Process for recovering heavy metal from waste catalyst of SCR
2016-06-27 17:18:51
1 Overview
With the end initial stage of denitrification treatment project of China's thermal power industry flue gas denitrification, SCR catalyst has been installed, has entered the normal state of operation management, future 1 ~ 2 years SCR catalyst regeneration and recovery use of enterprise will also be put into formal operation.
In the production process of catalyst regeneration and recycling, all kinds of heavy metal oxides from coal fly ash will be faced with the problem of pollution control. Therefore, how to effectively deal with these heavy metal pollutants, to avoid the occurrence of two or not affect the recovery of the two product sales, will be engaged in recycling and recycling companies in the top priority. Our country is rich in coal resources, according to the proven reserves of lignite, Anthracite bituminous coal accounted for 75% accounted for 12%, accounting for 13%. Among them, the raw materials of coal accounted for 27%, 73% of the power coal. Power coal reserves are mainly distributed in the north and northwest regions, accounting for 46% and 38% of the total country respectively, coking coal mainly concentrated in North China, anthracite mainly concentrated in two provinces of Shanxi and Guizhou. The total in the geographic distribution pattern of coal resources in China is more less, the poor rich North south. And mainly concentrated in the economy is not developed in Shanxi, Inner Mongolia, Shaanxi, Xinjiang, Guizhou, Ningxia, and other six provinces (autonomous regions), the total amount of coal resources to 4.19 trillion tons, accounting for 82.8% of the total national coal resources; as of the end of 1996, coal to maintain reserves of 8229 million tons, accounting for 82.1% of the national coal reserves and complete kinds of coal, coal is generally good. And in China's most economically developed, high industrial output, foreign trade is the most active, need energy most, consumption of coal quantity of Beijing, Tianjin, Hebei, Liaoning, Shandong, Jiangsu, Shanghai, Zhejiang, Fujian, Taiwan, Guangdong, Hainan, Hong Kong, Guangxi and other 14 southeast coastal provinces (cities,districts) of coal resources amount only 0.27 trillion tons, only 5.3% of the total national coal resources; as of the end of 1996, the coal reserves of only 548 million tons, only accounted for 5.5% of national coal reserves, resource is very poor. Among them, Shanghai under the jurisdiction of the scope, has not found the occurrence of coal resources; as of the end of 1996, a high degree of openness in Guangdong Province, the coal reserves of only 6 million tons, Tianjin for 4 million tons, 1 million tons of Zhejiang Province and Hainan Province less than 1 million tons. Not only the resources are few, but also most of the mining conditions are complex, the quality of the coal or lignite, not only the development cost, and the comprehensive utilization of coal is not high. Thermal power plant, the coal in the combustion process, which a large number of heavy metal oxide to volatile gaseous form out, then condensation in coal fly ash into subsequent flue gas purifying device, enriched in SCR catalyst, plus removal denitrification process of ammonia injection with flue gas SO3 formed (NH4) 2SO4 stickiness of the adsorbate,
exacerbated by heavy metal oxide in the catalyst aggregation.
2 main heavy metal pollutants on the surface of waste SCR catalyst
Because of China's many types of coal, thermal power plants around the country in the use of different types of coal, produced by waste SCR catalyst containing heavy metal pollutants difference is larger, mainly containing heavy metals lead, chromium, beryllium, thallium, arsenic and mercury. According to the China Environmental
Science Research Institute of our country part of coal-fired power plant flue gas waste removal denitration catalyst risk characteristics analysis results show that waste gas removal denitration catalyst, the main risk characteristics for leaching toxicity, including the leaching concentrations of beryllium, copper, arsenic is generally higher than the new de NOx catalyst leaching concentration; partial waste gas over the relevant requirements of the "dangerous waste identification standard leaching toxicity identification" (GB 5085.3-2007), exceeding the main reason is due to the denitration catalyst in flue gas de NOx process attached to a variety of harmful heavy metals in flue gas take off the leaching concentration of NOx catalyst in beryllium, arsenic, mercury. Therefore, this paper has chosen representative of heavy metals were analyzed, other not covered by heavy metals or toxic metal oxide in the treatment process can be excluding incidental, not the formation of "unexpected" secondary pollution and affect the quality of recycled products.
