Chromite

[gè tiě kuàng]

Magmatic minerals

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Chromite massive isometric (massive, equiaxed system) is a magmatic mineral, often occurring in ultrabasic rocks and coexisting with olivine; It is also found in placers. It looks like magnetite in appearance, generally in massive or granular aggregate. In the metallurgical industry, chromite is mainly used to produce ferrochrome and chromium metal. As the additive of steel, ferrochrome produces a variety of special steels with high strength, corrosion resistance, wear resistance, high temperature resistance and oxidation resistance. Chromium metal is mainly used to smelt special alloys with cobalt, nickel, tungsten and other elements. Chromite is a mineral in short supply, with small reserves and low output. In industry, ferrochrome, chrome spinel, chrome rich spinel, hard chrome spinel and other similar minerals are often collectively referred to as chromite.^[1]

Chinese name: Chromite
Foreign name: chromite
Hardness: 5.5-6.5
Specific gravity: 4.3-4.8g/cm3

Cleavage: Non cleavage
Belonging discipline: Mineral industry
See publications: Chemical Terms Science Press
Time of publication: 1991^[4]

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Mineral information

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brief introduction

Chromite It is a mineral, mainly composed of iron , magnesium and chromium Oxides: (Fe, Mg) Cr2O4 spinel One of. It is the only exploitable chromium ore with complex mineral composition, variable magnesium content and sometimes aluminum And iron. There are about 30 kinds of chromium minerals in nature, but only chromite is of industrial value. Chromite, aluminochromite and chromium rich spinel are common in China.

Chromite refractory, used as refractory Material, also used for making Chromium trioxide 、 Sodium dichromate 、 potassium dichromate Chromium compounds.

Ore composition

Chromite (Figure 2)

Chromite is an oxide mineral of chromium and iron. There are high carbon ferrochrome (4-8% carbon), medium carbon ferrochrome (0.5-4% carbon), low carbon ferrochrome (0.15-0.50% carbon), micro carbon ferrochrome (0.06% carbon), micro carbon ferrochrome (less than 0.03% carbon), metal chromium, silicon Chromium alloy 。 It is quite hard, with black and semi metallic luster. Chromite is the main source of metal chromium and can also be used as high-temperature refractory. Chromite is generally a massive or granular aggregate. The chemical composition of chromite is FeCr2O4, crystal genus Isometric Oxide minerals. Iron in the composition can often be partially replaced by magnesium. When Mg is the main component, it is called magnesiochromite. Normal spinel Type structure. Chromite Cr2O3 content is 67.91%. It is the main source of industrial chromium and can also be used to make high-temperature refractory materials, such as chrome bricks. Mohs Hardness 5.5～6， proportion 3.9～4.8。 It has weak magnetism. Chromite is only produced in ultrabasic or Basic rock Medium. large Chromite deposit Mainly produced in South Africa Transvaal zimbabwe Near Guilo, Russia Vader Lovesk region, Cuba and other places. The origin of chromite in China includes Tibet, Gansu and Shaanxi.

Mineral classification

Chromite (Figure 5)

Chromite is generally divided into three categories according to the use requirements in the market:

(1) High chromium chromite (for metallurgy), containing more than 48% of Cr2O3, with a large ratio of Cr2O3/FeO.

(2) High iron chromite (for chemical industry), containing more than 50% Cr2O3 and less SiO2 and Al2O3.

(3) High alumina chromite (used for refractory materials), containing 30%~35% Cr2O3 and more Al2O3 and MgO. The mineral composition of chrome ore used for refractories contains MgO% 26bull; Cr2O3, MgO% 26bull; Al2O3, containing FeO% 26bull; Cr2O3 and FeO% 26bull; Fe2O3 less is better. diabase Chromite for casting stone contains 10%~20% Cr2O3, 26 lt SiO2%; 10%.

purpose

In metallurgy, glass cement Magnesium chrome brick and chrome magnesia brick are used in industry. Because on Cement rotary kiln The use of chromium containing products on the surface will produce hexavalent chromium compounds. For environmental protection reasons, magnesium aluminum spinel products or dolomite Product substitution. In addition, it is used for refining metal chromium, producing ferrochrome, stainless steel, and manufacturing heavy metals chromate Etc.

