Chromium has a higher positive charge and a smaller radius of 0.052 nm than the same period of titanium and vanadium, so there is no simple Cr+6 ion in the crystal or solution, always in the form of oxides, oxyacids, etc. presence. CrO3 is an anhydride of chromic acid called chromic anhydride and has a melting point of less than 2O0 °C. Hexavalent chromium is an oxidant that pollutes the environment and is harmful to humans.
Alkaline media (such as in the presence of potassium, sodium, calcium), higher oxygen pressure, and suitable temperatures are important conditions for the production of hexavalent chromium.
Six-valent chromium pollution, as an internationally recognized environmental issue, is receiving more and more attention from all countries. The United States first established a hexavalent chromium emission limit standard. The Japanese Tap Water Act stipulates that the hexavalent chromium content in drinking water is below 0.05 mg/L. The industrial waste treatment law stipulates: (1) inorganic pollution test liquid produced by industrial waste or factory, hexavalent chromium below 0.5mg/L; (2) pollution test liquid produced by industrial waste or factory, 6 price The chromium is below 1.5 mg/L. China's environmental protection standard GBl8918—2O02 stipulates: the maximum emission concentration (daily average) of a class of pollutants is less than 0.1mg/L, and the hexavalent chromium is less than 0.05mg/L; the standard of pollutant control in sludge is the highest, and the total chromium content is the highest. Allowable values: 600 mg/kg dry sludge in acidic soil (pH ≤ 6.5) and 1000 mg/kg dry sludge in alkaline soil (pH ≥ 6.5).
The calcination test for adjusting the oxygen pressure of the magnesia chrome brick with a Cr2O3 content of 12% (adjusting the ratio of fuel to combustion air) has been shown. When the air excess coefficient is too large, the hexavalent chromium content is also high.
In a neutral atmosphere, the CaO-Cr2O3 system has a high melting point, and the formed trivalent chromium salt compound CaO·Cr2O3 (having two variants of α and β) has a melting point of 2170 ° C and no hexavalent chromium compound is formed. In the air, three kinds of hexavalent chromium compounds are formed in order from 1228 ° C to 808 ° C: 9CaO · Cr 2 O 3 · 4Cr 2 O 3 , 3 CaO - 2 Cr 2 O 3 · 2Cr 2 O 3 , Ca O · 0 O 3 . Oxygen pressure plays an important role in the production of hexavalent chromium from CaO-Cr2O3 materials. In alkaline medium, the most easily formed hexavalent chromium salt is K2Cr2O7 (potassium dichromate, commonly known as red strontium potassium), Na2Cr2O7 (sodium dichromate, commonly known as red strontium sodium), both of which are strong oxidants, corrosive and toxicity.
Raw materials for the manufacture of magnesia chrome bricks: Magnesia or chrome ore does not contain hexavalent chromium. However, the authors have chrome green (Cr2O3) and fused magnesia chrome sand with hexavalent chromium. Therefore, it is necessary to pay attention to the protection when crushing and grinding fused magnesia chrome sand and adding chrome green ingredients, mixing and forming.
When a mixed magnesia chrome brick slurry is used, a binder containing an alkali ion [Na+, K+], such as an alkaline pulp waste liquid, water glass, sodium phosphate or the like, is used, and a hexavalent chromium salt is formed in the fired product. The test shows that the pulp waste liquid with different Na2O content is used as a binder to make magnesia chrome bricks, and the detection of hexavalent chromium is found. When Na2O is high, the hexavalent chromium is also high.
The calcination test for adjusting the oxygen pressure of the magnesia chrome brick with a Cr2O3 content of 12% (adjusting the ratio of fuel to combustion air) has been shown. When the air excess coefficient is too large, the hexavalent chromium content is also high.
According to the data, the limit concentration of trivalent chromium can reach 0.5mg/m3 during the handling, masonry and cutting of magnesia chrome bricks. Long-term exposure can cause lung disease; the limit value of hexavalent chromium can reach 0.02-0.05mg/ M3, long-term exposure can cause kidney disease and cancer. Operators should pay attention to protection.
Under the reducing atmosphere at the time of steelmaking, the oxygen pressure is low, and the content of hexavalent chromium in the magnesia chrome brick is not increased. In the case of a large amount of heat medium (calcium, sodium, potassium), hexavalent chromium rises sharply. For example, magnesia chrome bricks used in cement rotary kiln, cement clinker react with magnesia-chrome bricks to form hexavalent chromium salt 3CaO·3A12O3·CaCaO4. In the environment where there is an alkaline heat medium such as a lime kiln or a calcined dolomite rotary kiln, a hexavalent chromium salt is produced after the magnesia chrome brick is used. These hexavalent chromium salts are dissolved in water, causing water pollution.
How to Solve Environmental Pollution Caused by Magnesia Chrome Bricks
The development and application of chrome free bricks is the most fundamental and permanent solution to the hexavalent chrome pollution. Magnesie alumina spinel bricks, magnesia calcium bricks, and magnesium zircon bricks are all very promising alkaline varieties.
The development and application of chrome free bricks has achieved remarkable results in the cement industry, which is the most hazardous area of ?hexavalent chromium pollution. Magnesia spinel bricks and MgO-CaO products have the largest market share and heat storage in glass kiln. There have also been significant progress in rooms and lime kiln. The difficulties encountered in the metallurgical industry are relatively large. In the secondary refining furnaces with high temperature, high thermal shock and severe slag erosion, especially in heavy ferrous metallurgical furnaces, the refractory materials are severely eroded by forsterite slag and developed effectively. The chrome free bricks have a long way to go. Therefore, while actively promoting non chromium, it is still an important issue to reduce and prevent the hazards in the production and use of magnesia chrome bricks.
