to ensure that the mother liquor and washing water are accurately introduced into the corresponding storage tank to avoid material leakage. Among them, the picture / image color recognition involves machine learning technology, which is common knowledge in the field and will not be repeated here.

[0011] The product filter press 3 and the 1# and 2# wastewater filter presses are all connected with hot pure water pipelines and hot compressed air pipelines, and the product filter press and the wastewater filter press are set up with one in operation and one in standby to avoid failures affecting production.

[0012] The cobalt hydroxide production and cobalt recovery process of the present invention is as follows: 1) After the cobalt hydroxide slurry is synthesized and aged in the reactor, it enters the homogenization tank for uniform mixing and is then pumped to the product filter press. The solid-liquid separation of the slurry is achieved by the product filter press. The cobalt hydroxide wet material containing chemical impurities is retained in the filter chamber of the filter press. The mother liquor of the filter press enters the mother liquor storage tank and returns to the liquid inlet end of the product filter press for secondary filtration; 2) Open the hot pure water pipeline valve, and use hot pure water to wash the cobalt hydroxide wet material in the product filter press for multiple times in all directions (see the prior art CN201210006785.4 for the washing principle) until the color of the washing water no longer changes; during the washing process, the washing water with a darker color at the beginning is discharged into the mother liquor storage tank, and the washing water with a lighter color at the later stage enters the washing water storage tank and is pumped to the wastewater filter press for subsequent treatment; The control principle of the electromagnetic three-way valve during drainage is as follows: if the monitoring camera observes that the wash water color becomes darker through the transparent glass tube, it will automatically switch to the mother liquid storage tank through the industrial control host computer, otherwise it will enter the wash water storage tank; 3) After multiple washings, the wet cobalt hydroxide material is still retained in the filter press chamber of the filter press. At this time, the hot compressed air pipeline valve is opened, and the wet cobalt hydroxide material is dried with hot compressed air and then unloaded for recovery; 4) The wash water in the wash water storage tank is pumped to the wastewater filter press for filtration and washing. Since the metal content in the wash water is relatively low, the filtrate and wash water produced by the wastewater filter press are temporarily stored in the wastewater storage tank. The filter residue is dried by hot compressed air and then discharged for recovery; 5) The wastewater temporarily stored in the wastewater storage tank is pumped into the ion exchange column, where the cobalt in the wastewater is further removed through ion exchange and then discharged into the external discharge pool.

[0013] The following is an example of cobalt recovery using the cobalt hydroxide production line of a factory of Jinchuan Group. The slurry of the same product synthesized by every 8 of the 24 kettles enters the same homogenizing tank and enters the same product filter press for washing. The mother liquor enters the mother liquor storage tank, and the wastewater in the mother liquor storage tank passes through the product filter press to make the slag in the mother liquor into products. During the washing process, a visible transparent glass tube is connected to the outlet of the product filter press, and the image recognition technology is used to determine whether the mother liquor and the washing water are mixed. When the washing water is mixed, the industrial control host computer automatically switches the product filter press outlet to the mother liquor tank, and then uses the product filter press to intercept the slag, so that the mixed slag is productized; the washing water enters the washing water storage tank, and then enters the wastewater filter press to intercept the slag, and the intercepted material of the wastewater filter press is washed with sodium and other operations, and the washing is unloaded for a specified time. The wastewater enters the ion exchange column again to further remove cobalt, ensuring that the cobalt content in the wastewater after cobalt removal is less than 1mg / l.

