We have learned that chromite ore (people often call it chrome ore) is the essential material of stainless steel and its wide application in metallurgy, refractory and chemicalindustries.
Here's the big question, how do we process chromite rocks for those wide applications?
Today we will focus on the first step in the chrome ore processing plant: making chromite concentrates.
Chrome ore properties
Among more than fifty kinds of chromium-bearing minerals found in nature, chromite is the sole commercial source.
The chromite ore is concentrated in South Africa, Zimbabwe, Kazakhstan, Russia, Finland, India. South Africa is the major chromite ore supplier.
Chromite is weekly magnetic. Its density ranges in 4.1~4.7g/cm3, its Mohs hardness fluctuates within 5.5~6.5, and its color differs from black to dark brown.
Its major gangue minerals are talc, olivine, serpentine, and pyroxene.
Chromite is insoluble in weak acids such as acetic acid, and boric acid, nor is it soluble in 1:1 HCl, 1:1 HNO3, 1:1 H2SO4, aqua regia and HF acid. But it dissolves in 1:1 phosphorus and sulfur mixed acid.
When calcined at low temperature, the dehydration of chromite turns from dark brown to blue-black. At high temperatures, chromite reacts with soda ash or sodium peroxide to convert Cr3 + into Cr6 +.
Understanding the mineral properties of chromite is the prerequisite for beneficiation.
A general chromite ore mining process consists of two sections:
Chrome lump ore must be crushed and ground into powder due to medium and fine dissemination grain sizes.
While chromite sand ore does not need the two steps. For example, most of the chromite mines in the Philippines and Vietnam are chromite sand. After desliming by washing, the grade of chromite sand will rise greatly, and it will be further improved by jigs and shaking tables.
The predominant method to concentrate chromite rocks is gravity separation. However, given the slightly different chromite properties around the world, magnetic separation, flotation, chemical beneficiation, and combined beneficiation methods can also be used flexibly.
Chrome ore gravity separation
The specific gravity of chromite ore is 4.1~4.7g/cm3, while that figure of associated gangue and iron silicate minerals is generally 2.7~3.2g/cm3. Therefore, we can utilize the difference in specific gravity between minerals to recover chromite with gravity separation equipment such as shaking tables, spiral chutes, and jigs. Shaking tables are more popular due to high separating accuracy.
Among the chrome beneficiation machines, jigs are for chromite rocks with medium and fine grain sizes, and shaking tables are for fine chromite ore. The combination of jigs and shaking tables is more prevalent in the chromite gravity separation plants for the best recovery effect.
Two problems should be paid attention to when treating low-grade chromite by gravity separation:
(1) The chrome lump ore is friable and will produce a large amount of fine-grained chromite ore that is difficult to recover by gravity separation. So, flotation equipment is required.
(2) Due to the huge difference in the grain size of chromite rocks, we will use multi gravity separators, i.e. different combinations of jigs, shaking tables, spiral chutes to form a complex chrome processing flowsheet for a better recovery result.
Chrome ore magnetic separation
Chrome ore has weak magnetism. Therefore, magnetic separation would be a good choice for separating it.
Low-intensity magnetic separation: This method is used to get rid of magnetite, increasing the Cr/Fe ratio and selecting the qualified chrome concentrate. This method means a lot to the unqualified chrome concentrate after gravity separation because it contains a small amount of magnetite.
High-intensity magnetic separation: In a high-intensity magnetic field chromite can be extracted as a magnetic product from the gangue material. It is mainly suitable for fine-grained chromite powder that cannot be effectively recovered by gravity separation.
However, the magnetic separation method is rarely used alone and is widely used in conjunction with gravity separation.
Chrome ore flotation
Generally, when the gravity separation and magnetic separation fail, flotation offers an alternative technique to concentrate the fine-grained and ultra-fine-grained chromite rocks and fine-grained chromite tailings which require fine grinding for liberation. But grinding will produce a large amount of ore slime.
If a cationic collector is used to extract chromite in the alkaline slurry, it needs to be deslimed in advance, but the fine sludge removed will take away a lot of chromite.
The use of an anionic collector in alkaline slurry can save the pre-desliming.
Chrome ore chemical beneficiation
One of the purposes of chemical beneficiation is to increase the Cr/Fe ratio of the chromite concentrate produced by the physical methods, and the other purpose is to treat certain chromium ores that cannot be treated by physical methods or that are uneconomical by physical methods.
The combined beneficiation method of physical-chemical methods and chemical beneficiation methods are one of the main trends in chromite beneficiation today. Chemical methods can directly extract chromium from the ore and produce chromium carbide and chromium oxide.
Chemical methods include selective leaching, oxidation-reduction, melting separation, sulfuric acid, and chromic acid leaching, reduction and sulfuric acid leaching, etc.
Before determining the beneficiation method, it is necessary to conduct a beneficiation test on the chromite ore, analyze the element types and the original ore structure of the chromite ore, and customize the targeted beneficiation method to obtain the ideal beneficiation index.