铬粉煤灰稳定法化学物质的选择和处理条件

1、Chromium Fly Ash Stabilization: Choices of Chemicals andTreatment ConditionsAbstractIncineration fly ashes are toxic and need stabilization/solidification before disposal. Chemical stabilization is a trend for fly ash treatment. In this paper, organic chelating agent, sodium sulfide, phosphate and c

2、opperas (FeSO47H2O) were compared from both their effect and costs. Hydrothermal conditions of the treatment process as well as normal temperature conditions were investigated for proper reaction condition choice. The comparison showed that copperas is comparable to organic chelating agent and other

3、 inorganic reagents by the effect; however, the price of the former is much cheaper. As for reactive conditions, the investigation proved that the hydrothermal conditions made good results both for phosphate and copperas; and hydrothermal conditions needs much lower chemical consumption.Keywords: In

4、cineration fly ash, heavy metals, chemical stabilization, reaction conditions1. INTRODUCTIONWith the development of MSW incineration in China, the production of fly ash increased greatly. Presently the capacity of MSW incinerators that in operation or will be put into operation in China has exceeded

5、 26000 tons per day, the production of fly ash is around 25% of the incinerated MSW, two years later the fly ash production will be over 5201300 tons per day. The annual production of fly ash will be over 284.7 thousand tons per year and they will take up space more than 474.5 thousand cubic meters.

6、 Take Shenzhen (a southern city in China) for example, in addition to the first-set-up incineration plant in China - Shenzhen Refuse Incineration and Multi-treatment Power Plant with capacity of 450 tons per day, the newly set up incineration plants hold incineration capacity of more than 2000 tons

7、per day, the fly ash production will be over 60 tons per day in two years. According to the national standard (GB18485-2001) the incineration fly ash is defined as hazardous solid waste; and it was also stipulated in related standards (GB 18597-2001 and GB 18598-2001) that fly ash cannot be stored i

8、n the incineration plants for long time and cannot be disposed or discharged out without treatment. It costs too much to dispose the fly ash and the security landfills are very difficult to find in a city like Shenzhen where economy is relatively more developed. Presently security landfill site in S

9、henzhen has Proceedings of the First International Conference on Thermal Treatment and Resource Utilization of Wastes November 21-23, 2005, Beijing, China415 almost arrived its lifetime, and fly ash from MSW incinerators are refused to send there. Therefore to find a practical method to dispose the

10、fly ash is highly urgent for the case of Shenzhen city. Besides Shenzhen, the conditions in other cities are very similar in China, for example, in Shanghai the existing incineration plants have a capacity of 2500 tons per day; two years later, this data will be increased to 5500 tons per day and th

11、e corresponding production of fly ash will be more than 200 tons per day.Considering the continuous development in fly ash stabilization/solidification researches, the treatment methods could be classified into four groups: Thermal treatment (melting/ vitrification sintering and heating), solidifica

12、tion, chemical stabilization, and separation /recycle. By the term “solidification”, the treatment is mainly based on physical encapsulation. By adding cement or other hydraulic binder into fly ashes a monolithic material is achieved, thus the specific surface of fly ashes as well as the hydraulic p

13、ermeability of the material is reduced. The price is reasonable, and the permeability of the final product can be varied by controlling the amount of cement or other binders used. But this process will pose many problems, for example, the increased mass for disposal and the possibility of heavy meta

14、ls leaching in a long run. Melting/vitrification processes are effective both for heavy metals and dioxins, by melting/vitrification, fly ash is melted at high temperatures of 1200 to 3000 into stable slag and secondary dust. The silica in the fly ash gains a Si-O2 network structure by melting. Some

15、 of heavy metals migrate into such network meshes and solidify there into a stable vitreous substance, whereby heavy metal leaching even under very low pHs is prevented. In addition to melting/vitrification process, thermal treatment also include sintering and heating processes operated in the tempe

16、rature range of 9001200. Treatment cost of melting/vitrification is inevitably high in comparison with other processes, according to Ref. 1, operating and management cost of melting is between 100 US$/ton and 150 US$/ton and the installation cost of different each melting processes (according energy