2.1 arsenic (As)
Coal is a complex natural mineral, the content of arsenic in all kinds of coal varies greatly, generally 3 to 45mg per kg. Most of the arsenic in coal is in the form of arsenic sulfide or arsenic sulfide (FeS2˙ FeAs2), and the small part is the organic matter. Due to the natural characteristics of coal itself is not even, so the change of arsenic in coal is also larger, arsenic content in 0.5 ~ 80ppm, in general, the southwest of China, especially in Guizhou coal is very high arsenic content.
During the combustion process, the high temperature and strong oxidation of coal will release arsenic. Arsenic is different in coal, and the ease of arsenic release in the process of coal combustion is different. The existence shape of arsenic in the combustion products determines its influence on the environment. If the combustion products into bottom ash, dust collector and fly ash into the atmosphere of the flue gas in three parts, arsenic in fly ash concentration was significantly higher than that in bottom ash concentration and with the ash particle size smaller, arsenic in which concentration increases, i.e. in ash content and ash particle size is inversely proportional to. According to the selected 3 typical samples in a thermal power plant, the distribution of arsenic in the combustion product is calculated
[5], see table 1.
From table 1, it can be seen that arsenic is mainly distributed in the electrostatic precipitator fly ash and flue gas, the pulverized coal fired boiler in the amount of fly ash is much larger than the amount of bottom ash, thus showing fly ash arsenic share far outweigh the share of bottom ash. And fly ash in the interception of the total amount of 40%~50%, so the inclusion of arsenic in the waste SCR catalyst is mainly adhered to the surface of the dust and catalyst.
2.2 Hg (Hg)
Mercury has a volatile, in the coal crushing and washing process will be lost (about 38.8%), during coal combustion, mercury in coal heated and volatilized existed in the form of mercury vapor in flue gas, in the furnace under the condition of high temperature, almost all the coal in the mercury (including inorganic and organic mercury) into elemental mercury and in the form of gas, stay in flue gas. It is estimated that the mercury content in the bottom ash is generally less than 2% of the total mercury. So the research on mercury emission in coal combustion process should focus on the form of mercury in flue gas. The forms of mercury in combustion flue gas mainly include gaseous elemental mercury (HgO), gaseous two (Hg2+) and particulate mercury (HgP) in three forms. The physical and chemical properties of different
forms of mercury in the atmosphere are very different. In the boiler combustion process, almost all of the coal in the form of HgO in the flue gas, part of the HgO in the flue gas cooling process is oxidation, in which HgCl2 based, HgO, HgSO4 and Hg (NO3) 2˙ 2H2O. Particulate solid mercury easily fly ash adsorption and after denitration device is trapped in the catalyst surface and in the hole, with the remainder with fly ash in the follow-up of dust removal and desulfurization equipment; removal of denitration device operating temperature for 280 DEG C ~410 DEG C, attached to the fly ash of mercury during this period can only be retained by 20% to 30%, the remaining 60% to 70% of the Mercury will enrichment in the dust removal and desulfurization unit (because the temperature is gradually reduced state), and ultimately with the flue gas is directly discharged into the atmosphere of mercury (form of HGO, with relatively stable form, it is difficult to be pollution control equipment to collect) less than 10% of the total mercury of gasification. Abandoned in SCR catalyst concentration of gaseous divalent mercury (Hg2 +), with characteristics of water soluble and in the recovery process in addition to ash cleaning process, will be removed most, by alkali leaching process into leaching solution accounted for only the total amount of mercury 4%~18%.
