Advanced catalytic oxidation pretreatment technology of high concentration organic pharmaceutical wastewater

At present, cyanide extraction of gold is the main method of gold, silver and precious metals smelting. With the attention of the state and society to environmental protection, the content of harmful substances in cyanide waste in cyanide plants is more and more strict. With the implementation of the Environmental Protection Tax Law in 2018, various enterprises have successively launched treatment processes suitable for their respective situations, and the core of each process is the treatment of cyanide in liquid.
Due to the gradual improvement of environmental protection requirements, at present, all gold concentrate cyanide enterprises are lean liquid closed-circuit cycle technology, and the heavy metals and total cyanide in the circulating water of the process are maintained at a high level. For example, the total cyanide content of circulating water in the cyanide process of a smelting plant in Shandong Gold is more than 10,000 mg/L, and the copper and zinc content can reach about 3,000 mg/L. Under such a high concentration of total cyanide, the cyanide tailing is generally treated by pressing the cyanide tailing to filter the tailing to make the tailing up to the standard, and then the cyanide in the washing water is treated. Because the data show that the treatment of cyanide per unit mass in the solution is much less than the direct treatment of unfiltered pulp.
At present, the treatment process of cyanide-containing wastewater is divided into oxidation method, precipitation method and recovery method, among which oxidation method is more used, mainly sulfur dioxide air method, chlor-alkali oxidation method, ozone oxidation method, electrolysis method and hydrogen peroxide oxidation method. The oxidation method generally has the problem of high cost of reagents, the introduction of other salts, secondary pollution and other problems, and the treatment effect of ferrous cyanide complex is poor. Generally, the complex cyanide cannot be completely removed by precipitation. The recovery method is generally sulfuric acid acidification recovery method or zinc sulfate - sulfuric acid method. In addition, there are ion exchange methods and semi-permeable membrane methods that are being studied and promoted. This method adopts two-step treatment, combined with the characteristics of oxidation method and precipitation method, and takes into account the cost and treatment effect better. It can meet the requirement of total cyanide content in washing water of cyanide slag in production, and has good popularization value.

1. Process flow and principle
1.1 Principle of the method
The main occurrence states of cyanide in cyanide-containing wastewater can be divided into simple complex cyanides such as free cyanide, copper zinc and stable iron-cyanide complexes. Among them, free cyanide and simple complex cyanide can be oxidized, which can be treated by oxidation, iron cyanide complex is very stable and can not be oxidized, so it can only be removed by precipitation.
The main reaction is as follows:

The actual chemical formula of the produced iron blue may also be NH4Fe, which will not be studied in this paper because it does not affect the treatment effect.
The flow diagram is shown in Figure 1.

In this process, ferrous is first used to react with ferrous cyanide root anion in wastewater under acidic conditions to produce insoluble ferrous cyanide and ferrous cyanide, and then filter and completely remove ferrous cyanide root in wastewater. The residual zinc cyanide complex, copper cyanide complex and simple cyanide in wastewater were removed by oxidizing with hydrogen peroxide and adjusting alkali with lime.

1.2 Specific operation process and key points

Add a certain amount of ferrous sulfate solution to the raw liquid to be treated, at this time the pH value is reduced to 6 ~ 7, then add sulfuric acid to adjust the pH value to 4.5 ~ 5, after the full reaction, add flocculant, use a filter press to separate the solid and liquid, add a small amount of hydrogen peroxide to remove the residual bivalent iron ions, and then add lime to adjust the pH to be weakly alkaline, continue to add hydrogen peroxide to remove the remaining cyanide complex ions.

Adding ferrous sulfate and then adjusting pH value with sulfuric acid can reduce the volatilization of hydrogen cyanide gas and improve the operating environment. When the pH value is controlled from 4.5 to 5, the precipitation effect and filtration effect are better, the volatilization of harmful gases is less, and the liquid copper ion content is basically not reduced, which can play a certain catalytic role in oxidation.

A small amount of hydrogen peroxide is first added to the solid cyanide filtrate to remove the remaining ferrous ions, because ferrous ions will react with the cyanide complex under alkaline adjustment to form the difficult oxidation of ferrous cyanide roots. At this time, the amount of hydrogen peroxide is less, so that the system does not contain ferrous as the standard, because excess hydrogen peroxide will quickly decompose in the presence of iron ions. Similarly, it is appropriate to control the pH value of 8 to 10 after adding lime, because more than 10 will accelerate the decomposition of hydrogen peroxide. According to the experiment, the pH value is controlled at 8 ~ 10, and the reaction speed is the fastest when there is copper ion.

2. Experimental situation

The laboratory has studied copper sulfate precipitation and SO2-air method, in view of cost and environmental considerations, considering other enterprise applications and the pilot test of the unit, the ferro-hydrogen peroxide two-step purification method was finally adopted as the preferred process.

In the early experiment, the liquid to be treated was obtained by diluting the lean liquid according to the ratio, which was determined by the mixing washing experiment and the data provided by the filter press manufacturer. At that time, the total cyanide grade of the liquid was estimated to be about 1500mg·L-1. The main components of the diluted liquid to be treated are shown in Table 1.