17、 source) is almost 1 million US$/ton. Power consumption is in the range of 6001250KWh/ton. Chemical stabilization is based on immobilization of the fly ash by adding a chemical reagent to form a mineral with low solubility. It has the merits of low energy consumptions; relatively low volume of the l

18、eft residues; moderate operation costs etc. Now chemical stabilization has become a trend for fly ash treatment.2. CHEMICAL STABILIZATION OF FLY ASHTo realize effective and economic operation it is very important to choose a suitable chemical as stabilizing agent and it is also very important to cho

19、ose the correct reaction conditions for a certain kind of chemical. There are many chemical reagents for this purpose: phosphates (orthophosphate PO) 2, sulphides3, ferrous sulphate 4,5, carbonate and several organic compounds6. The first three have been studied extensively and have been used in ful

20、l scale and pilot scale. The ferrox process based ferrous sulfate (FeSO.7HO) is one of the most successful chemical stabilization processes that can considerably reduce the leachability of heavy metals 5. The effects and costs of those reagents will be analyzed below.2.1 Chelating agentsThere are tw

21、o families of chelating agents for binding heavy metals. One family capture heavy metals to form dissolvable chelate compounds 7; another family bind heavy metals to form insoluble compounds. As it is too difficult to recover or dispose heavy metals when chelated in liquid phase, the chelates in the

22、 second family are more proper for fly ash stabilization.In Ref. 8 a kind of chelate was synthesized through the reaction by different types of polyamine or polyethleneimine and carbon disulfide in alkaline conditions. The produced heavy metal chelates can capture and encapsulate heavy metal ions ti

23、ghtly. As reported in Ref.8, when the dosage of this synthesized chelate agent reached to 0.40.6 wt% of the fly ash, the heavy metal concentration in the treated fly ash leachate can be lower than 1mg/l for most cases. With organic chelates as stabilizing agents for fly ash, the primary problem is t

24、he synthesis of the chelating agents themselves. Another problem is that when dioxin destruction is required simultaneously, organic chelates cannot be used.2.2 Sodium sulfideSodium sulfide can react with heavy metals to form sulfides with low solubility. For example, HgS, Ag2S, PbS are minerals wit

25、h extremely low solubility. It has been founded that the solubility of heavy metal sulfides decreases in the following sequence:Hg2+Ag+As3+Bi3+Cu2+Pb2+Sn2+Zn2+Co2+Ni2+Fe2+ Mn2+. It also reported that the addition amount of sodium sulfide should reach to 520%wt of the fly ash so that leaching of heav

26、y metals from treated fly ash can meet with the relative standards3. However, if Pb concentration in the treated fly ash leachate should be lower than 1mg/l,dosage of sodium sulfide may need to be over 20%wt in most cases3.2.3 PhosphatePhosphate radical (PO) can combine with more than 30 elements to

27、 form minerals naturally existing in environment. Those minerals are stable to pH and Eh changes. Especially those apatite minerals with the chemical formula of A(XO)( F , Cl , OH),bivalent cations like Ca2+ can be replaced by Pb2+, and radical of PO replaced by AsO ,thus heavy metals entered into c

28、rystal structure and were stabilized. When NaHPO·10HO was used as reagent, for the ash sample listed in Table 1, the stabilization results under different conditions were shown in Fig.1.Table 1 Typical heavy metal concentration and theirleaching in a fly ash sample(mg/kg)Heavy metalPbCrCdconcen

29、tration2255.2200.3131.0leachate3.8436.2920.124From Fig.1 it can be seen that when the addition of phosphate increased in stabilization treatment process (for example, from 6g P/kg fly ash increased to 20gP/ kg fly ash), the stabilization effect did not enhance accordingly. At the same time temperatu

30、re in the treatment process influenced the stabilization effect. When temperature increased, Cr leaching always decreased. Especially when less phosphate was used, higher treatment temperature gave better stabilization results than normal temperature; when more amount of phosphate was used, higher t