Chrome ore is an indispensable mineral resource for the development of metallurgy, national defense, chemical industry and other industries. It is mainly used for the production of stainless steel and various alloy steels and alloys in the metallurgical industry. It is also widely used in glass, ceramics, refractories and other aspects. include:

Chromite

Casting grade: used for large steel castings, casting molding sand, ladle drainage sand

Metallurgical grade: used to produce high carbon ferrochrome and low carbon ferrochrome

Fire resistance grade/chemical grade: for Production of magnesia chrome sand, magnesia chrome brick and other refractory materials

The grassland highland complex structure in South Africa is the area with the richest chromium ore reserves in the world, accounting for 76.1% of the known chromium reserves in the world. The west ore belt is centered on RUSTENBUG, while the east ore belt is centered on STEELPOORT and BUGERSFORT

Famous place of origin

Chromite is mainly produced in Brazil and Cuba, and its producing countries are mainly India, Iran, Pakistan, Oman, Zimbabwe, Turkey and South Africa, accounting for 80% of the world's production, of which South Africa The output of one country accounts for 40%. Chromite mainly exists in Sichuan, Tibet, Gansu, Qinghai and other provinces in China.

Structural characteristics

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chemical composition

(Mg, Fe) Cr2O4, with complex composition, has five basic components: Cr2O3, Al2O3, Fe2O3, FeO and MgO Isomorphism Replacement.

Chemical Formula

(Mg, Fe2+) (CR, Al, Fe3+) 2O4 or (Mg, Fe2+) O (Cr, Al, Fe3+) 2O3, the content of Cr2O3 is 18% ～ 62%.

Identification characteristics

Black, streaked dark brown, hard and produced in Ultrabasic rock Medium is the identification feature; Chromite, very similar in appearance magnetite The difference is that the magnetism is very weak, and the streaks are brown, which is different from the black of magnetite.

Genetic occurrence

Magmatism And is often found in ultrabasic rocks olivine Symbiosis; Also found in placer in

Crystal morphology

six Octahedron crystal

crystal structure

Crystal system and Space group ： Isometric ，Fd3m； Cell parameter : a0=8.325-8.344 angstrom; Powdered crystal Data: 2.52 (1) 1.46 (0.9) 1.6 (0.9)

physical property

Chromite (Figure 3)

fracture : stagger to flat

Color: black

Streaks : Dark brown

Transparency: Opacity

Gloss: Metallic luster

Others: Weak magnetism , with iron The higher the quantity, the stronger the magnetism

Optical property: plastid; n=2.08

Raw material characteristics

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Two main characteristics

Chromite (Figure 4)

Chromium has oxygen affinity and iron affinity, with strong oxygen affinity. Only when the fugacity of reduction and sulfur is high, it shows sulfur affinity. Chromium is trivalent under endogenous conditions. Cr3+and Al3+Fe3+ Ionic radius They are close to each other, so they can present extensive isomorphism. In addition, Mn, Mg, Ni, Co, Zn and other elements can be replaced in the same way as chromium silicate Minerals and Accessory mineral Chromium is widely distributed in. Under the condition of strong oxidation of supergene zone (alkaline medium), Cr3+is oxidized to chromium in the form of Cr6+ Acid radical ion , making inactive chromium ions Soluble Chromium anions of. When encountering highly polarized ions (such as Cu, Pb, etc.), it will form insoluble chromic acid minerals.

Discovered Types

In addition, there are a few hydroxide 、 Iodate Salt nitrides And sulfide. Chromium nitride and chromium sulfide minerals are only found in meteorites.

