Copper smelting arsenic-containing wastewater treatment

Domestic copper smelting enterprises have developed rapidly in the 1990s, and the increase in smelting capacity has increased the demand for raw material copper concentrate. For production needs, some companies have reduced the quality requirements for raw materials, especially the arsenic content of raw materials. The relevant national quality standards stipulate As <0.3% in raw materials, but the content of As in copper concentrate produced by some mines in China is higher, and As>1% in individual raw materials. The consequence is that it will bring difficulties to the environmental management of enterprises, and cause some enterprises to exceed atmospheric emissions and sewage discharge. This article focuses on the impact of the water environment. For the formation of arsenic-containing industrial wastewater in copper smelting enterprises and how to deal with the discharge of standards, and to ensure that no secondary pollution is caused, some understandings and opinions will be elaborated from my design experience and production practice.

1. Composition of industrial wastewater containing arsenic

(a) Sour acid

Arsenic in copper concentrate is generally in the form of copper sulfide, mainly in the presence of arsenic cuprite (3Cu ₂ S. As ₂ S ₃ ) and sulfur arsenic (Cu ₃ AsS ₄ ). The arsenic-containing minerals are basically insoluble in water during the mining process and are present in the copper concentrate. During the smelting process, arsenic in the copper concentrate enters the smelting flue gas due to the high temperature and exists in the form of As ₂ O ₃ . The smelting flue gas is made into sulfuric acid through a process of purification, dry suction and conversion. When the acid-making process adopts one turn and one suction, most of the As ₂ O _{3 in the} flue gas enters the acid-making tail gas, and is treated and recovered by the tail gas treatment system, so that the exhaust gas reaches the standard discharge. However, the existing tail gas treatment process has the problems of high treatment cost and difficulty in achieving exhaust gas emission. Therefore, most of the smelting gas-making acid-making enterprises adopt the two-turn two-suction acid-making process through technical transformation, so that the acid-making tail gas can reach the standard discharge. The flue gases As ₂ O ₃ and other impurities periodically withdrawn proceeds waste acid, and then acid treatment of sewage, which is useful metal recovery. Analysis of the arsenic content of some enterprises in the discharge of sulphuric acid is generally 3 ~ 10g / L, special circumstances up to 20g / L, and contains other harmful impurities. For example, the sulfuric acid components of Guiye and Jinlong Copper Company are shown in Table 1.

Table 1 Sour acid components and impurity content g/L

(2) Sewage

The industrial sewage of smelting enterprises mainly comes from electric dust washing, ground washing water in the sulfuric acid workshop and production water polluted at other working conditions. The water is large and the composition is complex. It contains harmful metal ions such as As, Cu, Pb, Zn and Cd, which need to be treated in depth to reach the standard discharge. The representative industrial wastewater composition is shown in Table 2.

Second, the treatment of arsenic-containing sewage

(1) Treatment of high arsenic and acid

1, the principle of processing

In chemical enterprises, the discharge of sulphuric acid in the production of sulphuric acid is generally achieved by multi-stage neutralization of lime milk, and the sulphuric acid in the sulphuric acid production of copper smelting enterprises must be sulphur-depleted to remove arsenic and copper ions due to the presence of high arsenic impurities. The neutralization method can be used to discharge the industrial wastewater. At present, domestic manufacturers of sulfuric acid treatment mainly use vulcanization→neutralization→oxidation process or neutralization→sulfidation→oxidation process. It has been verified by production practice and achieved satisfactory results. For example, the sewage treatment process adopted by Jinlong Copper Company is shown in Figure 1.

The reaction mechanism of each stage in the sewage treatment process is

1 neutralization reaction to form gypsum

CaCO ₃ +H ₂ SO ₄ =CaSO ₄ +H ₂ O+CO ₂ ↑

2 sulfurized copper

Cu ^{2 +} +S ^{2- 2-} CuS↓

3 Sulfide arsenic removal

3Na ₂ S+As ₂ O ₃ +3H ₂ O=As ₂ S ₃ ↓+6NaOH

2. Influencing factors

Since the sulfuric acid content in the sulphuric acid is about 100 g/L, pH ≈0, the pH is generally controlled to be 1.5 to 3.5 during the neutralization reaction, so the effect on the subsequent arsenic removal reaction is minimal. The arsenic in the sulphuric acid mainly exists in the form of trivalent arsenic, namely AsO ⁺ ion, and the potential-pH diagram of arsenic is analyzed. In the arsenic sulfide removal reaction, the oxidation-reduction potential should be controlled between -50 and +50 mv, which is proved by production practice. Under this controlled condition, the removal rate of arsenic can reach 95%, and the removal rate of copper can reach over 98%.

The stepwise vulcanization process is used to treat the sulphuric acid. The arsenic concentration in the treated reaction solution is generally less than 100 mg/L, which can recover the useful metals in the sulphuric acid and create conditions for the discharge of the sewage treatment station. However, the investment and processing cost of the vulcanization process equipment is relatively high. The cost of Na ₂ S in the treatment cost accounts for about 20% to 30% of the treatment cost, and the treatment cost per ton of acid is about 100 yuan. High investment and high cost have restricted the use of this process by some small and medium-sized enterprises. It has been shown that the cost of treating arsenic-containing acid by electrowinning is lower than that of vulcanization. At present, the scale of the test has been formed, and it is believed that it can be used quickly in production.

