Chrome Mining Process and Magnetic-Gravity Combined Dressing Process

The chrome mining processes include gravity separation, flotation, magnetic separation and chemical separation. The gangue minerals of chrome ore are usually composed of one or more polysilicates. The density of chrome ore is quite different from that of silicate. Therefore, gravity separation is widely used in chrome ore dressing. Gravity separation can obtain high-grade concentrate, but the recovery rate is relatively low. The magnetic-gravity combined separation process currently used in many chrome ore dressing plants has a good effect.

01Chrome mining process

1. Gravity separation

The principle of chrome ore gravity separation is to use the difference in mineral density for separation. The density of chromite (density 4.3-4.8 g/cm³) is significantly different from that of gangue minerals, and heavy minerals are separated by sinking through water flow, vibration or centrifugal force. The main equipment includes:

Jig: pulse water flow stratification, recovery of coarse chrome ore;

Shaking table: horizontal water flow and bed vibration to separate fine minerals;

Spiral chute: inclined water flow and centrifugal force to separate medium and fine ores.

The chrome mining gravity separation process is suitable for pre-enrichment of low-grade chrome ore, which is efficient and environmentally friendly.

2. Flotation

The principle of the flotation process is to use the difference in hydrophobicity of the mineral surface for sorting, which is suitable for processing fine-grained chromite (Cr2O3) with complex associated gangue.

The hydrophobicity of chromium minerals is enhanced by adjusting the pH (usually 8-10), adding fatty acid collectors (such as oleic acid), and combining with water glass to inhibit silicate gangue.

The flotation process is costly, and investment costs and economic benefits need to be considered.

3. Magnetic and electrostatic separation

The principle of magnetic separation of chromium ore is to use the difference in mineral magnetism. Chromite can be separated from gangue by a strong magnetic separator. Electrostatic separation uses the difference in conductivity and is suitable for the separation of ores containing conductive minerals (such as pyrrhotite). High-voltage electrostatic separators are used.

In chrom mining, magnetic separation is the mainstream process, and electrostatic separation is mostly used for special ore bodies or combined processes. The two can work together to improve the grade of concentrate, especially for the separation of fine particles or complex paragenetic chromium ores.

4. Chemical beneficiation

Chromium ore chemical beneficiation extracts chromium (Cr₂O₃) through acid leaching, alkaline leaching or redox reaction. It is suitable for low-grade ores, fine-grained ores that are difficult to physically separate. Common methods:

Acid leaching: sulfuric acid or hydrochloric acid dissolves chromite at high temperature to generate soluble chromium salts;

Alkali fusion-water leaching: ore and sodium carbonate are melted at high temperature and then leached in water to separate impurities;

Reduction roasting: Add a carbonaceous reducing agent to convert high-valent chromium into a low-valent state to increase the leaching rate.

Chemical beneficiation requires neutralization, electrolysis or precipitation equipment to recover chromium, which requires high equipment investment and high environmental protection requirements. Therefore, it is only suitable for chrom mining tailings resource recovery or high value-added products.

02Chrom mining magnetic-gravity combined dressing process

The magnetic-gravity combined beneficiation process combines the advantages of magnetic separation and gravity separation. First, a wet high-intensity magnetic separator is used to recover the target mineral: chromite; then a spiral chute is used for gravity separation. This method can not only ensure the grade of chromite concentrate, but also obtain a higher recovery rate. The specific steps are as follows:

1. Chromium ore mineral analysis

Cr exists in the form of independent minerals, most of which are found in chromite, so chromite is the main recovery target and is recovered by magnetic separation.

Enstatite, tremolite, diopside, biotite, etc. in the gangue minerals also have certain magnetism and are easy to enter the magnetic product, affecting the grade of the magnetic concentrate. However, the relative density of these minerals is quite different from that of chromite, so they can be separated by gravity separation.

2. Magnetic separation stage of chrom mining

The magnetic separation stage adopts the process flow of one stage roughing-one stage concentrating-one stage scavenging.

Roughing: The background field strength is 1.3T, and 1# high gradient separation conditions are used for separation. The grade of the roughing concentrate is increased by 13 percentage points, and a higher recovery rate is obtained.

Concentration: The concentrate obtained from the roughing enters the concentrating stage, and 2# high gradient separation conditions are used for separation. The grade of the concentrate is further increased by 3 percentage points.

Scavenging: The tailings obtained from the roughing enter the scavenging stage, and the tailings after scavenging also have recycling value.

3. Gravity separation stage of chromium ore dressing

Since the density of chromite is heavier than that of other gangue minerals, the gravity separation process can be used to separate chromite from other minerals.

The gravity separation process uses a spiral chute for two-stage gravity separation, and the final Cr2O3 concentrate grade can reach more than 40%, and the recovery rate reaches 70%, achieving a good separation effect.

The above introduces four commonly used processes for chrome mining, and focuses on the magnetic separation-gravity separation combined process. In actual production, it is difficult for a single beneficiation process to achieve a good separation effect, so the combined beneficiation method is a more practical method. Xinhai Mining can provide customers with beneficiation process design, beneficiation plant construction and operation services to help customers improve the grade and recovery rate of chrome concen