Integration of treatment and utilization of chromium slag
Chromium means of chromium iron ore filler, soda ash and calcium chromium by mixing manufacturing process, high temperature calcination, obtained after taking sodium chromate manufactured by gray-green residue washed with water, a strong alkaline substance. Due to the difference in raw materials and formulations used. The amount of chromium slag discharged per ton of sodium sulphate produced is not the same, about 2.0-3.0 tons. Depending on the raw materials used and the formulation, the elemental composition of the chromium slag discharged during the production process is also different.
Cr 6+ in slag has strong oxidizing property; water-soluble Cr 6+ has greater environmental pollution and harm. The harmless treatment of chromium slag is considered to be the bottleneck problem of the healthy development of China's chromium salt industry, and it is also a worldwide problem. . Since chromium slag contains elements such as calcium, magnesium , iron, aluminum , silicon, and chromium, this provides a possibility for the treatment and recycling of chromium slag. Shijiazhuang Yafu Chemical Co., Ltd. and Jinan Yuxing Chemical Plant are cooperative units. The company's technical team has found three effective ways to treat and utilize chromium slag.
First, cement curing method
(1) FeSO 4 reduces Cr 6+ in chromium slag
The chromium slag contains a large amount of alkaline substances such as periclase, calcium chromoaluminate, and basic chromium chromate, all of which are soluble in acid. If the chromium residue is under acidic conditions, these substances must be dissolved. As a result, the remaining amount of chromium slag does not reach the purpose of utilization. Therefore, we must reduce Cr 6+ in slag under alkaline or neutral conditions, and FeSO 4 can be used as a reducing agent to achieve this purpose. The main reaction formula is as follows:
FeSO 4 →Fe 2+ +SO 4 2 -
Alkaline conditions:
Fe 2 + +2OH - =Fe(OH) 2
CrO 4 2 - +3Fe(OH) 2 +4H 2 0=Cr(0H) 3 +3Fe(OH) 3 +2OH -
Neutral conditions:
CrO 4 2 - +3Fe 2 + +8H 2 O=Cr(OH) 3 +3Fe(OH) 3 +4H +
Thus, as long as FeSO 4 is mixed with the chromium slag in an aqueous solution, the reaction can be carried out regardless of the conditions, and the treatment process is greatly simplified. Cr 3 + is very toxic and is a trace element necessary for human body and organisms. Therefore, the treatment of chromium slag is to convert hexavalent chromium ions into trivalent chromium ions, which is the harmless treatment of chromium slag. FeSO 4 and the reducing agent is readily available and inexpensive, are we using FeSO 4 Yuxing plant byproduct of titanium white powder, water and sulfuric acid-rich, FeSO 4 · 7H 2 O content of more than 98.5%, and containing small amounts of spent acid titanium.
(2) Curing treatment of cement
60% of the chromium slag element composition is CaO, SiO 2 , Al 2 O 3 and Fe 2 O 3 , which are also the basic components of cement; they exist in the form of dicalcium silicate and calcium aluminosilicate in chromium slag. It is two of the four cementitious active compounds of cement. If there is no hexavalent chromium and periclase (free magnesium oxide, which accounts for about 20% of the chromium slag), the chromium slag can be directly used as a low-grade cement. Therefore, the removal of Cr 6+ and magnesium oxide is the key to solidify chromium slag with cement. In addition to the main reduction, FeSO 4 is also an accelerator of sulphate, which can stimulate the activity of cement. In addition, it can promote the transformation of magnesium oxide. To prevent its cracking and increase the stability of cement.
When chromium slag and FeSO 4 are in contact with water, they react with Cr 6+ in the chromium slag to remove chromium damage. The water soluble Cr 6+ can be reduced from 1000-2000ppm to less than 5ppm. It is mixed with cement to reduce the amount of chromium residue. The soluble hexavalent chromium can be sealed in the cement stone gel hard body along with the hydration and condensation hardening process of the cement. Even if a small amount of water-soluble hexavalent chromium is dissolved in the initial stage, the hardening and strength of the cement stone The growth of hexavalent chromium will decrease. Until this part of the hexavalent chromium is completely enclosed in the concrete and no longer dissolves. The performance is stable and the detoxification is thorough. After repeated measurements, the Cr 6+ concentration of the cement products is far below 5 ppm.
(3) Application examples
Chromium slag (Jinan Yuxing Chemical Plant) 100Kg, FeSO4·7H2O (Jinan Yuxing Titanium Dioxide Factory) 15Kg, water amount added to the mixer for 6 min, then add 425# Portland cement 25Kg for 3 min, used in our company In the civil construction, the concrete road is about 1 km and the ground is more than 600 square meters. The reduced chromium slag concrete is mainly used in the concrete cushion of the road and the ground, and then a 15-20 mm thick cement mortar surface is placed on the concrete cushion. The layer is such that the purpose of completely curing the residual water-soluble hexavalent chromium in the reduced chromium slag can be achieved.
Second, chromium slag desulfurization agent for coal-fired
Our company is located in Wuji County in the middle of the North China Plain. Wuji County is a traditional agricultural county. Most of the rural residents use honeycomb coal for cooking and heating. After the coal is burned, a large amount of SO2 and NOx gas will be generated, causing serious air pollution.