2.3 lead (Pb)
Lead in coal is one of the sources of lead pollution in the atmosphere, and the ash content of 20%~25% is produced after coal combustion. The average lead content of coal in our country is 15.55mg/kg, and the distribution of lead in the combustion products is shown in Table 2.
Lead dust deposition in the SCR catalyst surface and in the hole, most in the recycling process in addition to ash cleaning process will be removed, the remaining mixed in the catalyst lead will be accompanied by alkaline leaching slag into rich titanium material production process and in hydrochloric acid off sodium process is brought into the process of wastewater, so in waste catalyst recycling production, processing of lead mainly concentrated in wastewater treatment process.
2.4 chromium (Cr)
Chromium is a volatile heavy metal pollutant, which is separated from the coal during the combustion process of coal. When the flue gas is cooled, it is concentrated on the dust particles, and finally is stuck in the fly ash. Therefore, in the SCR waste chromium catalyst mainly concentrated in the catalyst interception of coal fly ash in, in the recycling process can through except lavation process removed. The follow-up into wastewater chromium can through removal by chemical precipitation.
2.5 beryllium (Be)
Beryllium oxide is the two sexes, Yi Shenghua, in the process of coal combustion is also entrained into the SCR waste catalyst through the fly ash. In the recovery process by except the lavation process can not be completely removed, and 30% to 40% of the beryllium will enter the leaching solution in alkaline leaching process, and ultimately into the wastewater system. Beryllium compounds in alkaline water can be precipitated in 5 days.
2.6 thallium (TI)
Thallium is a dispersed elements, the content of thallium in most of our coal in 0.01~2mg/kg, and high content of coal production in Guizhou, Sichuan and Yunnan area, can reach 10~100mg/kg, has the typical characteristics of regional pollution. Because of the environmental cycle and toxic enrichment time of thallium (20~30 years), the ecological effects and toxic effects caused by environmental degradation of thallium have obvious hysteresis, so the pollution prevention and control should be paid attention to. During the combustion process of thallium in coal, 10%~40% is enriched in fly ash and its main form is TI+, TI+ can be dissolved in acidic water or water containing SO42-, AsO42- and Cl- ions. Therefore, the TI in the waste recycling process mainly enters the dust collecting and washing circulating water of the ash cleaning process.
3 the occurrence of heavy metals in the process of recovery and utilization of waste SCR catalyst
3.1 the occurrence of heavy metal in the process of recovery
The contents of heavy metals in various kinds of products in the recovery process of the typical SCR waste catalyst were illustrated in the recovery process, and the data were shown in Table 3. From table 3 data shows, the heavy metals are not in tungstate products deposition; and vanadium products in the entrainment of heavy metals although many kinds, but were not very high, if the further processing can also enhance the quality of; rich titanium material in the heavy metal type and content completely does not affect the as the subsequent processing of titanium material. Dust accumulation, arsenic and phosphorus containing precipitated slag and two toxic precipitation as the main heavy metal enrichment carrier, and therefore must be sent to the effective disposal of hazardous waste landfill. The content of
all kinds of heavy metals in the process of recycling technology is very low, which can meet the requirements of process water use, and will not have a negative impact on the quality control of the production process.
Treatment process of heavy metal pollutants in the production of 4 SCR waste catalyst
4.1 treatment of various kinds of heavy metal pollutants
Treatment technology of arsenic compounds in 4.1.1
The conventional treatment methods for arsenic compounds include the following:
(1) chemical precipitation method
Arsenic can form the insoluble compounds with many metal ions, such as arsenate or arsenite ion and calcium, ferric iron, ferric aluminum plasma can form insoluble salts, after filtration can remove arsenic liquid phase. Because the solubility of arsenite generally than arsenate is much higher, is not conducive to the conduct of precipitation reaction, so in many practical design need the oxidation of trivalent arsenic pentavalent. The most commonly used oxidant is chlorine is also available with activated carbon as catalyst by air oxidation. Many kinds of precipitant, the most commonly used is the calcium salt, iron salt, magnesium salt, aluminum salt, sulfide etc..
