IMMERSION HEATERS IN MINING INDUSTRY
A centuries-old industry, mining has come a long way in defining and improving process efficiency. Even still, operations require complex processes and heavy equipment. The mining industry relies on heat for various processes, including heating, cooling, and soaking ore.
Early mining processes soaked ore in water and then heated it, generating a large amount of contaminated water. This water is treated to remove the excess contaminates. Modern process technologies now use corrosive chemicals, increasing the efficiency of extraction. These chemicals need precision temperature control. Otherwise they may become volatile, unstable, or even flammable.
The need for temperature maintenance without exposing these corrosive chemicals to open flame is fulfilled by the electric immersion heaters which can maintain the process temperature within +/-1˚C.
Immersion Heaters in Copper Mining
Natural ore which contains copper can be found around the world. Once copper extraction is complete, it generates one of the highest purity metals which is widely used in electrical applications. Copper is mined in the open pit mines, underground mines, or a combination of both.
The ore extracted by mining is crushed. Crushing reduces the size, increasing the extraction of raw metal as it passes through various process stages. The crushed ore is concentrated through a froth filtration process in which froth is skimmed and is rich in raw metal.
The first heating process is roasting. The concentrated ore is heated in the presence of air between 500˚C- 700˚C. This removes the volatile impurities like arsenic and mercury while the sulfur is removed as the sulfur dioxide. In very small-scale processes and development projects, circulation immersion heaters heat the air. Larger commercial processes use boilers and smelters.
The remaining solid is called calcine and is smelted where flux is added. Flux facilitates melting the calcine. The calcine is heated to 1200 ˚C where it melts and converts to matte which is a mixture of copper and iron sulphide in a liquid form. Hot air is blown through the matte thus forming the blister copper.
The lister copper is further purified in the electrolytic refining. In modern processes the ore in the froth filtration process is heated with immersion heaters. Then they are used to warm the process fluids to float the copper. Due to presence of sulphides, deionized water and acid content it is important to put some thought into the material selection of the sheath. This serves to prevent corrosion and premature failure.
Incoloy is recommended as a sheath material while Alloy 800 is usually a good all-rounder material for heating element. For applications containing corrosive elements it is better to use the Alloy 825. The most cost-effective option is to use the welded tube. Although seamless elements are highly recommended in high temperature and high corrosive environments.
Immersion Heaters in Gold Mining
Gold mining has changed a lot in the last 170 years. It is now a large-scale activity involving huge machines and complex industrial extraction processes. The main process to extract gold is the cyanide leaching process. This is used where there are large enough gold deposits to make the process economically feasible.
Crushed rock excavated from open pit mining or underground mining is mixed with the sodium cyanide solution. This extracts the gold in the crushed rock, accumulating it in the seepage tank. From there leaching agent, zinc, is added to filter the gold. This process is highly controversial due to its environmental impacts in the form of release of cyanide compounds which are poisonous and easily flammable.
The cyanide leaching process is being replaced by the Borax process which is more environmentally friendly. The crushed rocks, which contain the gold particles, are melted in the presence of sodium borate. Sodium borate reduces the melting point of the ore and decreases the viscosity of the molten fluid. This reduces the power requirements to melt the ore and is a greener process.
Immersion heaters: A greener choice
The conventional process of melting ore requires high energy consumption with difficulty in the process control, especially temperature control. Immersion heaters offer the same process specifications in terms of temperature and are more precise in terms of temperature control and can quickly reach target temperature without high energy consumptions thanks to their efficient and compact design.
Another heating option is using microwave energy. Darker compounds are heated to a temperature of 1000 ˚C. The lighter colored compounds are heated slowly thus achieving much higher temperatures then the darker colored compounds.
Design of the System
Flanged heaters are used in enclosed vessels to prevent the exposure of the volatile gases to the atmosphere. Flanged immersion heaters are designed for heating the liquids and gases in the tanks and pressure vessels making them ideal for use in heating the slurry containing the ore.
It also makes them maintenance friendly due to the ability of quick turnaround if the heater needs to be replaced. It is important to mention that in the presence of hydrogen sulphide and cyanide-based gases care must be taken while designing the element. Usually it is recommended to utilize the explosion proof element operating at temperatures below the flash point of the flammable gases which are either product or the byproduct of the processes and thermostat must be set with a safety factor to limit the temperature much below the flash point temperature.