31、emperature did not give better results as expected, this may be due to the formation of different type of apatite. Generally phosphate is not an effective stabilizing reagent for Cr. And when phosphate should be chosen, higher temperature condition should be adopted to decrease the dosage.Fig.1 Stab

32、ilization effect of three typical heavy metals during and after treatment process with phosphate (L/S3:1 during treatment process; L/S10:1 during leaching process）2.4 Ferrous sulphateThe reaction mechanism of ferrous sulphate stabilizing heavy metals in the fly ash can be referenced to 4,5. When dif

33、ferent amount of ferrous sulphate is used for different size of the fly ash shown in table 2, the stabilization effect of heavy metals is shown in Table 3.Table 2 Heavy metal contents in another fly ash sample (mg/kg）Heavy metals size(mm)CrCdPbZnAsHgCud<0.1360.17100.772044.823503.741683.13965.160

34、.1<d<0.2324.76831844.2173.660.221741.94741.940.2<d<0.25251.2865.271563.641554.7446.40.059231.72d>0.25202.8853.181315.411369.2210.8ND145.21It should be noted that heavy metals in process water is not so difficult to control, more attention should be paid to the leachate. From Table 3 i

35、t can be seen that 10g Fe/kg fly ash is enough to stabilize the coarse part of fly ash, according to the experiences of Shenzhen refuse incineration and multi-treatment power plant, the coarse part (d>0.1mm) amounts to more than 60%wt of total EP fly ash production. More than 20g Fe/kg fly ash of

36、 copperas may be needed for fine part of fly ash. When the mass of copperas is adopted as basis for calculation, for per kilogram un-sieved EP fly ash, 100 gram copperas is needed for EP fly ash from incineration plant. 2.5 Comparison of different reagentsIn order to choose the proper chemical reage

37、nts for stabilizing fly ashes, the above four chemical reagents are compared in Table 4 by effect and prices.From Table 4 it can be seen that when copperas is chosen, the cost for the stabilizing agent is lowest. However, as cation instead of anion is the stabilizing part, salt concentration in the

38、fly ash treatment system increased greatly thus more water is needed than other chemicals. But more amount of process water consumption itself cannot increase operation cost significantly, at the same time salts are removed from fly ash system together with process water, which is benefit for the fi

39、nial disposal of the treated fly ashes.2.6 Reaction conditions choiceFrom above discussion, copperas can be chosen for fly ash chemical stabilizing agent. Also from Fig.1 it has been founded that higher treatment temperature is benefit for reducing the dosage of stabilizing agent. In order to choose

40、 proper reaction conditions for copperas, for the same fly ash sample in Table 1, higher temperature and normal temperature stabilization effects are compared in Fig. 2.From Fig.2 it can be seen that under higher temperature when the dosage of iron was only 5gFe/kg fly ash (corresponding to 2.5wt% o

41、f copperas usage) fairly good stabilization effected has been achieved; while as the dosage increased, better stabilization effect did not appear for higher temperature. In addition to the less usage of copperas, another benefit of higher treatment temperature is to shorten the treatment time and sa

42、ve the power of the blender. It is also easily to realize high temperature treatment in incineration plant where steam is available.Fig.2 Stabilization effect of three typical heavy metals with copperas (L/S3:1 during treatment process; L/S10:1 during leaching process）3. CONCLUSIONS1. The production

43、 of incineration fly ash is increasing greatly in China. Chemical stabilization is one of the most favorable methods for fly ash treatment before the final disposal.2. Four chemical agents: organic chelating agent, sodium sulfide, phosphate and copperas were compared for their stabilization effect a

44、nd economy. The comparison showed that copperas is better than all the other three both for the stabilization effect and economy.3. The higher reaction temperature in the treatment process (200) gave better results than normal temperature conditions especially when the dosage of chemical reagent was lower. This was proved both by phosphate and by copperas as stabilization reagent.ACKNOWLEDGEMENTSThe research work was supported by Shenzhen City Governmental Scientific and Technological Fund (Contract no: 2002-k3-98).REFERENCE1 Shin-ichi Sakai. Municipal sol