With centralized incineration has become the main way of urban living garbage disposal. Incineration safe disposal and utilization of fly ash has become a scholar of common concern. Due to a variety of environmental factors shaped ring in the landfill process. Heavy metals in waste incineration fly ash migration and transformation may occur. So how to deal with the development of incineration ash society more and more attention. Prior to final disposal incineration ash must be stable, curing, because incineration fly ash contains a higher concentration of the leaching of lead, cadmium and other heavy metals, incineration ash of course also a special hazardous waste destined for landfill. In recent years, many cities are on the incineration ash curing treatment, and achieved good results.
The characteristics of waste incineration ash
The solid residue after incineration of garbage generated roughly 30% -35% by weight of the garbage, bottom ash which accounted for 25% -30%, the rest is fly ash, accounting for about 5% [2].Fly ash after burning heat recycling system, flue gas purification system to collect the material.Fly ash production and waste type, burning conditions, incinerator and flue gas treatment process related.Less heavy metals in waste incineration bottom ash produced, especially volatile metals such as: Pb, Zn, Hg, Cd less content, mainly because some metals such as lithology pro: Ni, Cr, Cu, etc., are generally considered no poison can be used as raw material for reuse.Such as Denmark, the Netherlands waste incineration bottom ash more than 90% of used car parks, dams, embankment fill material such as asphalt and concrete and aggregates, Germany and France, respectively, 60% and 45% of incineration bottom ash used as roadbed and municipal engineering [3] materials.Analysis showed that: waste incineration ash is not chemically inert substance, which can contain flooding out of Cd, Pb, Zn, Cr and other harmful heavy metals and salts.If direct incineration ash landfill or mishandled, in the natural environment due to the effect of acid rain and other factors, the acidic environment will gradually leaching out heavy metals, re-enter the environment, polluting groundwater and crimes against humanity [4].Meanwhile, the fly ash dioxin is also potentially important environmental pollutants. Since waste incineration fly ash and other heavy metals and dioxins difficult natural degradation, so its impact on the environment very seriously.
Results Characteristics of heavy metals in fly ash and abroad show: Heavy metals having a high boiling condensation easily even in the combustion process, thereby forming the core of the fly ash, and volatile heavy metals at high temperature will decrease as the temperature of condensation in the fly ash surface, heavy metals in fly ash with fly ash particle size decreases and increases. Leaching of toxic heavy metals in fly ash and fly ash particle size, surface area, pH value, primarily dependent on the presence of heavy metals in fly ash morphology [5].Table 1 lists the hazardous waste leaching toxic identification standard.
Incineration ash solidification processing required to meet the following requirements:
(1) Cured after treatment by curing the formed should have excellent resistance to leaching resistance, wet and dry, penetration resistance, frost resistance, and must also have sufficient mechanical strength, the best effect is obtained by curing the curable can be reused as resources.
(2)Must take into account the material and energy curing process that requires the use of the energy consumption as low as possible, but also should take into account the capacity increase ratio, Capacity ratio is an important indicator for the evaluation of the curing method, while increasing really depends on the size of the infiltration capacity ratio and an acceptable level of salt curing toxic and harmful substances in the body.
(3)Curing process to be as simple and easy to operate, easy to use.
(4)Source curing agent should be more widely used, but cheap and easy to get.
(5)To lower the cost of curing, so that it can be widely promoted.
The current implementation of the stabilization process still has the following three methods:
1.Cement curing technology: cement curing is fly ash, cement by mixing an appropriate amount of water to make a method of curing, the curing mechanism is in the process of hydration of cement, by adsorption, chemical absorption, sedimentation, ion exchange, passivation and other means, the ultimate form of heavy metal hydroxides or hydrated silicate complexes remain in the surface of the cement cures to form CSH gel, while cement is added also provides an alkaline environment of heavy metals, inhibiting heavy metals in the fly ash leachate. Fly ash cement solidification technology is a relatively mature hazardous waste treatment technology has obvious advantages in terms of economics and operational, but the amount of cement is high, resulting in high capacity increase solidified over time, solidified part of the gradual dissolution of toxic substances may be long-term, the existence of a potential threat to the environment.
2.Melting and solidification technology: the melting and solidification technology mainly to small mixing fly ash and glassy, after mixing granulation molding, melting at 1000-1400 ℃ high temperature, usually around 30min (melting time depending on the nature of the fly ash may be) , fly ash upon the physical and chemical state changes, cooling cured to form a cured body of glass, with a dense crystalline structure of the vitreous, to ensure stability of heavy metals. Melting and solidification technology for residue volume reduction rate, curing effect is good, but the fatal flaw is part of the toxic substances will evaporate, exhaust gas treatment measures must be taken. Therefore, the more complex the system, high operating costs.
3.Chemical stabilization techniques: chemical stabilization technology is the use of chemical agents, toxic and hazardous substances by chemical reaction into a low solubility, low migration and low process toxic substances. The stabilizer is commonly used inorganic and organic. Inorganic mainly Na2, S and phosphoric agent, an organic chelating agent primarily polymer material, fly ash insoluble drug is mixed with the complexing group, ash soluble metal (Cd, Pb, etc.) with complexing agents in the radical reaction, the formation of stable complexes, thereby fixing the fly ash, fly ash in order to reduce the possibility of leaching of harmful ingredients. Treatment with these agents ash, generally can achieve good results, this method has a simple process, less investment in equipment, etc., but will produce high concentrations of inorganic waste, the need for further processing.
In summary, the use of cement solidification and stabilization combined process chemical treatment, incineration ash is an effective way to present the best practicable, on the basis of the company after years of painstaking experiments and practice, successfully developed the following process technology solutions, products made leachate tests are in line with the national emission standards.
Fly ash curing systems Process Description
After the fly ash from the incineration plant flue gas treatment systems into the storage bin , quantitative transported to screw conveyor, then chelated by a screw conveyor to the mixing device , cement silo of cement delivered to the screw conveyor is also quantitatively under cement silo machine , the same machine sent by the chelating spiral kneading apparatus , according to the design in the ratio of fly ash and cement mixing device for mixing chelation , and chelating agents diluent pump and water supply systems at the same time start to chelate mixing device chelating agent and water supply . Fly ash , cement, chelating agents and water mixed in a kneading apparatus chelation , heavy metals in fly ash with chelating agents reacted chelate thus be stabilized . Chelation mixing device out of the fly ash and cement slurry is stabilized after delivery device by belt conveyors, pallet conveyor, block making machine, block , made of cement blocks after the last conservation between conservation . Evaporate a lot of water conservation process , and then transported by a special brick car away. Brick carrier transported to designated locations landfill, thereby completing the entire fly ash stabilization solidification process. The ratio of the chelating agent is from 0.5 to 3% . The actual amount to be determined according to the use of fly ash in the final test component ; cement ratio of 10% to 20% ; at 10% design design. Fly ash handling output capacity by 100 tons / day design.
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technological process
