One of arsenic contamination common, serious toxicity to humans; arsenic poisoning is cumulative poison, in recent years, also found arsenic or carcinogens. Arsenic contamination in the environment is mainly caused by industrial waste, including arsenic fumes containing arsenic metal ore mining, roasting, smelting, chemical, coking, thermal power, paper, leather and other production process emissions, waste water, waste gas, waste resulting The pollution, which is the highest in arsenic emissions from metallurgical and chemical industries, is the main source of environmental pollution. In the production process of the metallurgical industry, about 30% of arsenic enters the waste water and waste gas. Therefore, the final treatment of the arsenic-containing waste residue formed from the waste water has been an important research topic for metallurgical and environmental protection workers. This paper briefly introduces the treatment technology of arsenic-containing waste residue at home and abroad.
First, stabilization technology
The stabilization process is a process that uses additives to alter the engineering properties of the waste (eg, permeability, compressibility, strength, etc.), even if the waste is converted to a non-flowable solid. This process turns harmful pollutants into low-solubility, low-toxicity, and low-mobility materials to reduce waste hazards. When treating toxic arsenic slag and sludge at home and abroad, most of them are chemically stabilized, that is, chemically reacted to form relatively insoluble, stable metal arsenate and arsenite under natural conditions, including common Calcium arsenite, calcium arsenate, iron arsenate, and the like. Soluble arsenic can be formed by such compounds with many metal ions, use of this feature, often precipitated calcium, iron, magnesium, aluminum salts and the like precipitant sulfide, arsenic and then the liquid phase can be removed by filtration . According to this, in the treatment of the arsenic-containing waste residue and sludge, it is pretreated, and the arsenic is leached by a solution such as hot water or acid and alkali, and then the leachate is stabilized. The methods commonly used at home and abroad in recent years are calcium salt and iron salt precipitation methods.
(a) calcium salt precipitation method
Calcium salt precipitation method is a commonly used stabilization method because of low cost and simple process. Jinzhe Nan et al., When processing arsenic antimony soda residue on the use of calcium salt precipitation, the reaction equation for the experiment:
4Ca 2+ +2AsO 4 3- +2OH - =Ca 3 (AsO 4 ) 2 •Ca(OH) 2
4CaOH + +2AsO 4 3- =Ca 3 (AsO 4 ) 2 •Ca(OH) 2
The process is: hot water leaching of arsenic arsenate slag - calcium oxide arsenic. The test is carried out by hot water leaching, so that more than 96% of the strontium enters the leaching slag, and more than 97% of the arsenic enters the leaching solution, so that the separation of arsenic and antimony is well realized, and then the leaching solution is immersed in the leaching solution by the lime arsenic arsenic method. When the calcium-arsenic equivalent ratio exceeds 1.85 and the test temperature is 85 °C, the sedimentation rate of arsenic reaches over 95%. After the arsenic test, the arsenic calcium slag with higher arsenic is obtained.
Also used a large amount of precipitation of calcium salts of phosphorus and arsenic slag generated during the processing wolframite concentrates Preparation ammonium tungstate and tungsten oxide. The process used in his experiment was to pressurize the phosphorus arsenic slag under alkaline conditions, and react the magnesium tungstate in the phosphoric acid slag with sodium hydroxide to form magnesium hydroxide, which in turn reacted with sodium arsenate in the solution to form arsenic acid. Sodium, the reaction equation is as follows:
MgWO 4 + 2 NaOH = Na 2 WO 4 + Mg(OH) 2 ↓
3Mg(OH) 2(S) +2Na 3 AsO 4(aq) =Mg 3 (AsO 4 ) 2(S) +6NaOH (aq)
Experiments have shown that this reaction must be carried out under conditions of high alkali. Under high alkali conditions, Mg 3 (AsO 4 ) 2 is partially dissolved into Na 3 AsO 4 , and Na 3 AsO 4 reacts with Ca(OH) 2 as a decomposition product of scheelite in the concentrate under alkaline pressing conditions. A more insoluble calcium arsenate and a complex salt such as NaCaAsO 4 are formed. This causes the pollution factor arsenic to solidify in the tungsten slag and realize the conversion of the harmful phosphorus arsenic slag to the harmless tungsten slag.
Vandacasteel, who successfully dealt with pulverized coal ash arsenic. According to the test and analysis, the success is that it forms Ca 3 (AsO 4 ) 2 in the cured product. It is also found that if the waste is oxidized in advance, the amount of arsenic filtered from the stabilized product can be reduced. One order of magnitude.
In addition, in recent years, a new method, mineral precipitation, has been developed abroad. At pH 12, H 3 PO 4 and CaO are added to the arsenic-containing leaching solution to form a stable Ca 10 (As x P y O 4 ) 6 (OH) 2 precipitated.
