At present, the main raw materials for smelting ferrochromium alloy are chromium lump ore and chromium fine ore. Due to the requirement of air permeability of furnace burden in the smelting process, chromium lump ore is usually more suitable for smelting. However, with the large demand for resources in the contemporary ferrochromium alloy industry, there are fewer and fewer chromium ores suitable for direct furnace entry and the price is getting higher and higher. At the same time, with the increase of chromium ore mining and the decrease of crude ore grade, the output of chromium fine ore is getting higher and higher. In smelting, compared with chromium lump ore, chromium fine ore is easy to lead to poor thermal permeability of ore and cannot be directly put into the furnace, but it has better characteristics than chromium lump ore in other aspects, such as high taste and low price.

Chromium fine ore lump-making is an effective way to solve the application of chromium fine ore. Due to the high melting point of Cr2O3 in chromium fine ore, the pellet process commonly used in sintering process is generally not adopted. The processing methods include preheating and reduction method, cold briquetting and cold pressing pellet method, etc. Among them, cold briquetting method has been widely used due to its advantages of reasonable process flow, simple processing equipment, less energy consumption, low investment and low cost.

1. Basis for selecting binder

Binder plays a decisive role in the cold briquetting process of chromium powder ore, which is generally divided into two types: organic binder and inorganic binder. Organic binders mainly include molasses, asphalt, humic acid, carboxymethyl fiber, water-soluble resin, etc. Inorganic binders mainly include cement, sodium silicate, bentonite, lime, etc. Through a large number of experiments, it is found that under normal circumstances, inorganic binder is cheap, but its strength at normal temperature or medium and low temperature is poor. The price of organic binder is relatively high, and the bonding effect is good at normal temperature, but the decomposition of organic matter at higher temperature leads to a sharp decrease in pellet strength. Therefore, in the selection of binder, cost control and pelletizing effect factors are comprehensively considered.

In the production of ferrochromium alloy, the ratio of magnesium to aluminum of raw materials entering the furnace is a key process index. Al2O3 in slag has influence on slag viscosity. If Al2O3 in slag is too high, slag viscosity will increase, which is not conducive to slag discharge. However, Al2O3 can increase the resistivity of slag and is conducive to deep insertion of electrodes, so a certain content is required. Most of the lining of submerged arc furnace for smelting ferrochromium alloy is magnesium lining. Low magnesium content and high aluminum content in the burden will cause the burden to erode the lining and shorten the service life of submerged arc furnace. Therefore, in actual production, the magnesium-aluminum ratio of the burden should be greater than 1. On this basis, the binder containing magnesium should be considered first.

2. Experimental design and methodology

Two kinds of magnesium-containing substances and one kind of carbon-containing organic matter were selected as binders, and Turkish ore powder and South African ore powder were selected as experimental ore powder. A total of 10 schemes were designed. The chromium powder ore is fully mixed with the binder, and a cylinder with a diameter of 20mm and a height of 20mm is made by using a self-made sample pressing abrasive tool. The strength is tested after being placed at room temperature for 2 days.

3. Test and analysis of compressive strength of cold briquettes

The strength of the samples at normal temperature and high temperature was tested by using an adjustable atmosphere high temperature compression testing machine. The equipment can continuously measure 10 samples in reducing, neutral or oxidizing atmosphere, with the highest service temperature of 1500 ℃ and the short-term temperature of 1600 ℃. The internal structure of the equipment is shown in Figure 1.

1. Pressurized cross beam   2. Sensors    3. Sealing device     4. Pressure bar     5. Electric heating element     6. Sample    7. Sample carrier    8. Power plant

The compressive strength test of the sample is divided into two types: normal temperature test and high temperature test, with the temperatures of 25 ℃ and 1000 ℃ respectively. No matter under normal temperature or high temperature conditions, the sample has certain strength, the normal temperature compressive strength can reach 1.457 MPa, and the high temperature compressive strength can reach 1.352 MPa.

4. Conclusions

4.1 The compressive strength of cold briquette chrome ore pellets reached 1.457 MPa and 1.352 MPa respectively at room temperature and 1000 ℃ when the new type of magnesium-containing binder was added to 5%. From the practical application effect, this strength can completely meet the need of direct smelting of artificial lump ore.

4.2 Low melting point compounds containing Ca or Na can be formed between chromium fine ore particles, which can improve the strength of cold briquettes.

4.3 Chromium powder with different gradation can significantly improve the density of cold briquettes, thus improving the normal temperature and high temperature strength of cold briquettes.