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Treatment of arsenic-containing wastewater
Many of ore gold ore containing arsenic, often arsenopyrite (arsenic pyrite) exists in the form of inconvenience to the extraction of gold is usually pretreated. It is easy to produce arsenic-containing gas and arsenic-containing wastewater in the pretreatment process. The arsenic-containing wastewater mainly contains oxides of arsenite, arsenate and arsenic. If it is not treated and discharged into the surface water body, it will cause serious harm to farmland irrigation, human and animal drinking, and fishery water. For example, the As concentration of the overflow water in the tailings reservoir of Hunan ××× gold mine is 0.7mg/L, and the cyanide dam of Xinjiang ×× gold mine is 1.43mg/L, which exceeds the standard of national sewage As concentration of 0.5mg/L (total arsenic). .
Arsenic in waste water treatment, the more commonly used lime method, a curing method, soft manganese ore method, transformation method and the like. The Changchun Gold Research Institute has introduced chemical precipitation and flocculation methods. Currently, many gold mines are adopting a process to comprehensively recover arsenic.
I. Arsenic-containing wastewater treatment method
(a) Lime method
Generally applicable to acidic wastewater with high arsenic content. Adding lime milk to react with arsenic or arsenite ions to form insoluble calcium arsenate or calcium arsenite precipitate.
The wastewater is added with lime milk mixed sedimentation. When the dosage of lime milk is appropriate and the reaction is completed completely, the effluent water quality can reach the secondary standard of comprehensive wastewater discharge (GB8978-88). The advantage of this method is that the operation unit is low in cost; the disadvantage is that the amount of sediment is large, the treatment effect on trivalent arsenic is poor, and secondary pollution is easily caused.
(2) Lime-iron salt method
It is generally used for wastewater treatment with low arsenic content and pH close to neutral or weakly alkaline. The use of arsenate, arsenite can form stable complexes with iron, aluminum and other metals, and metals such as iron hydroxides, aluminum, etc. in addition to adsorption of arsenic precipitate.
When pH>10, arsenate, arsenite and hydroxide are replaced, so that a part of arsenic is dissolved in water, so the end point is preferably controlled at pH<10.
(3) Vulcanization method
Under acidic conditions, arsenic is present in the form of a cation which, when added to the vulcanizing agent, forms a poorly soluble As 2 S 3 precipitate. The vulcanization method can reduce the arsenic content in the wastewater to less than 0.05 mg/L. However, the precipitation by vulcanization needs to be carried out under acidic conditions, otherwise the precipitate is difficult to be filtered, and excess sulfur ions in the supernatant need to be treated before being discharged.
(4) Pyrolusite method
Using the pyrolusite method (natural manganese dioxide), the trivalent arsenic is oxidized to pentavalent arsenic, and then the lime milk is added to form a manganese arsenate precipitate, namely:
H 2 SO 4 +MnO 2 +H 3 AsO 3 →H 3 AsO 4 +MnSO 4 +H 2 O
3H 2 SO 4 +3MnSO 4 +6Ca(OH) 2 →6CaSO 4 ↓+3Mn(OH) 2 +6H 2 O
3Mn(OH) 2 +2H 3 AsO 4 →Mn 3 (AsO 4 ) 2 ↓+6H 2 O
Specific practice: the wastewater is warmed to 80 ° C, aerated for 1 h, then the milled soft manganese ore powder is oxidized for 3 h, and finally 10% lime milk is added, the pH value is adjusted to 8-9, the precipitation is 30-40 min, and the effluent water quality Arsenic can be reduced to less than 0.05mg/L.
(5) Comprehensive recovery method
At present, many mines are in the process of evaporating, concentrating, crystallization, and centrifugal dehydration of arsenic-containing wastewater to obtain sodium arsenate, that is, a comprehensive recycling process. This kind of treatment measures not only reduces the arsenic pollution in the water environment, but also benefits the disease and has certain economic benefits.
(6) Other methods
Some mining methods using phosphate dearsenification; some with activated carbon, active aluminum arsenic adsorption; and some arsenic removal by reverse osmosis; ion exchange arsenic removal, the effect is good.
Second, the development of arsenic-containing wastewater treatment methods
The treatment of arsenic-containing wastewater in gold mines in China is still in the research stage, and the research results on arsenic-containing acidic wastewater in other industries have been made. The arsenic-containing wastewater in gold mines is mostly alkaline wastewater.