The deficiency of the calcium salt precipitation method is that the solubility of the calcium salt is large, and the concentration of calcium must be excessively excessive, and the concentration of arsenic can be lowered to a low level, which requires consumption of a large amount of flocculant, and the amount of residue after treatment is greatly increased. .
(2) Iron salt precipitation method
Iron salt is also a common method for removing arsenic. Ferric chloride is often used as a flocculant to join water. Under the high pH condition, a large amount of iron hydroxide colloid is generated while the iron arsenate is formed, and the arsenate and iron hydroxide in the solution can also be adsorbed and coprecipitated, so that a high arsenic removal rate can be obtained. .
Fang Zhaoyu et al. neutralized the nitric acid catalytic leaching solution of high arsenic refractory gold ore with NaOH solution to combine arsenic in the leaching solution with trivalent iron ions to form stable iron arsenate. It is found that the final pH of the NaOH neutralization precipitation is preferably 5-7, because at high pH, ​​some of the iron arsenate precipitation will be converted into iron hydroxide or goethite, thereby releasing arsenate and causing an increase in arsenic content in the solution.
Sun Fengqin et al. used the iron salt precipitation method in the treatment of arsenic- cobalt- nickel concentrate produced by cyanide slag flotation. In the experiment, they first leached the arsenic-containing cobalt-nickel concentrate with bacteria and then oxidized the arsenic-containing mineral by the oxidation of bacteria. The reaction equation is as follows:
FeAs 2 +2O 2 +H 2 SO 4 +2H 2 O → FeSO 4 +3H 3 AsO 3
CoAs 2 +2O 2 +H 2 SO 4 +2H 2 O → CoSO 4 +2H 3 AsO 3
2FeAs 2 +13/2O 2 +Fe 2 (SO 4 ) 3 +2H 2 O → 4FeAsO 4 +3H 2 SO 4
CoSO 4 +2Fe 2 (SO 4 ) 3 +2H 2 O → CoSO 4 +2FeAsO 4 +2H 2 SO 4
It can be seen from the above reaction that bacterial leaching can continuously produce high-sulfur sulfate and sulfuric acid, and environmentally-contaminated arsenic precipitates in the form of scorodite (FeAsO 4 ).
JU-YONG KIM et al. used As(III) and Fe(III) adsorption coprecipitation when dealing with arsenic slag produced by Korean metal ore. Most of arsenic and iron produced stable iron-containing precipitates. Q. Wang et al. pointed out that solidified arsenic is formed by adsorption of co-precipitation of As(V) and Fe(III) into arsenic-containing ferrihydrite and As(III) and Fe(II) to form scorodite precipitate. Because the arsenic-iron co-precipitation to form arsenic-containing hydrous deposits is quite stable, this method is currently the most widely used method for immobilizing arsenic in the world. PMSwash et al. demonstrated that the stability of scorodite precipitation is at least comparable to that of Fe/As>3 arsenic-containing ferrihydrite precipitates by column leaching experiments, which is better than the stability of fixed arsenic compounds used in the metallurgical industry. Stone precipitation is a good fixed arsenic compound. The fixation of arsenic by scorodite precipitation is the development trend of arsenic-containing materials.
Second, curing technology
Curing technology is a harmless treatment method that physically or chemically fixes or contains harmful solid waste in an inert solid matrix to make it chemically stable or sealed. Curing technology can be divided into curing curing, self-binding curing and melt curing (glass curing) according to curing agent. Encapsulation curing can be divided into cement curing, lime curing, plastic material curing, organic polymer curing and ceramics according to the rubber coating material. Cured. At present, the stabilization methods commonly used in the treatment of arsenic-containing waste slag and sludge at home and abroad are cement curing, organic polymer curing, plastic material curing and melt curing.
(1) Cement and organic polymer curing
Cement curing is a treatment method that uses cement as a curing agent to cure hazardous waste. When curing, the cement and the waste water or additional added water hydrates to form a gel, which contains the harmful particles in the waste and gradually hardens into a cement solidified body. Cement curing is one of the main methods of dealing with toxic and hazardous wastes internationally. The US Environmental Protection Agency also refers to cement curing as the best technology for handling hazardous waste.
The curing of organic polymer is to completely mix the monomer of an organic polymer with waste in a specially designed container and add a catalyst to stir evenly to polymerize and solidify.
Zhao Meng and other cements used in the treatment of arsenic-containing sludge solidified, and after the formation of the spherical solidified block, the solidified block was also subjected to leaching experiments: after the hardened block was hardened for 7 days, it was immersed in a leaching agent (tap water) for 7 days. Then, the concentration of arsenic in the leaching solution is measured, and the result is that the leaching concentration of arsenic is far lower than the requirement of the leaching concentration of 1.5 mg/l of the identification of hazardous wastes - identification of leaching toxicity of GB5085.1-1996, and with the cement As the ratio increases, the leaching concentration is further reduced. The Golder Association of Australia also used cement curing methods for arsenic-containing calcine waste. The leaching experiment was also carried out later, and the results were the same as those of Zhao Meng.