Due to different origins, raw coal contains different amounts of organic sulfur, inorganic sulfur, carbon and organic matter, etc. The chromium slag contains elements such as Na, Ca, Mg, Cr, Fe and Al, which not only have oxidative but also catalytic combustion effects. The redox characteristics of the two can be used as a sulfur-fixing agent for raw coal under certain combustion conditions, and the raw coal is used as a detoxifying reducing agent for chromium slag. This solves the problem of pollution control of chromium slag and the cost of sulfur fixation of coal burning, and achieves integration of treatment and utilization.
Since the hexavalent chromium in the slag is mainly in the form of sodium chromate tetrahydrate and calcium chromate, the main reaction formula is as follows:
2Cr 6+ + 3S 2- + 3OH - = 3S+Cr(OH) 3
2C+O 2 =2CO (1)
2Na 2 CrO4·4H 2 O+3CO=Cr 2 O 3 +2Na 2 O+3CO 2 ↑+4H 2 O↑ (2)
2CaCrO 4 +3CO=Cr 2 O 3 +2CaO+3CO 2 ↑ (3)
Detoxified coal chromium slag, its hexavalent chromium content can reach below 8PPm, in line with the second-level standard specified in the chromium salt industrial pollutant standard GB4280-84, and the stability is good, long-term open storage of hexavalent chromium is not obvious "Recovery" phenomenon.
Application examples
Raw coal: Chromium slag = 90:10 The above materials are pulverized to <3mm, and an appropriate amount of water is aged for two days, and some of the sulfur in the coal is reacted with Cr6+ in the chromium slag to be solidified, then 10% clay and appropriate amount of water are added, and stirred. Uniform, extruded by a honeycomb machine. The sulfur fixation rate is up to 68%, and the slag can be used as a concrete cushion material after being crushed.
Third, chromium slag as biogas desulfurizer
Biogas energy is widely distributed in rural areas of our county. Biogas is produced on the spot using manure, orange poles, weeds, waste residue and waste. Biogas contains a certain amount of hydrogen sulfide, and sometimes contains a small amount of organic sulfur, which is a highly toxic and harmful substance. Inhalation of 0.1% hydrogen sulfide in the air can be fatal. It has a strong corrosive effect on gas pipelines, instrumentation, and combustion equipment. Its combustion product, sulfur dioxide, is also a highly corrosive gas, and it can produce "acid rain" when it enters the atmosphere. In order to ensure human health and protect the atmosphere, and prolong the service life of gas equipment, desulfurization must be carried out.
Removal of hydrogen sulfide gas more, wherein the iron oxide is a classic method and effective desulfurization methods, the advantage of simple process, easy to operate, low energy consumption, still widely used. The chromium slag is shown in Table 1. The Fe 2 O 3 content is 10.6%. Our company uses FeSO 4 ·7H 2 O to reduce Cr 6+ in the chromium slag, so that the chromium slag is treated with Fe 2 O 3 in the chromium slag. Detoxification, even if residual Cr 6+ is re-reduced by H2S in the biogas, the detoxification is thorough, and the desulfurization process is carried out in an alkaline liquid membrane.
Desulfurization principle of iron oxide desulfurizer
When the gas containing H 2 S passes through the desulfurizing agent, first the H 2 S molecules diffuse to the surface of the particles and then dissociate in the water film:
H 2 S →H + +HS -
HS - →H + + S 2-
The dissociated HS - , S 2- replaces - OH and =O in O = Fe - OH to form S = Fe - SH, which is the hydrate of Fe2S3 and FeS.
FeSO 4 → Fe 2+ +S0 4 2-
Cr 2 0 7 2- + 3Fe 2 + +14H + = Cr 3+ + 3Fe 3+ +7H 2 O
Fe 2 O 3 ·H 2 O +3H 2 S = Fe 2 S 3 ·H 2 O +3H 2 O
Fe 2 O 3 ·HO +3H 2 S = 2FeS+S+4H 2 O
2Cr 6+ + 3S 2- + 3OH - = 3S+Cr(OH) 3
Production example:
FeSO 4 ·7H 2 O 100Kg, chromium slag 25Kg, 10kg of wood chips and water added to the mixer for 6 min, add 25kg of slaked lime and mix well, mix for 10 min, extrude into strip solids by molding machine, dry and activate into yellow strips. Product.
summary:
1. Cement solidification is a method for solidifying reduced chromium residue based on hydration and hydraulic gelation of cement. It combines reduced chromium residue with ordinary cement to form a solidified body with a certain strength to achieve reduced chromium. The purpose of leaching the hazardous component of residual Cr 6+ in the slag. This method has a large amount of chromium slag and is one of the best applications. It can save about 20 yuan per cubic meter for civil construction.
2. The use of chromium slag as a sulfur-fixing agent solves the problem of pollution control of chromium slag and the cost of sulfur fixation of coal burning, and has certain environmental and economic benefits.
3. The preparation of desulfurizer by using chromium slag is a good comprehensive utilization method for waste treatment and harming, and has good desulfurization effect on biogas. After desulfurization, the biogas H2S content is from 3000-5000 mg/ m 3 is reduced to below 20 mg/m 3 , in line with national emission standards, and detoxification of chromium slag is the most thorough. This work has good environmental and economic benefits.
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