Chromite is also called chrome ore. Chromite is produced in black lumps and granules, and sometimes small octahedral crystals can be seen. Chromite has a semi-metallic luster, a Mohs hardness of 5.5, and is brittle. The density depends on the composition of the spinel and the nature of impurities, generally between 4.2 and 4.8. The iron ore is non-magnetic or weakly magnetic, and the one with higher iron content is more magnetic. The Mohs hardness is 5.5-6, and the refractoriness is greater than 1900℃. The composition of natural chromite ore in nature is complex and varies greatly. The chromite ore often mentioned in the industry is actually a mixed crystal composed of six kinds of spinels. The general chemical formula can be expressed as (Mg, Fe) O. (Cr, Al, Fe) 2O3. In industry, similar minerals such as chromite, chromium spinel, chromium-rich spinel and hard chromium spinel are commonly referred to as chromite.
Chromite sand has a good resistance to alkaline slag and does not react chemically with iron oxide. In the refractory industry, it is mainly used to produce magnesia-chromium raw materials and refractory bricks. The thermal conductivity of chromite sand is several times greater than that of silica sand, and the chromite sand itself undergoes solid-phase sintering during molten metal pouring, which is beneficial to prevent the penetration of molten metal. In the foundry industry, chromite sand is mainly used as the mold, core surface sand and anti-sticking sand coating and paste for large steel castings and various alloy steel castings. It has excellent corrosion resistance. Chromite sand is also the only resource for the preparation of chromium and chromium-containing compounds. my country’s chromite resources are poor and the ore grade is low, mainly distributed in North China, Northwest China and Tibet. In the foundry industry, currently only Shangnan Molding Material Factory produces chromite sand, with an annual output of only a few hundred tons. Although the chromite ore in Tibet has been mined, it is difficult to transport. In order to meet the needs of domestic foundry production, the country has been importing chromite ore from South Africa for the past 10 years.
Heating changes of chromite
(1) Oxidation weight gain: As mentioned above, FeO is a common state of iron in chromite. When chromite is heated in the air, FeO will be oxidized to Fe2O3 from about 200 degrees. , This oxidation can be completed by keeping it at 1000 degrees for 1 hour. The reaction formula is: 2(FeO·Cr2O3)+1/2O2——Fe2O3+2Cr2O3 Oxidation of chromite increases its weight.
(2) Heating expansion: The characteristic of chromite expansion increases significantly above 850 degrees Celsius, but shrinks again at high temperatures. The chromite ore fired at 1400 degrees Celsius and 1800 degrees Celsius from different origins showed a sharp shrinkage from 1200 degrees Celsius and 1500 degrees Celsius respectively. This phenomenon is very harmful to chromium refractories. Studies have shown that adding MgO to chromite can eliminate this shrinkage phenomenon, because chromite can absorb MgO in a solid solution and cause lattice expansion.