Chromium minerals with industrial value belong to chromium spinel minerals Chemical Formula It is (Mg, Fe2+) (Cr, Al, Fe3+) 2O4 or (Mg, Fe2+) O (Cr, Al, Fe3+) 2O3, whose Cr2O3 content is 18% ～ 62%.

Valuable mineral

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The content of Cr2O3 in chromium minerals with industrial value is generally more than 30%, and the common ones are:

Ore composition

The chemical composition is (Mg, Fe) Cr2O4, which is between ferrochromite (FeCr2O4, containing about 9% FeO32, and about 91% Cr2O367) and magnesite (MgCr2O4, containing MgO20.96%, and 79.04% Cr2O3). Usually, some people also call ferrochromite and magnesite chromite chromite chromite chromite chromite chromite. Chromite is an equiaxed crystal system, with small octahedron crystals, usually granular and dense massive aggregates, black color, brown streaks, semi metallic luster, hardness of 5.5, specific gravity of 4.2~4.8, and weak magnetism. Chromite is Magma Genetic minerals occur in ultrabasic rocks. When ore bearing rocks are weathered, chromite is often transferred into placer ores.

Chromium rich spar

Also known as chromium Iron spinel Or aluminochromite. The chemical composition is Fe (Cr, Al) 2O4, containing Cr2O332%~38%. Its shape physical property The origin, occurrence and use are the same as those of chromite.

Hard chrome spinel

The chemical composition is (Mg, Fe) (Cr, Al) 2O4, containing Cr2O3 32%~50%. Its shape, physical property, origin, occurrence and use are also the same as chromite.^[2]

geographical distribution

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The 56 ferrochrome mining areas identified in China are distributed in 13 provinces, cities and autonomous regions. Tibet has the largest reserve of 4.251 million tons, accounting for 39.4% of the total reserve in China. The second is Inner Mongolia, with reserve of 1.744 million tons, accounting for 16.5%; Xinjiang has a reserve of 1.652 million tons, accounting for 15.3%; Gansu has 1.496 million tons of reserves, accounting for 13.6%. The total reserve of the above four provinces (regions) is 9.143 million tons, accounting for 84.8% of the total reserve in China. The remaining 9 provinces (cities and autonomous regions) including Beijing, Qinghai, Hebei, Jilin, Hubei, Shaanxi, Shanxi, Sichuan, and Yunnan have total reserves of only 1.636 million tons, accounting for 15.2% of the total reserves in China. In terms of administrative regions, it mainly focuses on Southwest Region (4.263 million tons, accounting for 39.6%) Northwest Region (3.76 million tons, accounting for 34.4%), North China (2.749 million tons, accounting for 25.5%), while the Northeast and Central South administrative regions only account for 0.5%, East China At present, no chromite reserves have been identified.

Smelting process

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Ferrochrome is a ferroalloy composed of chromium and iron, which is an important alloy additive for steelmaking. Chromite for smelting ferrochrome is generally required to contain Cr2O340~50%, and the ratio of chromium to iron is greater than 2.8. The "charging grade ferrochrome" containing 50% chromium produced in large quantities uses ores containing Cr2O3 and low ratio of chromium to iron.

Ferrochrome is divided into carbon ferrochrome (including charging grade ferrochrome), medium carbon ferrochrome, low carbon ferrochrome, micro carbon ferrochrome, etc. according to different carbon content. Silicon chromium alloy and nitrided ferrochrome are also commonly used. Ferrochrome is mainly used as an alloy additive in steelmaking, which was added in the later stage of steelmaking refining in the past. Low and micro carbon ferrochrome must be used to smelt low carbon steel such as stainless steel, so refining ferrochrome production was once developed on a large scale. Due to the improvement of steelmaking process, when producing stainless steel and other steel grades with AOD method (see refining outside the furnace), carbon ferrochrome (mainly charging grade ferrochrome) is used for charging, so only low and low-carbon ferrochrome needs to be added in the later period to adjust the composition, so the focus of ferrochrome production is to refine carbon ferrochrome.

