Metal sulfide ores (such as pyrite, chalcopyrite, galena, and sphalerite) are important raw materials for the sulfuric acid and metallurgical industries. Flotated concentrates (such as sulfur concentrate, copper concentrate, nickel concentrate, molybdenum concentrate, and zinc concentrate) need to be dried before entering subsequent roasting and smelting equipment. In China, with the construction of large-scale pyrite roasting and non-ferrous metal smelting facilities, there is an urgent need to improve the single processing capacity of concentrate drying equipment and implement deep drying of concentrates.

1. Drying characteristics and drying requirements of concentrate

Wet copper sulfide concentrate is generally gray brown in color, with a particle size of about 200 mesh (74 m). Due to its high metal content and high bulk density, it has strong wear resistance to equipment. The W (H2O) of the concentrate produced by flotation method is generally 10% to 15%. When W (H2O) is around 10%, the concentrate is relatively loose; When w (H: O) is higher, the concentrate is more viscous. In addition, sulfides in the concentrate are prone to acidic corrosion during heating.

The water content of the concentrate entering the furnace has a significant impact on the smelting operation. The general smelting process requires a W (H: O) of 6% to 8% for the incoming copper concentrate, while some require even lower values. For example, flash furnace smelting requires a W (H: O) of less than 0.3% for the incoming copper concentrate. Excessive water content can cause poor flowability of the concentrate, block the feeding hopper L2, and also hinder the smelting of non-ferrous metals. In addition, smelting operations also have certain requirements for the particle size of the dried concentrate.

2 Drying methods and equipment

The drying of metal sulfide concentrates is generally divided into two methods: direct drying and indirect drying.

2.1 Direct drying

Direct drying of concentrate refers to the drying process in which the drying medium comes into contact with the concentrate during the drying process, such as cylindrical drying and air flow drying. Direct drying generally uses hot air or hot flue gas as the heating medium, and performs primary or secondary drying according to process requirements. In the early days, smelters generally used cylindrical drying or air flow drying, with air flow drying divided into two-stage (squirrel cage disperser+drying tube) and three-stage (short cylindrical dryer+squirrel cage disperser+drying tube). Although these two methods have a large processing capacity for concentrate and uniform product particle size, they both have the disadvantages of low thermal efficiency, large footprint, and complex processes.

In addition, due to the vertical conveying of materials, the power consumption of the rotary kiln in the airflow dryer is very high; The squirrel cage disperser, due to its high-speed rotation (250-300 r/rain), has a rotor that is prone to wear and tear, requiring a large amount of maintenance work. Generally, it needs to be welded every 2-3 weeks. Due to the above reasons, in recent years, some manufacturers have started using rotary flash dryers to dry certain concentrates (such as molybdenum concentrates). However, rotary flash dryers generally use high-temperature gas as the drying medium, which can easily form SO: and cause corrosion to the equipment. In addition, the lower agitator has a high speed, which is prone to wear and tear. In addition, the rotary flash dryer also has the problems of low thermal efficiency and small processing capacity. It should be pointed out that the direct drying method generally has exhaust gas treatment problems. Its exhaust gas flow is large, the dust content is high, and improper treatment can easily cause environmental pollution.

2.2 Indirect drying

Indirect drying of concentrate refers to the drying process in which the drying medium does not come into contact with the concentrate during the drying process. Its equipment includes multi-layer steam coil dryers, disc dryers, blade dryers, and steam tube rotary dryers. Indirect dryers generally use saturated steam as the heating medium, which has a much higher thermal efficiency than direct drying, and has a smaller exhaust gas volume and lower dust content, making the process relatively simple. Multi layer steam coil dryer is the earliest indirect dryer used for drying concentrates. It consists of a rotor with a multi coil structure and a fixed shell, and the concentrate advances forward with the rotation of the rotor. Due to the strong wear and tear of the concentrate, it is prone to coil damage, protective plate and scraper wear, and maintenance work is very frequent. Generally, the dryer should be shut down and inspected once a week. The disc dryer was once used to dry sulfur concentrate sand, but many production failures occurred: the material on the upper layers of the drying disc formed scars, the material easily fell directly from the outer edge of the drying disc to the bottom disc and short circuited, and the rake blades were easily worn, causing the rake rod to break. These issues directly affect the drying effect and the normal operation of the drying equipment. In the end, manufacturers are forced to pre dry the materials before sending them to a disc dryer for drying, which not only prolongs the process flow but also increases investment. In addition, its processing power is also relatively small. The paddle dryer is particularly suitable for drying viscous and paste like materials. According to experimental research, the blade dryer has a good drying effect on the concentrate. Due to the self-cleaning effect of the blades, the material has almost no bonding, and the flow state is good. The moisture content and particle size of the exported products can meet industrial requirements. However, due to the characteristics of the material, the blades are prone to wear and consume a large amount of power, making them unsuitable for large-scale installations. The steam pipe rotary dryer has gradually become the preferred equipment for concentrate drying due to its energy-saving and environmentally friendly advantages, large single processing capacity, and low power consumption. The following focuses on introducing the steam pipe rotary dryer

3 Steam pipe rotary dryer

A certain drying technology limited company has been focusing on the localization of steam pipe rotary dryers for high-density polyethylene (HDPE) plants since 1992. Through continuous improvement and optimization, it has developed its own patented technology. The company has successively completed the localization of large-scale HDPE and PTA (para phthalic acid) steam pipe rotary dryers, and successfully applied this technology to the drying of wet concentrates in the metallurgical industry.

3.1 Structure and Working Principle

As mentioned earlier, the steam pipe rotary dryer is an indirect heating rotary dryer, and its structure is shown in Figure 1. The steam pipe rotary dryer is mainly composed of simplified components, steam pipes, feed screw, feed end seal, discharge end seal, steam system, transmission system, and support components. The steam heat exchange tubes are arranged in concentric circles and arranged inside the cylinder, running through the entire dryer. The heat required for drying is provided by the steam flowing into the heat exchange tube. In addition, a small amount of carrier gas (air) is introduced into the simplified cylinder to carry the moisture evaporated during the drying process outside the cylinder.

3.2 Drying process flow

The steam pipe rotary dryer uses medium and low pressure saturated steam as the heating medium, and the drying temperature is relatively low, so it will not cause the decomposition and oxidation of sulfides in the concentrate. The process flow of the copper concentrate rotary dryer is as follows: wet concentrate enters the dryer through a spiral feeder, and the dryer cylinder has a certain slope. The material rolls forward as the cylinder rotates, while exchanging heat with steam pipes arranged around the cylinder to be dried. The dried concentrate is discharged through the discharge box; A small amount of carrier gas enters the interior of the drying equipment from the discharge end, carrying the evaporated moisture during the drying process and is discharged from the feed end; After being separated and dedusted by a bag filter, the exhaust gas is discharged and the collected dust is returned to the feeding screw. This process has the advantages of simple process, low equipment quantity, low investment, high thermal efficiency, low dust and harmful gas emissions, and easy operation and maintenance management.