According to the reaction formula Fe (CN) 64-+2Fe2+ = Fe2Fe (CN) 6↓, in theory, the number of moles of ferrous sulfate heptahydrate is twice the amount of iron in the liquid, converted to mass, liquid iron content: added amount = 1∶6.5, the production site does not have the ability to determine the iron content, so the amount of cyanogen contained in the liquid is usually the basis for the addition of chemicals. According to the experiment, the ratio of ferrous sulfate and total cyanide content is 5.5 times better, and the residual iron ion in the liquid is about 300mg/L.

2.1 Experimental process

Take 1kg of the liquid to be treated, add 8.2g ferrous sulfate heptahydrate, adjust the pH value to 5, stir for 1h, and filter. The filtrate test results are shown in Table 2.

According to the equation, the mass ratio of hydrogen peroxide and cyanogen is 1∶1. In fact, due to the existence of reducing substances such as thiocyanogen, the amount of hydrogen peroxide is much higher than the theoretical value. Taking 27.5% hydrogen peroxide as an example, it is appropriate to treat the sample with the amount of hydrogen peroxide about 40 times of the total amount of cyanide, and the treatment effect is stable under the condition of the amount.
Take 1kg of primary treatment solution, add 34g hydrogen peroxide, and filter after full reaction. The filtrate test results are shown in Table 3.

2.2 Influence of reaction time and alkalinity on treatment effect
It has been shown that a weakly alkaline system is generally adopted for the oxidation of cyanide roots with hydrogen peroxide. Take the filtrate after a reaction filtration, adjust the pH value of one part to 8 ~ 10, the other part is not adjusted, the pH value is about 5, add the same amount of hydrogen peroxide, stir the reaction, and determine the concentration of cyanide root, the results are shown in Table 4.

The experimental results show that the reaction rate of alkaline adjustment is fast, about 1h.
Considering that hydrogen peroxide is easy to decompose in the presence of alkali, iron is easy to decompose in the presence of ferrous, and ferrous is easy to react with cyanogen, the pilot conditions were basically determined as follows: the amount of ferrous sulfate treated once was 5.5 times the total amount of cyanogen, the time was 1h, and the end pH value was 5; The amount of hydrogen peroxide in the secondary treatment is 40 times the total amount of cyanide, the reaction time is 1.5h, the pH value is above 8, and the hydrogen peroxide is added in different times.
3. Pilot test situation
In the pilot test, three schemes were tested, namely, two-step copper precipitation, copper precipitation-oxidation and ferrous precipitation-oxidation.
The two-step copper precipitation method is to add excess copper sulfate first, fully react and filter, and the total cyanogen of the filtrate is about 80 ~ 100mg/L. Sodium metabisulfite was added to the filtrate and copper sulfate was added according to the situation, and the cyanide roots in the liquid were removed by the cyanide precipitation of copper and cuprous, and the total cyanide was finally reduced to less than 20mg/L. Copper precipitation-oxidation method is to first add copper sulfate, produce precipitation filtration and decyanation, filter filtrate and then oxidize with hydrogen peroxide, in which excess copper is used as reaction catalyst, the total cyanide content of the oxidized liquid reaches the standard, and the filtrate is recycled. The process of ferrous precipitation-oxidation is described in this paper.
The main drug consumption of the three schemes in the pilot test is shown in Table 5.

Due to the high price of copper sulfate, the cost of ferrous precipitation-oxidation method is much lower than the two methods using copper sulfate. Finally, ferrous sulfate precipitation-oxidation method was used as the production process.
4. Production situation
4.1 Device Introduction
Due to the relatively tight policy, after a short pilot test, the design and construction of transitional production facilities, due to the original plan as a transitional program for short-term use, equipment selection and so on. However, a better method has not been found, so the process has been used for more than two years, because the equipment selection is not suitable, the operation is quite inconvenient, and the equipment failure rate is high.
A section of the reaction tank adopts three Φ3×4 reaction tanks, which are made of FRP anti-corrosion inside, and one tank is used as the buffer tank of the filter press. The two-stage reaction tank uses three Φ4×5 tanks, one as a buffer tank. The lime is added intermittently by buffer tank. There are also three liquid storage tanks for storing twice pressed filtrate and raw liquid to be treated. Filter a section of filtrate with two 150m2 filter presses. The processing capacity is about 140m3 / shift.
The production agents are ferrous sulfate, sulfuric acid, polyacrylamide flocculant, hydrogen peroxide, copper sulfate, lime. Because the amount of lime in the second stage reaction is not much, the liquid solid content after the reaction is not high, so the filtration is cancelled directly for washing operations, such a disadvantage is that the treated wash water storage tank will have precipitation, and there will be siltation for a long time, but because the storage tank uses the lower opening, the amount of siltation will not increase indefinitely, and will not affect production.
4.2 Production Effect
After commissioning, the treatment effect is stable, the existing problem is that the plate and frame filter press will run for a period of time after each discharge, the main reason is that the flocculant is not used correctly, but because the site can not be improved, the running muddy part will return to a process to re-filter. The dosage of main agents and the final process treatment effect are shown in Table 6 and 7.

5. Conclusion
The scheme distinguishes the easily released cyanide and stable complex cyanide contained in the liquid, and has a wide range of application, which can deal with high concentration of cyanide, and can deal with the liquid containing iron cyanide and ferrous cyanide root. If the copper content in the liquid is high, the copper slag can be recovered separately during the two-stage treatment. In the process of hydrogen peroxide oxidation, the cyanide in the liquid is preferentially oxidized, and the influence on the thiocyanogen is small, so that the reagent is relatively saved.