Chromite (Figure 6)

Carbon ferrochrome is smelted by reducing electric furnace with coke As reducing agent, silica or bauxite do flux 。 slag The composition is generally SiO227-33%, MgO30～34%，Al2O326～30%，Cr2O3<9.0%。 Due to the formation of Chromium carbide The product contains 4~9% carbon. The modern reduction furnace for smelting ferrochrome has a capacity of 10000~48000 KVA, and is generally closed and fixed, with smelting power consumption of 3000~4000 kW? Hour/ton. The electric furnace for smelting silicon chromium alloy is similar to the ferrochrome reduction electric furnace. There are two smelting methods: one-step method and two-step method. Chromite for one-step process Silica Coke smelting with flux. The two-step process uses carbon ferrochrome, silica and coke as raw materials for slag free smelting, and the smelting process is generally similar to ferrosilicon production. Smelting power consumption per ton is 3000~4000 kW? Around the clock.

Medium, low and micro carbon ferrochrome is generally made of silicochromium alloy, chromite and lime, refined and desilicated by 1500~6000 KVA electric furnace, and operated by high basicity slag (CaO/SiO2 is 1.6~1.8). Low and micro carbon ferrochrome is also produced in large scale by heat exchange method. During production, two electric furnaces are used, one for smelting silicon chromium alloy and the other for melting slag composed of chromite and lime. The refining reaction is carried out in two buckets in two stages: ① after the slag of the slag furnace is injected into the first bucket, the silicon chromium alloy that has been preliminarily desilicated in the other bucket is added. Because the excess amount of the slag oxidant is large and the desilication is sufficient, micro carbon ferrochromium with silicon content less than 0.8% and carbon content as low as 0.02% can be obtained. ② The molten slag (containing about 15% of Cr2O3) after reaction in the first bucket is moved to the second bucket, and the silicon chromium alloy (containing 45% of silicon) produced by the silicon chromium electric furnace is heated into the slag. After reaction, the silicon chromium alloy (containing about 25% of silicon) with initial desilication is obtained, which is added into the first bucket for further desilication. The slag containing Cr2O3 less than 2-3% can be discarded.

Refining medium and low carbon ferrochrome by oxygen blowing, using liquid carbon ferrochrome as raw material, adding a small amount of lime fluorite Slagging, adding silicon chromium alloy or ferrosilicon before tapping to recover chromium in slag. The converting of low-carbon ferrochrome is possible under certain vacuum degree.

Vacuum solid decarbonization Method refining, using ground high carbon ferrochrome as raw material, in which a part of ground high carbon ferrochrome is oxidized and roasted as oxidant, added with water glass or other adhesives, pressed into lumps, dried at low temperature, placed in a car bottom vacuum furnace vacuum degree 0.5～10 MmHg At 1300~1400 ℃, heating and reducing for 35~50 hours can obtain low-carbon ferrochromium with carbon content less than 0.03% or even less than 0.01%.

Chromium is fused into iron, steel and many superalloys through the intermediate medium: ferrochrome. The method is to use carbon and/or silicon to reduce ferrochrome ore by pyrometallurgy in a high-temperature electric arc furnace. Ferrochrome is essentially an alloy of iron and chromium, and a considerable amount of carbon and silicon are artificially added.

A Brief History of Mining

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Chromite (Figure 7)

Chromium was discovered by French chemist William Faullin in 1798. Chromite was first discovered in Russia in 1799 Ural In mountainous areas, the discovery and development of this mine became the main source of chromite supply in the 18th century, when chromium was mainly used in the chemical industry. After the discovery of chromite in Maryland in 1827 Virginia Chromite was also discovered one after another, making the United States one of the limited suppliers of chromite in the world at that time. In 1860, Turkey discovered a large deposit, which was supplied to the international market. Turkey was the main source of chromite supply until the discovery of chromite in India and Rhodesia in 1906. Up to now, chromite has been found in more than 40 countries and regions in the world, with a total reserve of 3.7 billion tons and an output of more than 10 million tons.

