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   5 . 6  Secondary treatment of wastewater
 After acidification the acid waste water treatment method typically amounts recovered cyanide-containing 5 ~ 50mg / L, far from the emission standards set by the state, wherein the heavy metals Zn, Cu, but also seriously overweight, it is necessary for further processing, often referred to as Secondary treatment.
  6  Secondary treatment method and process of wastewater
   After acidification recovery process wastewater treatment, CN -, Cu, Zn, etc. not standard, it is necessary for further processing, if the processing by blow-off method, the effect is poor, and therefore, the need for secondary processing more effective.
 The cyanide plant that treats cyanide-containing wastewater by acidification recovery often has a large amount of alkaline flotation wastewater, and its flow rate is about 10 to 30 times that of cyanide-containing wastewater . If this water is used, the cyanide can be diluted to reach the target, and The pH value of the wastewater is neutralized to 7 to 9 to precipitate heavy metal ions, which is ideal for the purpose of waste treatment.
 If there is no such condition, but the tailings pond does not leak and is relatively large, then after the wastewater is diluted, although the CN- does not meet the standard, the drainage can be ensured by natural purification in the tailings pond. In addition to these two methods, The wastewater produced by the acidification recovery method can be treated by the sulfur dioxide - air method and the chlorine oxidation method, but the latter is costly and the treatment effect is not very satisfactory.
 From the composition of the waste water, although CN - very low concentrations, SCN - is very high, and therefore, when a high consumption of chlorine by oxidation with chlorine, which proved significantly more than the cost of the recovery process of the acidified profitability. Moreover, the CN- standard can not be achieved, and the residual cyanide is generally in the range of 2 to 10 mg/L . If the sulfur dioxide - air method is used, since the copper content in the wastewater treated by the acidification recovery method is only 2 to 10 mg/L , it is necessary to add copper salt as a catalyst to increase the cost. Further, power consumption is large, the cost is not high, although oxidation with chlorine, but may also counteract acidification profit recovery method, the method further sulfur dioxide air ─ SCN - Removal of poor. In recent years, the treatment of this wastewater with activated carbon has a good effect and low cost. The use of hydrogen peroxide as a secondary treatment is better than the sulfur dioxide - air method, and the equipment is simple, but hydrogen peroxide is often expensive. .
   7  Acidification recovery method application example
 A domestic mine adopts acidification recovery method to treat gold concentrate cyanide depleted liquid. The process flow is shown in Figure 6-5 . The device uses secondary copper and secondary copper to separate copper slag process, which effectively reduces the stripping tower. The blockage is carried out by two stripping processes, so that the residual cyanide of the treated wastewater is greatly reduced. The treatment effect is shown in Table 6-1 . The drug consumption and recovery of NaCN and copper slag are shown in Table 6-2 .
Cyanide-containing wastewater → warming tank → mixed acid → solid-liquid separation → copper slag drying → sale
↑ ↑ ↓
Steam or electricity  → HCN HCN sulfate absorbent occurs → NaCN
HCN occurs → HCN absorption → NaCN
↓
Waste liquid
Figure 6-5 Schematic diagram of industrial equipment for acidification recovery
Figure 6-3 Schematic diagram of industrial equipment for acidification recovery
Table 6-1 Composition concentration of wastewater before and after acidification recovery ( mg/L )
Wastewater composition   CN —  SCN —  Cu    Zn Fe    Pb ? Au pH
Lean liquid ( before treatment ) 1000 to 3000 900 to 1300 600 to 900 100 to 300 5 to 50 0.5 to 10 0.03 to 0.1 >10
Waste liquid ( after treatment ) Â 10 to 50 350 to 700 5 to 20 100 to 250 Â Â micro- Â Â Â Â Â Micro ? 0.02 ~ 0.05 <2
Table 6-2 Acidification recovery method consumption and recycling products
 project     sulfuric acid    Caustic soda    Electricity         steam     Recovery of NaOH Cu slag
 Unit kg/m 3 kg/m 3 kw·h/ m 3 kg/ m 3 kg/ m 3 kg/ m 3
 Consumption 5.79 1.79  4.26   40 1.82   1.4
Gold ore cyanide wastewater acidification recovery method (3)
: ËŽÌ¥; mso-hansi-font-family: ËŽÌ¥; mso-bidi-font-family: Song "> It is impossible to convert all into CuSCN , but to generate a certain amount of CuCN precipitate, which reduces cyanide recovery and builds energy The investment in the fully enclosed sedimentation tank (concentrater) for precipitating acidic cyanide-containing wastewater for 5 hours is also large. If the liquid-solid separation is carried out after the stripping, the floc of the precipitate is destroyed and emulsified during the stripping process. In order to make the sedimentation slower, it is necessary to build a larger sedimentation tank. The sedimentation time should not be less than 18 hours, but at this time, the acidity of the wastewater is low, and the cement pool can be used. Because the cyanide content in the wastewater is low, it will not make the surrounding The staff is poisoned, so most of the foreign countries use the stripping to separate the sediment. The multi-cell conversion can solve the problem of sediment drying.