Steps Involved in the Production of Refractories, along with Emissions and Controls

Steps Involved in the Production of Refractories, along with Emissions and Controls

Materials called refractories serve as ‘refractory lining in blast furnaces’ for high-temperature and other processing machinery. Refractories need to be resistant to abrasion, high temperatures (over 538 °C (1000 °F), and chemical corrosion. There are two broad categories of both clay and non-clay refractories. Manufacturing of refractory materials has a six-digit source categorization code (SCC), which is 3-05-005. Fireclay (hydrous silicates of aluminium) and alumina (57 to 87.5 per cent) are used to make clay refractories. Kaolin, bentonite, ball clay, and common clay are some other clay minerals utilized in the manufacture of refractories. Some other nonclays used to make nonclay refractories include the following:

  • Alumina (87.5 percent)
  • Mullite
  • Chromite
  • Magnesite
  • Silica
  • Silicon carbide, and
  • Zircon.  

The main types of refractories produced

The top 10 refractory companies in India produce two main types of refractories. These include formed objects and unformed granular or polymeric mixtures. Bricks and shapes are considered to be prefabricated products. These materials are comprehensively used to make walls, arches, and floor tiles of different high-temperature process equipment. Mortars, gunning mixes, castables (refractory concretes), ramming mixes, and plastics are examples of unformed compositions. After application, these compounds undergo in-place curing to create a monolithic internal structure.

Major steps involved in the manufacture of refractories

The 4 steps involved in the manufacture of refractories include processing of raw materials, forming, firing, and final processing. Processing raw materials entails crushing and grinding the raw materials, followed, if necessary, by size classification, calcining, and drying the raw materials. The refined raw material can then be dry-mixed with additional minerals and chemicals, packaged, and transported as a finished product. Some refractory materials do not require all of these procedures.

Forming or molding 

The process of molding involves combining the basic ingredients and shaping them into the required forms. This procedure typically takes place in damp environments. In order to create the ceramic bond that gives the finished product its refractory qualities, the refractory material must be heated to high temperatures in a periodic (batch) or continuous tunnel kiln. The finalized product is milled and sandblasted during the penultimate processing stage. After any necessary thermal expansion, this process ensures that the product remains in the proper size and form. The final packing and tar and pitch impregnation of some products may also be considered final processing.

Describing two more refractory process types is also necessary. First is the creation of fusing products. The refractory raw materials are melted in an electric arc furnace using this method, and the molten components are then poured into molds that create sand. Another type of refractory process is ceramic fibre production. This procedure involves melting calcined kaolin in an electric arc furnace. The molten clay is either put into an air jet and quickly blown into thin strands, or it is fiberized in a blow chamber with centrifuge equipment. 

The ceramic fibre may then be sent to an oven for curing, which gives the fibers more structural stiffness, after the blow chamber. Oils are utilized during the curing process to lubricate the machinery used to handle and shape the fibres. Similar processes are used to create mineral wool and ceramic fibre for refractory materials.

Emissions and Controls

Particulate matter (PM) is a matter of serious concern while making refractory materials. Particulate matter emissions happen throughout the raw material crushing, grinding, screening, calcining, and drying processes. They also happen during the tar and pitch operations, refractory brick finishing (grinding, milling, and sandblasting), and the drying and firing of unfired "green" refractory bricks.

The most common method of reducing emissions from crushing and grinding processes is to use fabric filters. To reduce PM emissions from calciners and dryers, wet scrubbers are utilized first, then product recovery cyclones. Electric arc furnaces and refractory fire kilns are the main sources of PM emissions. Kiln emissions of particulate matter are often uncontrolled. To limit kiln emissions, the producers of refractory materials now employ a multi-stage scrubber. A baghouse typically regulates the emissions of particulate matter from electric arc furnaces. At one plant that utilizes an ionizing wet scrubber, particle removal of 87 per cent and fluoride removal of more than 99 per cent have both been documented.

Sulfur dioxide (SO2), nitrogen oxides (NOx), carbon monoxide (CO), carbon dioxide (CO2), and volatile organic compounds (VOC) are among the pollutants released during the calcining and kilning processes as a result of combustion. The sulfur content of some clays and the plaster applied to refractory materials to promote brick setting both affect the emission of SOx. Fluorides in the raw materials cause fluoride emissions to happen during the kilning process. Tables 11.5-1 and 11.5-2 give the emission factors for the filterable PM, PM-10, SO2, NOx, and CO2 emissions from rotary dryers and calciners processing fire clay. Particle size distributions for filterable particle emissions from fire clay processing equipment include rotary dryers and calciners.

Numerous nonclay refractories, such as chrome-magnesite (chromite-magnesite), magnesia-chrome, and chrome-alumina, include chrome. These kinds of refractories are produced using operations that release chromium compounds, including ore crushing, grinding, material drying and storing, and brick burning and finishing. The drying, calcining, and fire processes of all types of refractory materials also release a variety of metals in trace quantities. Some of them include:

  • Aluminum
  • Beryllium
  • Calcium
  • Chromium
  • Iron
  • Lead
  • Mercury
  • Magnesium
  • Manganese
  • Nickel
  • Titanium
  • Vanadium, and
  • Zinc. 

Concluding Remarks

Offering world-class products for heat management, RHI Magnesita India provides a wide range of high-quality refractory products and mixes to its customers in Steel, Cement and lime, Non-Ferrous Metals, Glass, and other industries. Some of the specialty products manufactured and supplied by the company include refractories for electric arc furnace, Isostatically pressed Continuous Casting Refractories, Ladle to Tundish Shrouds, Monoblock Stoppers, Submerged Entry Nozzle/ Submerged Entry Shrouds, Convertor Tap Hole Sleeve, Slide Gate Plates, Nozzles and Well Blocks, Tundish Nozzles, Bottom Purging Refractories and Top Purging Lances, Slag Arresting Darts, Basic Spray Mass for Tundish, working lining, Castables, High Alumina Bricks, and Magnesia Carbon Bricks.