Treatment of cyanide-containing sewage from carbon slurry gold extraction plant by sodium hypochlorite method

The cyanide-containing tailings slurry of the cyanide carbon pulp gold extraction plant has a high water content, high cyanide content, and difficult solid-liquid separation. The sewage treatment is liquid chlorine oxidation. From the perspective of production practice, the law is feasible. However, there are outstanding problems: the secondary pollution of chlorine is serious, chlorine phenomenon often occurs, the consumption of chlorine is large, the control is difficult, the supply of liquid chlorine is tight, and normal production is not guaranteed. The Guangzhou Institute of Nonferrous Metals uses a self-made sodium hypochlorite generator to propose the use of fresh sodium hypochlorite to remove cyanide.

First, the principle of sodium cyanide removal of cyanide

Under alkaline conditions, the cyanide in the oxidizing wastewater of NaClO can be divided into two stages. First, the cyanide is oxidized to cyanate and further oxidized to carbon dioxide, ammonia and nitrogen.

According to the nature of this segmentation reaction, in the treatment of cyanide-containing wastewater, the oxidation reaction is controlled until the first stage is completed, and then CNO ^- hydrolysis is carried out into CO ₂ and NH ₃ (referred to as incomplete oxidation); of NaClO, so CN ^- completely oxidized to CO ₂ and N ₂ (referred to as fully oxidized).

Second, the test and results

The test is to simulate the current process conditions of the mine: the preparation contains CN ^- concentration 200mg/L, pH 12～13, and the sodium hypochlorite concentration is 10～12g/L. The effects of sodium hypochlorite dosage, sodium hypochlorite concentration and reaction time on the removal of cyanide are observed.

At the time of NaClO11g/L and reaction time of 60 min, the amount of sodium hypochlorite was CN ^- (mass ratio) = 2.5:1 (take 3:1). The tail liquid contained CN ^- was 0.33 mg / L. The concentration of sodium hypochlorite is limited by the concentration of electrolyzing equipment and brine, which directly affects the equipment selection and treatment cost. It can be seen from the experiment that the NaClO concentration changes from 9.3g/L to 13g/ at NaClO:CN ^- = 3 ^: 1 and 60min. L, has almost no effect on the cyanide removal effect. Observation of the reaction time is actually sufficient for 30 minutes. Therefore, the butyl NaClO:CN=3:1, NaClO11g/L, and the time of 30 min were selected as the optimum process parameters for the expansion test, and the cyanide removal effect was satisfactory.

Third, the processing cost estimate

It can be seen from Table 1 that according to the daily treatment capacity of Zhangjiakou 470t, the main raw material and power consumption of the cyanide-containing slurry treated by the sodium hypochlorite method is about one-third lower than that of the alkali liquid chlorine treatment.

Table 1 Comparison of the cost of sodium hypochlorite method and alkali liquid chlorine method

Sodium hypochlorite method mainly consumes	Amount / yuan	Alkali law mainly consumes	Amount / yuan
Salt 16t·d -1 Lime 5t·d -1 Power consumption 1920kW·h·d -1	640.0 250.0 384.0	Liquid chlorine 1.2t·d -1 Lime 10t·d -1 Power consumption 138kW·h·d -1	1200.0 500.0 28.0
Water 40m 3 ·d -1	8.0	Washing base	224.0
total	1282.0	total	1952.0
470t per day		470t per day
Processing fee per ton	2.73	Processing fee per ton	4.15

By controlling the appropriate process conditions and using incomplete oxidation, the amount of chlorine injected can be minimized. The test results show that the cyanide content of the discharged wastewater is up to the national standard. The method also has the characteristics of quick reaction, and can reduce the number of reaction tanks compared with the current alkali liquid chlorine method.

The sodium hypochlorite method is added in the form of an aqueous solution, which is simple in operation, easy to control, complete in reaction, low in residual chlorine in the tail liquid, avoids washing operation, and eliminates the problem of "secondary pollution" in the liquid chlorine method. At the same time, the amount of lime can be reduced by nearly half.