At present, there is also a kind of volcanic ash cement curing in foreign countries, that is, a solidified material with aluminosilicate as the main component - volcanic ash
To cure the arsenic-containing waste residue. It has been reported in the literature that the arsenic-containing sludge is treated with a combination of volcanic ash and lime. Although the treated product still exhibits a soil-like appearance, the leaching test confirms that the stabilization process significantly reduces the arsenic leaching rate.
Tri T. Hoang conducted experiments on arsenic-containing mixed wastes, such as calcium thioaluminate (CSA) cement curing, magnesium phosphate (MP) cement curing, polyester resin (OPE) curing, and epoxy vinyl ester resin (EVE) curing. In the experiment, the cured body after curing was also tested by TCLP and SPLC methods. The result of the test was that CSA, OPE, and EVE had good curing stability in addition to MP curing, and the durability and hardness of the cured body were OPE. EVE cured body is better, which shows that OPE, EVE curing will have a good prospect. The arsenic sulfide precipitate obtained by treating a arsenic-containing wastewater by a smelting plant in a smelting plant in Japan is solidified by organic polymerization and then piled up on site.
Cement curing is widely used in industry because of its simple curing process, low equipment and operating cost, and good strength, heat resistance and durability of the cured body. However, cement curing also has certain disadvantages: the solidified body has a high leaching rate and needs to be coated; the cement solidified body has a relatively high capacity; some wastes need to be pretreated and added with additives to increase the processing cost ] .
The advantage of organic polymerization curing is that it can be operated at room temperature; the amount of catalyst added is small, and the final product volume is smaller than other curing methods, and it can handle dry slag as well as wet mud. The disadvantage is that it is not safe enough. Sometimes the strong acid catalyst used will dissolve heavy metals during the polymerization process, and requires the use of corrosion-resistant equipment; the cured body has poor aging resistance; and the solidified body is loose and needs to be placed in a container for disposal, which increases the disposal cost.
(2) Plastic material curing
Plastic material curing can be divided into thermosetting plastic curing and thermoplastic curing according to the properties of the materials used. Thermosetting materials are commonly used for curing. The curing of thermoplastic materials is to use a molten thermoplastic ( asphalt , paraffin, polyethylene, polypropylene, etc.) mixed with hazardous waste at high temperatures to stabilize it. At present, the most commonly used thermoplastic curing technology at home and abroad is asphalt curing technology.
Asphalt curing is based on the use of asphalt-based materials as a curing agent, which is uniformly mixed with waste at a temperature of a harmful temperature to produce a saponification reaction, so that harmful substances are contained in the asphalt to form a solidified body, thereby being stabilized. Asphalt is a hydrophobic material. The complete asphalt solidified body has excellent water resistance, good adhesion and chemical stability, and has high corrosion resistance for most acids and alkalis. Therefore, asphalt solidification has better stability. Sex. Q. Wang et al. used asphalt solidification for the stabilization of arsenic-containing slag. He also proposed that the arsenic-containing waste residue can also be frozen.
The advantage of curing of the thermoplastic material is that the leaching rate of the cured body is lower than other curing methods, and the compatibilization ratio is small; the curing has good barrier property to the solution and is highly resistant to microorganisms. The disadvantage is that the cured substrate has flammability, and the product has suitable packaging; the thermoplastic material is expensive, the operation is complicated, and the equipment cost is high.
(3) Melt curing
Melt curing technology is also known as glass curing technology. The method combines the waste to be treated with fine glassy materials, such as glass cullet and glass powder, and after mixing and granulating, melts at a high temperature to form a glass solidified body, and ensures the permanent stability of the solidified body by the dense crystal structure of the glass body. . LG Twidwell et al. performed glass curing on arsenic-containing slag and proved to be stable for long-term storage through experiments.
The advantage of glass curing is that the formed glassy material has higher durability, better anti-bleeding property and stronger acid corrosion resistance than the cement cured product, because the waste component has become a component of the glass and helps the glass to solidify. The leaching rate of the body is the lowest, and the capacity ratio of waste is not large. The disadvantage of this method is that the process is complicated, the material requirements of the equipment are high, and the processing cost is high.
In addition, in recent years, the treatment of arsenic-containing waste slag at home and abroad is also a fire curing method - high-temperature calcination of arsenic-containing slag, such as calcium arsenic slag and iron arsenic slag. When there is an experimental result table, the higher the calcination temperature, the lower the solubility of the arsenic slag after calcination. In recent years, several copper smelters in Chile have adopted the fire curing method when dealing with arsenic calcium slag, and have achieved good results. Liu Zheng et al. used the high-temperature fire method to cure the arsenic-containing waste residue generated during the high-arsenic-cobalt ore enrichment process, and achieved good results.
The solidified and stabilized arsenic-containing waste slag and sludge must also be considered for final disposal, so that the solid waste is separated from the biosphere to the maximum extent.
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