[0014] Example 1 The 8-10μm slurry from the synthesized reactors 1-8 is pumped into the 200m 3The mother liquor is then washed in the product filter press, and the mother liquor is discharged into the mother liquor storage tank; the mother liquor is then fed into the product filter press to intercept the slag and turn it into products; the washing water enters the washing water storage tank. During the washing process of the product filter press, the color of the water outlet from the transparent glass tube is monitored by image recognition. If the color of the water outlet turns black or becomes turbid, the industrial control host will make a judgment on the mixing, automatically control the opening of the electromagnetic three-way valve entering the mother liquor tank, and automatically close the water inlet valve entering the washing water tank, so that the mixed water enters the mother liquor storage tank. The water and slag in the mother liquor storage tank are then intercepted, washed, and dried by the product filter press. After the components are tested to meet the product index requirements, they are unloaded. If the surveillance camera identifies that there is no leakage in the transparent glass tube section of the product filter press outlet water, normal washing is carried out and the washing water enters the washing water storage tank. The water in the washing water storage tank passes through the No. 1 wastewater filter press for primary filtration and slag interception. After the filter press is full, the material of the production line wastewater filter press is washed with the connected hot pure water. After the washing is qualified, it is quickly blown dry with hot compressed air, and samples are taken for testing before disassembly. The test results are: the particle sizes are 1.6μm, 3.8μm and 5.4μm respectively, the sodium impurity content is 0.008%, the cobalt content is 63.22%, Fe 0.1%, Ca 0.005%, Ni0.001%, Cu0.001%, As0.001%, Zn0.001%, Pb0.0013%, and the moisture content is 0.35%. From the above test results, it can be seen that the material recovered from the wastewater filter press has uneven particle size and does not meet the particle size of the cobaltous hydroxide product, but the composition meets the raw material composition of cobaltous oxide, so the material is used as the material for calcining cobaltous oxide. The wastewater after the 1# wastewater filter press enters the 1# wastewater storage tank, and then uses the 2# wastewater filter press for secondary slag interception. After the 2# wastewater filter press is full of slag, it is washed with hot pure water and quickly blown dry with hot compressed air. After unloading, sampling and testing are carried out. The test results are 1.2μm, 3.3μm and 4.1μm, the sodium element impurity content is 0.004%, the cobalt element content is 63.45%, Fe 0.05%, Ca 0.003%, Ni0.001%, Cu0.001%, As0.001%, Zn0.001%, Pb0.0009%, and the water content is 0.38%. After the two-stage wastewater filter press intercepts the slag, the wastewater enters the 2# wastewater storage tank, and then is pumped into the ion exchange column to further remove the ionic cobalt in the wastewater. The cobalt content of the wastewater after de-weighting is 0.3mg / l, and it is discharged for the next step of treatment. Calculations show that the direct recovery rate of cobalt metal is 99.7% and the recovery rate is 99.98%.

[0015] Example 2 The 18-20μm slurry from the synthesized reactors 9-16 is pumped into the 200m 3The product filter press is then used for washing, and the mother liquor is discharged into the mother liquor storage tank; the mother liquor then enters the product filter press to intercept the slag and make it a product, and the washing water enters the washing water storage tank. During the washing process of the product filter press, the color of the water outlet from the glass tube is monitored by the monitoring camera at all times. If the color of the water outlet becomes black or turbid, the industrial control host will make a judgment on the mixing, and then automatically open the electromagnetic three-way valve opening of the mother liquor tank, and then automatically close the opening of the washing water tank, so that the mixed water enters the mother liquor tank. The water and slag in the mother liquor tank are then intercepted, washed and dried by the product filter press. After the detection component meets the product index requirements, it is unloaded. If the transparent glass tube section is not mixed by the real-time monitoring of the surveillance camera, normal washing is carried out and the washing water enters the washing water storage tank. The water in the washing water storage tank passes through the No. 1 wastewater filter press for primary filtration and slag interception. After the filter press is full, the material in the No. 1 wastewater filter press is washed with the connected hot pure water. After the washing is qualified, it is quickly blown dry with hot compressed air, and samples are taken for testing before disassembly. The test results are: the particle sizes are 3.2μm, 4.1μm and 6.6μm respectively, the sodium impurity content is 0.006%, the cobalt content is 63.38%, Fe 0.1%, Ca 0.004%, Ni0.001%, Cu0.001%, As0.001%, Zn0.001%, Pb0.0011%, and the moisture content is 0.44%. From the above test results, it can be seen that the material recovered from the wastewater filter press has uneven particle size and does not meet the particle size of the cobaltous hydroxide product, but the composition meets the raw material composition of cobaltous oxide, so the material is used as the material for calcining cobaltous oxide. The wastewater after the 1# filter press enters the 1# wastewater storage tank, and then uses the 2# wastewater filter press for secondary slag interception. After the filter press is full of slag, it is washed with hot pure water and quickly blown dry with hot compressed air. After unloading, sampling and testing are carried out. The test results are 1.8μm, 2.4μm and 3.5μm, the sodium element impurity content is 0.0038%, the cobalt element content is 63.26%, Fe0.03%, Ca0.004%, Ni0.001%, Cu0.001%, As0.001%, Zn0.001%, Pb0.0007%, and the water content is 0.46%. After the 2# wastewater filter press intercepts the slag, the wastewater enters the 2# wastewater storage tank, and then is pumped into the ion exchange column to further remove the ionic cobalt in the wastewater. The cobalt content of the wastewater after de-weighting is 0.2mg / l, and it is discharged for the next step of treatment. The direct recovery rate of cobalt metal is 99.72% and the recovery rate is 99.98%.