Although China found some clues of chromite in Jilin, Ningxia, Hebei and other places before 1949, it did not make in-depth investigation and research. There are only two mining spots in China, one is Kaishantun in Jilin, and the other is small in Ningxia Songshan The former has been plundered by the Japanese invaders. After the founding of New China, due to the needs of industrial development, the search and exploration of chromite began. In the early 1950s, the Ministry of Heavy Industry of Northeast China organized teams to go to Kaishantun and the Ministry of Geology organized teams to work in Xiaosongshan Mountain of Ningxia, Gaositai and Damiao of Hebei. In Beijing in the 1960s Miyun In Subei, Gansu Province, a general survey of chromite was carried out, and it was finally found that Miyun County Fangmayu chromite and Dawaoerji chromite in Subei. But our country Chromite resources The real breakthrough should be said to be after the discovery of chromite in Xinjiang and Tibet. Chromite work was carried out in Xinjiang in the late 1950s and 1958 Radioactivity measurement Found Salto Haichromite, which was used by gravity magnetic force And drilling methods to find whale chromite. From 1964 to 1966, the Ministry of Geology organized a war in Xinjiang. In 1970, the Whale Mine was completed and put into operation, which was the only chromite mine with regular well construction and development at that time. Tibet chromite was discovered in the late 1950s and early 1960s. After years of work, the largest chromite deposit, Luobusha chromite, has been discovered and Tibet has become a major producer of chromite.^[2]

Resource status

Chromite (Figure 8)

By the end of 1996, chromite had been proved in China mining area 56 places, with a total proven chromite ore reserve of 13.149 million tons, of which grade A+B+C accounts for 43%, 5.652 million tons. After deducting mining and losses over the years, the remaining chromite ore reserves are 10.779 million tons, of which grade A+B+C accounts for 34%, which is 3.684 million tons. The reserves of chromite increased rapidly in 1957 proven total reserves Only 181000 t, increased to 2233000 t in 1965, 11.907 million t in 1985, and 13.149 million t in 19996. The total amount of chromite resources is 44 million tons, and the resource potential is 31 million tons. In 1995, the world's chromite reserves were 3.7 billion tons, Reserve basis 7.4 billion tons, mainly concentrated in South Africa (3 billion tons of reserves, 5.5 billion tons of reserves base), Zimbabwe (140 million tons of reserves, 9.3 billion tons of reserves base) Kazakhstan (320 million tons of reserves, 320 million tons of reserve base), Russia (4 million tons of reserves, 460 million tons of reserve base), and other countries with more reserves include Finland, India, Brazil, Turkey, etc. (Table 3.4.2). If compared with the reserve base of these countries in terms of A+B+C reserves, China is far behind them.

1. Small scale and scattered distribution of deposits

At present, no large chromite deposit with a reserve of more than 5 million tons has been found, and only four medium-sized deposits with a reserve of more than 1 million tons have been found. They are Lobsa in Tibet, Daaoerji in Gansu, Sartuohai in Xinjiang, and Hegenshan (3756 mine) in Inner Mongolia. The rest are small deposits with reserves below 1 million tons. It is the Lobsa deposit with the largest reserves, with 3.96 million tons of reserves distributed in 7 ore groups and more than 100 ore deposit The largest ore body is only 325m long.

2. The distribution area is uneven, and the development and utilization conditions are poor

As mentioned above, 84.8% of the remaining reserves of chromite deposits are distributed in remote provinces (regions) such as Tibet, Xinjiang, Gansu and Inner Mongolia, with long transportation lines and inconvenient transportation.

3. The reserves of lean ore and rich ore account for half and half respectively

Among the existing reserves, Lean ore Reserves account for 46.3% (4.993 million tons) and rich ores account for 53.7% (5.786 million tons). Rich ores are mainly distributed in Tibet and Xinjiang, accounting for 73.5% and 13.8% of the total rich ores respectively. From the perspective of use, metallurgical reserves account for 37.4% of the total reserves, chemical reserves account for 38.4%, and fire-resistant reserves account for 24.2%.