(2) co precipitation method
Arsenic in industrial waste water can be removed by co precipitation of heavy metals. Co precipitation has two kinds of functions, one is the soluble ion is a large amount of precipitation of solid adsorption, two is a large number of particles are precipitated by solid condensation or net capture. Co precipitation can reduce arsenic by about 90%. Can be used for co precipitation of ferric chloride, calcium hydroxide, sodium sulfide including material and aluminum.
(3) biochemical method
In recent years, biochemical process for treatment of arsenic wastewater research progress has been made. The experimental results show that activated sludge process of arsenic (V) removal very quickly. Within 0.5 hours approximately 80% removal, arsenic and sludge contact in a short time can be substantial removal of, in the 1 ~2H gradually reached the equilibrium state, after the removal amount increases less. The reason is that the activated sludge on the metal adsorption to surface adsorption and by highly specific trace delivery system into the cell absorption. Extracellular polymer, cell wall ion gene (phosphate and hydroxyl group) on metaladsorption is mainly adsorbed on the surface, its characteristics is is rapid and reversible. And energy metabolism, intracellular uptake by ion and cell surface
membrane enzyme, hydrolase combining reality, so the speed is slow.
(4) adsorption method
Can be used for removal of arsenic adsorbent has a lot of, such as activated carbon, zeolite, sulphonated coal, alumina production waste red mud. Zeolite is rich in domestic resources, the zeolite used as the adsorbent of the zeolite should be treated by alkali, so that it can greatly improve the adsorption of arsenic. Use the product of calcium silicate and calcium bentonite of calcium hydroxide reacts with bentonite, low price, simple processing technology, the arsenic removal rate can reach 99.9%. Red mud is the waste of alumina production, the group is iron, aluminium, titanium and other elements, by hydroxide prepared from sulfuric acid or hydrochloric acid treatment, the frozen made particle size of adsorbent 1~5mm, for adsorption of arsenic.
(5) ion exchange method
Ion exchange method is suitable for the treatment of waste water with low concentration, low content of arsenic ion, simple composition and high recovery value. The type of resin is better than OH, and the pH should be adjusted to about 7 before the wastewater treatment. The iron and molybdenum type cation resin can also remove the arsenic ion in wastewater.
(6) ion flotation
Surfactant in liquid junction has the ability of adsorption of arsenic and the nature of the removal of arsenic in the water called ion flotation. Containing arsenic wastewater is added to and opposite charge collector and generate water soluble complex, or insoluble precipitate, which is attached to the bubbles and float to the surface as the dross recovery. British by flocculant foam flotation method, selection of ferric hydroxide flocculant, using sodium dodecyl sulfonic acid as collector, arsenic is removed to below 0.5mg/l.
Treatment technology of 4.1.2 mercury compounds
The common treatment technology of inorganic mercury in water is the following:
(1) precipitation method
With mercury in wastewater by adding sodium sulfide treatment, due to the strong affinity of Hg2 + and S2 -, can generate solubility minima of HgS and from the solution to remove, so the sulfide precipitation is the most frequently reported a precipitation process. The precipitation method can be combined with flocculation, gravity sedimentation, filtration or dissolved air flotation and other separation processes. The subsequent operation can increase the removal effect of mercuric sulfide precipitation, but did not improve the deposition efficiency of dissolved mercury itself.