[0016] Example 3 The 4-5μm slurry from the No. 17-24 synthesis reactor was injected into the 200m 3The product filter press is then used for washing, and the mother liquor is discharged into the mother liquor storage tank; the mother liquor then enters the product filter press to intercept the slag and make it a product, and the washing water enters the washing water storage tank. During the washing process of the product filter press, the color of the water outlet from the glass tube is monitored by the monitoring camera at all times. If the color of the water outlet becomes black or turbid, the industrial control host will make a judgment on the mixing, and automatically control to open the electromagnetic three-way valve connected to the mother liquor storage tank. After the valve is opened, the washing water enters the mother liquor storage tank, and then the system will automatically close the valve entering the washing water tank, so that the mixed water enters the mother liquor tank. The water and slag in the mother liquor tank are intercepted, washed and dried by the product filter press. After the detection component meets the product index requirements, it is unloaded. If the monitoring camera observes that there is no mixing in the visible glass tube section, normal washing is carried out and the washing water enters the washing water storage tank. The water in the washing water storage tank passes through the No. 1 wastewater filter press for primary filtration and slag interception. After the filter press is full, the material in the production line wastewater filter press is washed with the connected hot pure water. After the washing is qualified, it is quickly blown dry with hot compressed air, and samples are taken for testing before disassembly. The test results are: the particle sizes are 0.8μm, 1.4μm and 2.5μm respectively, the sodium impurity content is 0.007%, the cobalt content is 63.24%, Fe 0.1%, Ca 0.005%, Ni0.001%, Cu0.001%, As0.001%, Zn0.001%, Pb0.0011%, and the moisture content is 0.48%. From the above test results, it can be seen that the material recovered from the wastewater filter press has uneven particle size and does not meet the particle size of the cobaltous hydroxide product, but the composition meets the raw material composition of cobaltous oxide, so the material is used as the material for calcining cobaltous oxide. The wastewater after the 1# wastewater filter press enters the 1# wastewater storage tank, and then uses the 2# wastewater filter press for secondary slag interception. After the filter press is full of slag, it is washed with hot pure water and quickly blown dry with hot compressed air. After unloading, sampling and testing are carried out. The test results are 0.6μm, 1.24μm and 2.1μm, the sodium element impurity content is 0.0045%, the cobalt element content is 63.24%, Fe 0.04%, Ca 0.005%, Ni0.001%, Cu0.001%, As0.001%, Zn0.001%, Pb0.0007%, and the water content is 0.48%. After the 2# wastewater filter press intercepts the slag, the wastewater enters the 2# wastewater storage tank, and then is pumped into the ion exchange column to further remove the ionic cobalt in the wastewater. The cobalt content of the wastewater after de-weighting is 0.45mg / l, and it is discharged for further treatment. The direct recovery rate of cobalt metal is 99.72% and the recovery rate is 99.98%.