4. Open pit mining is few, and the small and easy to mine chrome rich iron ore has been mined out

Only about 6% of chromite reserves are suitable for independent open mining, and most of them need pit mining. Some small, rich and easy to exploit chromite ores have been mined out. For example, Xinjiang's Whale and Tibet's Dongqiao chromite ores were closed in 1983 and 1982, respectively. The former mined 310000 tons of chromite ores and the latter 176300 tons of chromite ores.

5、 Genetic type of deposit single

The known chromite deposits are mainly late magmatic deposits. However, some famous large and super large magmatic early segregation deposits with layered characteristics in the world have not been developed in China.^[3]

Economic indicators

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Requirements for metallurgical industry

Metallurgical grade chromium ore is mainly used for smelting various ferrochrome alloys. Chromium ore used for smelting ferrochrome can be divided into 4 grades according to different smelting purposes:

Grade	Cr2O3 （%）	Cr2O3 /FeO	P（%）	S（%）	SiO2（%）	Blockness mm	purpose
Ⅰ	≥50	>3	－	－	<1.2		Ferrochromium nitride
Ⅱ	≥45	≥2.5~3	<0.03	<0.05	<6		Medium and low carbon ferrochrome
Ⅲ	≥40	≥2.5	<0.07	<0.05	<6	40~50	Electric furnace carbon ferrochrome
Ⅳ	≥32	≥2.5	<0.07	<0.05	<6	40~75	Blast furnace carbon ferrochrome

In addition to the above composition requirements, the block size for smelting carbon ferrochrome in blast furnace is required to be 40~75mm, and that for smelting carbon ferrochrome in electric furnace is 40~50mm.

Metallurgical grade chromite ore can also be used to smelt metal chromium. At present, there are two ways to smelt metal chromium in China: pyrometallurgy and hydrometallurgy. Requirements for hydrometallurgical smelting of metal chromium: chromium ore or concentrate contains Cr2O3 ≥ 38% Cr2O3/FeO>2、SiO2<12%、Al2O3<10%， In addition, ore size less than 180 mesh should account for more than 80%.

Fire resistant industrial requirements

In refractory industry, chrome ore is mainly used to make magnesia chrome brick, chrome brick and chrome aluminum brick.

Chromium ore used for refractory production is divided into two grades. First class products are used as Natural refractory , quality requirements: Cr2O3 ≥ 35% SiO2≤8%、CaO≤2%。 The second grade product is used to produce chrome bricks and chrome magnesia bricks. The quality requirements are: Cr2O3 ≥ 30% ～ 32% SiO2≤11%、CaO≤3%。

For the above two grades, the ore fragmentation is required to be between 50~300mm, and no inclusion of more than 5~8mm is allowed in the ore.

Requirements for chemical industry

In the chemical industry, chromium ore is mainly used to produce dichromate (chromate), and then it is used as raw material to produce other chromium compounds. Industrial requirements for chrome ore used for chromate: Cr2O3 ≥ 30%, Cr2O3/FeO ≥ 2~2.5, and a small amount of SiO2.

Requirements for cast stone industry

Chrome ore used to produce diabase cast stone, the quality requirements are: Cr2O3 ≥ 10% ～ 20%, SiO2 ≤ 10%.

Industrial requirements for other chrome ores

Grade		Cr2O3（%）	Cr2O3/FeO	SiO2%	CaO%	Blockness mm	purpose
Fire resistance rating	Ⅰ	≥35		≤8	≤2	50~300	Natural refractory
Ⅱ	≥30~32		≤11	≤3	50~300	Chrome brick
Chemical grade		≥30	≥2~2.5	a few			Dichromate
Cast stone grade		≥10~20		≤10			Diabase cast stone

There are two fire resistance grades, and no inclusions larger than 5~8mm are allowed in the ore.^[3]

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