(2) ion exchange method
Shuji macroporous ion exchange agent of mercury containing wastewater treatment has a good effect. The hydrophobic group on the resin has strong adsorption ability to mercury ion, and mercury can be adsorbed on the resin, and can be used for the elution of concentrated hydrochloric acid, and the quantity of the mercury ion can be recovered quantitatively. Mercury containing wastewater treatment in the back of the water content can be reduced to 0.05mg/L. In addition, the use of selective adsorption of mercury chelating resin treatment of mercury containing wastewater is also being applied, and has achieved some results. In the majority of inorganic mercury ion exchange processing, first of all need to add chlorine and chlorate (oxidation of metallic mercury) or chloride, to form a band of negative charge of
mercury chloride complexes, followed by anion exchange resin removal. The ion exchange method is mainly used in chlor alkali plant wastewater treatment with high chloride content.
(3) coagulation method
The coagulation of wastewater for mercury removal treatment, including aluminum sulfate coagulant used (Ming Fan), salt and lime. The processing method of inorganic and organic mercury has achieved certain success. In coagulation method in the study of mercury, the first in domestic sewage added 50~60ug/L inorganic mercury and then with ferric chloride or alum coagulation and filtration, two methods can make waste water containing mercury reduces the amount of 94% to 98%. With the high concentration of mercury in the treatment of 500ug/L, the removal rate of mercury is 70%.
(4) reduction method
The inorganic mercury ion can be transformed into metal mercury by reduction, and then the solid is separated by filtration or other techniques. Reduction agent has many kinds, including iron, bismuth, tin, magnesium, copper, manganese, aluminum, lead, zinc, hydrazine, stannous chloride and sodium borohydride as.
4.1.3 Treatment technology of lead compounds
For the soluble lead in wastewater, it is generally first to lead to the formation of lead deposits and then removed. The precipitant used lime, caustic soda, soda and phosphate etc.. They react with the lead ion to form Pb (OH) 2, PbCO3 or Pb3 (PO4) 2 precipitate. In addition, alum, ferrous sulfate and ferric sulfate, coagulation,adsorption and ion exchange methods have also been used in the treatment of lead in wastewater. In the process of treating lead containing wastewater by precipitation process, the sediment is usually PbCO3 or Pb (OH) 2. The shape of the lead deposit depends on the amount of carbonate (or added) in the wastewater, as well as the pH
value that is controlled by the process. However, the carbonate content in the original acidic waste water is lower, so the sediment is usually Pb (OH) 2 unless the carbonate is added in the treatment of these waste water. Since the PbCO3 ratio of Pb (OH) 2 has a better crystal structure, and pH was neutral, the solubility of PbCO3 was lower than Pb (OH) 2. So it has better sedimentation and dewatering performance. In the treatment of lead carbonate precipitation, the optimal amount of carbonate dosage (equivalent to CaCO3) is 200mg/L, the best pH value is 7.5~9.0. When carbonate dosage or pH value is over 9, the effect of precipitation is decreased.
Treatment technology of 4.1.4 chromium compound
By adding lime or caustic soda to form chromium hydroxide precipitation in the form of, or concentration and recovery by ion exchange, trivalent chromium can be removed. Trivalent chromium can react with caustic soda or lime to form insoluble Cr (OH) 3 precipitate removed. Due to the effect of pH on the solubility of Cr (OH)
3, the effect of pH was best when was 8.5~9.5.
Treatment technology of 4.1.5 beryllium compounds
Beryllium has been used in industry for more than 50 years, with the development of the aerospace industry and the atomic energy industry, the use of beryllium has become more and more extensive. Beryllium and its compounds are very toxic to humans, especially in animal experiments, beryllium has become one of the environmental pollutants. According to the information reported, beryllium chloride and beryllium sulfate in the water is more stable, its initial concentration after 5 days of time can only be reduced by 30%~35%. Add water to be the compound to be 10 days after the precipitation, but in the alkaline environment can accelerate the precipitation, and in 5 days can be all precipitation. In our country, the maximum allowable discharge concentration of beryllium is 0.005mg/L.