Claims

1. A method for improving the cobalt yield in the production process of cobalt hydroxide, characterized in that: The method is equipped with a cobalt hydroxide production system, which comprises a synthesis kettle, a homogenizing tank, a product filter press, a wastewater filter press, an ion exchange column and an industrial control host computer which are arranged in sequence, wherein the liquid outlet end of the synthesis kettle is connected to the liquid inlet end of the homogenizing tank, the liquid outlet end of the homogenizing tank is connected to the liquid inlet end of the product filter press, the liquid outlet end of the product filter press is connected to a section of transparent glass tube, an electromagnetic three-way valve is installed at the end of the transparent glass tube, and the electromagnetic three-way valve is respectively connected to a mother liquid storage tank and a wash water storage tank, the liquid outlet end of the mother liquid storage tank is connected to the liquid inlet end of the product filter press, the liquid outlet end of the wash water storage tank is connected to the liquid inlet end of the wastewater filter press, the liquid outlet end of the wastewater filter press is connected to the wastewater storage tank, the liquid outlet end of the wastewater storage tank is connected to the liquid inlet end of the ion exchange column, and the liquid outlet end of the ion exchange column is connected to an external discharge pool; A monitoring camera is installed directly above the transparent glass tube, and the monitoring camera is connected to the electromagnetic three-way valve through an industrial control host computer; The product filter press and the wastewater filter press are both connected with a hot pure water pipeline and a hot compressed air pipeline; The cobalt hydroxide production and cobalt recovery process is as follows: 1) After the cobalt hydroxide slurry is synthesized and aged in the reactor, it enters the homogenization tank for uniform mixing and is then pumped to the product filter press. The solid-liquid separation of the slurry is achieved by the product filter press. The cobalt hydroxide wet material containing chemical impurities is retained in the filter chamber of the filter press. The mother liquor of the filter press enters the mother liquor storage tank and returns to the liquid inlet end of the product filter press for secondary filtration; 2) Open the hot pure water pipeline valve and use hot pure water to wash the cobalt hydroxide wet material in the product filter press multiple times in all directions until the color of the washing water no longer changes; During the washing process, the initial darker wash water is discharged into the mother liquor storage tank, and the later lighter wash water enters the wash water storage tank and is pumped to the wastewater filter press for subsequent treatment; The control principle of the electromagnetic three-way valve during drainage is as follows: if the monitoring camera observes that the wash water color becomes darker through the transparent glass tube, it will automatically switch to the mother liquid storage tank through the industrial control host computer, otherwise it will enter the wash water storage tank; 3) After multiple washings, the wet cobalt hydroxide material is still retained in the filter press chamber of the filter press. At this time, the hot compressed air pipeline valve is opened, and the hot compressed air is used to dry the wet cobalt hydroxide material and then unload it for recovery; 4) The wash water in the wash water storage tank is pumped to the wastewater filter press for filtration and washing. Since the metal content in the wash water is relatively low, the filtrate and wash water produced by the wastewater filter press are temporarily stored in the wastewater storage tank. The filter residue is dried by hot compressed air and then discharged for recovery; 5) The wastewater temporarily stored in the wastewater storage tank is pumped into the ion exchange column, where the cobalt in the wastewater is further removed through ion exchange and then discharged into the external discharge pool.

2. A method for improving the cobalt yield in a cobalt hydroxide production process as claimed in claim 1, characterized in that: The wastewater filter press includes a 1# wastewater filter press and a 2# wastewater filter press, and the wastewater storage tank includes a 1# wastewater storage tank and a 2# wastewater storage tank. The 1# wastewater filter press is connected to the 1# wastewater storage tank, and the other end of the 1# wastewater storage tank is connected to the 2# wastewater filter press, the 2# wastewater filter press is connected to the 2# wastewater storage tank, and the 2# wastewater storage tank is connected to the ion exchange column.

3. A method for improving the yield of cobalt in a cobalt hydroxide production process as claimed in claim 1, characterized in that: In step 5), the cobalt content in the wastewater finally entering the external discharge pool is less than 1 mg / L.

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