Dephosphorization by phosphate method requires pre-oxidation of arsenic; removal of arsenic by sodium sulfide, its dosage is large; adsorption of arsenic by activated carbon and activated aluminum is good (except arsenic rate is 90%), and regeneration, life and cost of adsorbent Need to be further studied; the use of reverse osmosis method to remove arsenic, high cost; the use of electric flocculation method to remove arsenic, its high cost, suitable for deep purification of a small amount of wastewater; ion exchange resin method to remove arsenic, the effect is very good, but the problem of elution regeneration Not well solved. The Changchun Gold Research Institute proposed that the chemical precipitation method, the flocculation method and the conversion method obtained good results. The wastewater contains arsenic 0.70～2.80mg/L, pH 7～7.5, and the treatment is required to reach the industrial wastewater discharge standard of 0.5mg/L or less.
(1) Chemical precipitation method
The basis for treating arsenic-containing wastewater with CaO as a precipitant is that the solubility product of calcium arsenate is small (K sp =[Ca 2 + ] 3 AsO 3 - 4 ) 2 =10 - 18.2 . When CaO is added to the wastewater, the arsenic removal rate is 70% to 76% at a pH of 12, which indicates that this method can be used in some gold mines where the arsenic is not too high (As1.05 mg/L). Some mines do not have enough depth to treat arsenic-containing wastewater. For example, arsenic pyrite, arsenic exists mainly in trivalent and pentavalent oxidation states. In order to improve the effect of CaO treatment of arsenic-containing pulp, add phosphoric acid 250g/m 3 or flocculant. 1g/m 3 can guarantee the treated wastewater <0.5mg/L.
(2) Flocculation method
1. Treatment of arsenic-containing wastewater by iron salt method
The arsenic in the wastewater exists in the form of AsO 3 - 4 and AsO 3 - 3 and has the property of adsorbing to the hydroxide. Removal of arsenic by iron salt method:
As 2 O 3 +3H 2 O=2H 3 AsO 3 =2H 2 O+2AsO 2
H 3 AsO 3 +2FeCl 3 +H 2 O=2FeCl 2 +H 3 AsO 4 +2HCl
H 3 AsO 4 +FeCl 3 =FeAsO 4 +3HCl
In addition, AsO 3 - 4 and AsO 3 - 3 are also adsorbed on the flocculent precipitate of Fe(OH) 3 to produce coprecipitation. When FeCl 3 is used at pH 8, the As content can be lowered to <0.1 mg/L, and the drug consumption is 0.6 kg/m 3 .
The effect of removing arsenic by FeSO 4 is also ideal, and since FeSO 4 is lower in price than FeCl 3 , it is more practical.
2. Treatment of arsenic wastewater with aluminum salt
The aluminum salt is only effective for As (V), and the arsenic removal rate is 95%. The removal mechanism is that the floc formed by the hydrolysis of the aluminum salt adsorbs arsenic. The arsenic removal effect of polyaluminum chloride and aluminum sulfate is very similar, and Al 3 + is added in an amount of 50 times that of As. Aluminum salts are not as good as iron salts. The aluminum salt consumption is 1 kg/m 3 .
(3) Conversion method
1. Treatment of arsenic-containing wastewater with Na 2 S—FeSO 4
Treatment of arsenic-containing wastewater by sulfide sulphate is a relatively reliable method to eliminate the reverse dissolution of arsenic slag and to prevent arsenic from oozing out of slurry solids. The test proved that the best effect of removing arsenic was at pH 6-7. At pH = 6, the drug consumption was Na 2 S·9H 2 O, 1.5 kg/m 3 and FeSO 4 ·7H 2 O, 2.987 kg/m 3 .
2. Treatment of arsenic-containing wastewater with ferrous sulfide
The use of ferrous sulfide as a conversion agent to convert arsenic (V) and arsenic (III) in wastewater into sulfide precipitates by conversion of insoluble sulfides, and partial removal of heavy metals Cu 2 + and Ca 2 + Pb 2 + , Hg 2 + , Zn 2 + . The preferred results of the test are pH 6-7, treatment time 15 min, particle size 35-60 mesh, and FeS drug consumption 1 kg/m 3 .
(4) Alkaline chlorination
Gold mines use arsenic-containing flotation gold concentrates. There are few direct cyanidation processes. Most of them use arsenic-containing pulp or wastewater produced by flotation-calcination-cyanide process or gold mud cyanidation process. At the same time as the wastewater, arsenic can be removed. The amount of bleaching powder is 6 kg/m 3 of wastewater.
The above four methods for treating arsenic-containing wastewater have the best effect and low cost. CaO precipitation, aluminum salt coprecipitation, sodium sulfide-sulfate and ferrous sulfide conversion can all remove arsenic and meet industrial wastewater discharge standards. China's arsenic-containing gold ore amounts to tens of millions of tons, and the pollution control of arsenic is of universal significance. At present, the use of arsenic in gold mines in China is below 1%. For the fixation of high-arsenic ore and arsenic slag containing 1% to 8% of arsenic, further research is needed.