Treatment technology of 4.1.6 thallium compounds
Thallium (TL) is a typical dispersed elements, is widely used in such as defense, aerospace, electronics, communications, health, an important field, has become a high
technology support material and an important part of, the demand is also increasing. However, due to the toxicity of thallium in mammals is far greater than Hg, Pb, As and so on, the human lethal dose is only 10~15mg/kg, so people pay more attention to thallium pollution. Thallium in nature in most cases is a monovalent, few cases of trivalent. TI+ occupies almost all of the Eh-pH space in the water, only in the presence of very strong oxidation conditions TI3+. At present, the research on thallium treatment mainly focuses on water and soil. For thallium containing water, the main treatment measures are: 1) the use of thallium is easy to be "sponge adsorption" adsorption nature, in the polluted water to join MnO (solid) and other adsorbents, reduce the rate of thallium activity and make it. 2) low temperature, oxidation and
alkaline conditions, thallium from monovalent to trivalent transformation, so it can be in water pollution with the addition of oxidizing agent and an alkaline substance (such as lime, etc.), and pay attention to the control of temperature and decreasing of thallium activity.
Treatment process of heavy metal pollutants in the recovery of 4.2 SCR waste catalyst
According to the occurrence of heavy metals in the waste catalyst, three processes were used to collect and process the SCR waste catalyst.
(1) blowing and washing ash removal process through the treatment process, can remove the waste catalyst entrained more than 90% of the Coal-fired Fly Ash, prevent heavy metals through the fly ash into the subsequent processing process.
(2) and alkaline leaching solution purification process with MgCl2 solution on alkaline leaching solution of impurity removal and purification, removal of the leaching solution in 99% of silicon compounds, 98% of the arsenic compounds, 99% of the phosphorus compounds, 85% of the mercury compounds, 60% of thallium and its compounds,further reduce these harmful impurities to recover the purity of the product influence.
(3) process wastewater treatment process
This process is the final disposal of heavy metal in the whole process. The process of chemical removal of wastewater is complex, not only the precipitation of heavy metals, but also the precipitation of other ions, and the presence of a variety of metal ions in the co precipitation phenomenon. The reaction principle is：
FeCl3 + 3NaOH = Fe(OH)3↓+ 3NaCl
CaCl2 + Na2CO3 = CaCO3↓+ 2NaCl
MgCl2 + 2NaOH = Mg(OH)2↓+ 2NaCl
Ti4+ + 4OH- = Ti(OH)4↓
As3+ + 5OH- =As(OH)5↓
Al3+ + 3OH- = Al(OH)3↓
Pb2+ + 2OH- = Pb(OH)2↓
Hg2+ + S2- = HgS↓
Cr3+ + 3OH- = Cr(OH)3↓
Process steps are:
1) the process waste water is heated to 50 degrees C ~60 C, adding a concentration of 20% Na2S solution, adding the amount of Hg in the process of the total amount of
the theoretical reaction, the reaction after 120min filter. Filter slag for the two toxic precipitation.
2) the filtrate is heated to 50 degrees C ~60 C, adding NaOH 400g/L solution under the stirring, the solution of the pH to adjust to 8~9, after the reaction of 240Min filter. Filter slag for the two toxic precipitation.
3) the filtrate is heated to 50 degrees C ~60 C, stirring to join the Na2CO3 305g/L solution, the amount of Na2CO3 in solution with the concentration of 0.25~0.6g/L as the standard, the reaction after 120min filter. Filter residue for industrial CaCO3 precipitation.
4) after removing the impurity from the filter, the filtrate is filtered through the reverse osmosis unit, and the water is returned to the main flow. Two times of salt concentration wastewater using multi effect evaporation crystallization production industry NaCl crystal.
5 Conclusion
At present, our country has put the waste flue gas denitrification catalyst into the scope of hazardous waste management, recycling of resources must be strictly carried out two times of pollution control, especially for heavy metal pollution control. In this paper, from the perspective of engineering practice, the paper lists the heavy metal pollution treatment technology of SCR waste catalyst in the recycling process, which is used for reference in the industry.