(2) Treatment of arsenic-containing sewage

1, the principle of processing

Copper smelting enterprises have sewage treatment stations to treat sewage from the sulfuric acid plant and sewage from the whole plant. Generally, the sewage entering the sewage treatment station of the plant is characterized by large processing capacity and complex composition. For example, the comprehensive sewage quality of Jinlong Copper Company and Jinchang Smelter is shown in Table 2.

Table 2 Sewage water quality components

Heavy metal ions, especially arsenic ions, bring certain difficulties to the selection of wastewater treatment processes. According to GB8978-1996, the arsenic emission concentration is 0.5mg/L. In the design selected process index, the total removal rate of arsenic ions should reach 99%, so that the treated water can reach the standard discharge. The use of a simple lime milk neutralization process does not guarantee water quality compliance. In recent years, large-scale production of copper smelting enterprises and enterprises transformation of environmental control, acidic arsenic-containing waste water treatment are used in the two-stage lime milk and processing of iron salts arsenic removal process, the production proved that the process is a line Effectively, when the arsenic ion reaches the standard discharge, other heavy metal ions can reach the standard discharge.

The process flow is shown in Figure 2.

The reaction mechanism of the process is:

A neutralization reaction control PH=7~8

Ca(OH) ₂ +H ₂ SO ₄ =CaSO ₄ ↓+2H ₂ O

2H ₃ AsO ₃ +Ca(OH) ₂ =Ca(AsO ₂ ) ₂ ↓+4H ₂ O

The oxidation reaction oxidizes Fe ^{2 +} to Fe ^{3 +} and As ^{3 +} to As ^{5 +} to form iron salt and ferrous salt, respectively.

4Fe(OH) ₂ +O ₂ +2H ₂ O=4Fe(OH) ₃

2Fe(OH) ₃ +3As ₂ O ₃ =2Fe(AsO ₂ ) ₃ ↓+3H ₂ O

Fe(OH) ₃ +H ₃ AsO ₄ =F3AsO ₄ +3H ₂ O

Fe(OH) ₃ +H ₃ AsO ₄ =FeAsO ₃ +3H ₂ O

2. Influencing factors

1PH effect

The second stage neutralization control PH = 10 to 11, so that the iron arsenic salt and the calcium salt in the above reaction are completely precipitated under alkaline conditions. In the above reaction, to ensure that the removal rate of arsenic reaches 99%, the key is to control the conditions of the second-stage neutralization reaction. According to the test data on arsenic removal, see Figure 3 and Figure 4, but the two-stage neutralization reaction control PH= At 9 to 11 hours, the effluent As <0.5 mg/L can be obtained.

The influence of 2Fe/As

Analyze the relationship between different pH values ​​and iron salt co-sinking curve. When Fe/As>10, the arsenic in the treated water is <0.5mg/L. In the production, the different arsenic-containing acidic water is treated according to the above control parameters and reaction conditions. Adjustments have achieved good results.

3 influence of coagulant

The addition of a coagulant after the above reaction contributes to the rapid precipitation of the neutralized product, and the PAM has a good adsorption and bridging effect, so that the iron arsenic salt and the calcium salt can be quickly precipitated in the concentration tank.

4 Impact of equipment

Two sections of neutralization and iron salt removal process for arsenic-containing wastewater treatment, the advantages and disadvantages of the treatment effect are related to the advanced reliability of the process equipment and measurement and control facilities. The key equipment and instruments adopt advanced products at home and abroad, which can meet the requirements of the entire treatment process. The operation laid a solid foundation.

Third, the treatment of neutralization slag

The main components of the neutralized slag after dehydration are gypsum and iron arsenic salts, containing other heavy metal basic salts (Cu(OH) ₂ , Zn(OH) _{2 ,} etc.). At the current stage, it is difficult to recover the useful metals therein, and the production cost is high. high. In order not to cause secondary pollution, the neutralization slag must be properly disposed of. It is usually landfilled in a permanent slag yard.

Fourth, the conclusion

(1) Treatment of arsenic-containing sewage in copper smelting enterprises The use of vulcanization method and lime milk two-stage neutralization and iron salt arsenic removal process can achieve the expected goal, but the treatment of sulphuric acid has a high treatment cost and needs to be treated. Application, at present, there are already experimental electro-integration methods in China to treat arsenic-containing and sulphuric acid. The cost is lower than the vulcanization method, which will bring obvious economic benefits to enterprises.

(2) At present, the arsenic-containing industrial wastewater in copper smelting enterprises has achieved discharge standards after treatment, but there is still much work to be done in terms of returning water for treatment and reducing treatment costs. These work have clear links with the enterprise system and management level. . Doing these tasks can significantly improve the economic and environmental benefits of the company.

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