Magnesia - Forms, Applications and Production Processes

Fused Magnesia (MgO) is normally manufactured by the electric arc melting of caustic calcined magnesia, deadburned magnesia or raw magnesite in furnaces at temperatures in excess of 2750°C, producing a refractory product whose altered crystalline structure is such that its characteristics and performance are superior to competing materials.

Magnesite

Magnesite (MgCO₃), the naturally occurring carbonate of magnesium (Mg) is one of the key natural sources for the production of magnesia (MgO) and subsequently fused magnesia. Magnesite occurs in two distinct physical forms: macrocrystalline and cryptocrystalline. Cryptocrystalline magnesite is generally of a higher purity than macrocrystalline ore, but tends to occur in smaller deposits than the macrocrystalline form.

At present, there is only one producer of fused magnesia in Australia, QMAG, majority-owned by Australian Magnesium Corporation. QMAG is a producer of refractory grade fused magnesia.

High Grade Magnesia Production

Historically, and due principally to the small size of most known cryptocrystalline deposits, production of high grade magnesia products was mainly by extraction from natural brines or seawater (synthetic MgO), a high cost and energy intensive process. High quality deposits, provide an alternative source of supply to the high cost seawater-sourced magnesia.

Uses of Fused Magnesia

Magnesia products (calcined, deadburned and fused) are widely used in a range of market applications.

Calcined Magnesia

Is used in agricultural and industrial applications, eg, as a feed supplement to cattle, fertilisers, electrical insulations, industrial fillers, and in flue gas desulphurisation.

Deadburned Magnesia

Is used almost exclusively for refractory applications in the form of basic bricks and granular refractories. Deadburned magnesia has the highest melting point of all common refractory oxides and is the most suitable heat containment material for high temperature processes in the steel industry. Basic magnesia bricks are used in furnaces, ladles and secondary refining vessels and in cement and glass making kilns.

Fused Magnesia

Fused magnesia is superior to deadburned magnesia in strength, abrasion resistance and chemical stability. Major applications are in refractory and electrical insulating markets. Producers of fused magnesia commonly fall into one of two categories: those producing refractory grades and those producing electrical grades. Few producers serve both markets on a mainstream basis.

Refractory Grade Fused Magnesia

The addition of fused magnesia grains can greatly enhance the performance and durability of basic refractories such as magcarbon bricks. This is a function of a higher bulk specific gravity and large periclase crystal size, plus realignment of accessory silicates. Refractory grade fused magnesia has exacting specifications and is normally characterised by the following:

•         Generally high magnesia content (minimum 96 per cent MgO and up to/exceeding 99 per cent MgO)

•         Low silica; lime:silica ratios of 2:1

•         Densities of 3.50 g/cm³ or more

•         Large periclase crystal sizes (>1000 microns)

Due to its excellent corrosion resistance, refractory grade fused magnesia is used in high wear areas in steel making, eg, basic oxygen and electric arc furnaces, converters and ladles.

Ultra high purity (>99 per cent MgO) grades have been used in high-tech applications such as optical equipment, nuclear reactors and rocket nozzles.

Electrical Grade Fused Magnesia

Fused magnesia is also used as an electrical insulating material in heating elements. Although electrical grades of fused magnesia have very tight specifications, they do not necessarily require the highest MgO contents or densities. Impurities such as sulphur and iron are particularly undesirable, but the product should contain sufficient silica to enhance its electrical properties. The following are characteristic of electrical grade fused magnesia:

•         Low levels of boron, sulphur, iron and trace elements.

•         Lime: silica ratios of 1:2 (opposite to refractory requirements).

•         Used as electrical insulating material in ceramic sheaths for heating elements.

Producers manufacture three categories of fused magnesia, each related to the environment of application:

•         High Temperature (up to and in excess of 950°C) requiring high purity fused magnesia of 94-97 per cent MgO and low silica and calcium contents, eg, for stove grills.

•         Medium Temperature (up to 800°C) with magnesia contents of 93-96 per cent MgO, eg, for elements in ovens.

•         Low Temperature (<600°C) with <90 per cent MgO, eg, immersion elements.

Electrical grade cements can be produced by blending electrical grade fused magnesia and plasticisers and hardeners for use in hot plates, toasted sandwich makers and electric irons. Electrical grade fused magnesia can be given a uniform silicon coating for greater resistance to moisture absorbance during heating element manufacture; this also improves the cold insulation resistance of low duty elements exposed to conditions of humidity. Electrical grade magnesia is tested for its electrical and thermal properties, eg, high electrical resistivity and high thermal conductivity.

Fused Magnesia Production Process

Magnesite (magnesium carbonate MgCO₃) is converted into magnesia by the application of heat which drives off carbon dioxide (CO₂), thereby converting the carbonate to the oxide of magnesium (MgO).

Magnesite, from both natural sources (primarily magnesite) and synthetic sources (seawater, natural brines or deep sea salt beds), is converted into caustic calcined magnesia by calcining to between 700°C and 1000°C, driving off 96-98 per cent of the contained carbon dioxide. Caustic calcined magnesia is both an end product and an intermediary step in the chain of magnesia products.

Further calcining of magnesite at higher temperatures between 1750-2200°C results in the largely inert product, deadburned magnesia. Heating to this level drives off all but a small fraction of the remaining carbon dioxide to produce a hard crystalline non reactive form of magnesium oxide known as periclase. Deadburned magnesia exhibits exceptional dimensional stability and strength at high temperatures.

Fused magnesia is produced in a three phase electric arc furnace. Taking high grade magnesite or calcined magnesia as raw materials, 12 hours is required for the fusion process at temperatures in excess of 2750°C. The process promotes the growth of very large crystals of periclase (>1000 microns compared with 50-100 microns for dead burned magnesia) with a density approaching the theoretical maximum of 3.58g/cm³.

In fused magnesia production, the main constraints on capacity are the size and number of electric arc furnaces, and the cost of energy. The manufacture of fused magnesia is very power intensive with electricity consumption varying between 3500-4500 kWh/tonne; fused magnesia producers often quote total capacity based on utilising off peak power.

Raw Materials

Commercially acceptable magnesite should contain at least 95% MgCO₃. The most important magnesite deposits in New South Wales, Australia are located at Thuddungra (about 30 km northwest of Young). Other known deposits are located at Fifield (northwest of Condobolin), Lake Cargelligo and Attunga; these are smaller and less pure than the Thuddungra deposits.

Markets

The emergence of China as a major producer of low priced fused magnesia has impacted on the market share and profitability of Western manufacturers of the product. However, it is the increased availability of competitively priced Chinese fused magnesia that has bolstered global demand for this refractory raw material, and higher grade fused magnesia has developed a niche market to some degree due to its lower iron content.

Figure 1. Fused Magnesia Process Flow Chart

The 1990’s heralded a period of expansion in the fused magnesia sector in response to accelerating consumer demand, with several new players entering the market. Outside of China, key producers of fused magnesia in the refractory market include QMAG of Australia, Baymag of Canada, Tateho Dead Sea Fused Magnesia of Israel and Kombinat Magnesit of Russia. In 2000, with the growing dominance of the Chinese producers, the fused magnesia business has become an intensely competitive environment, with profit levels much tighter than they were previously.

There are now over 500 magnesia producers in China and over 100 companies produce fused magnesia. Whilst difficult to obtain accurate figures on Chinese production, it is estimated that fused magnesia production in China is in the order of 500 000 tpa of which 250 000-300 000 tpa are destined for the export market. However, despite a more competitive environment and declining specific refractory consumption in steel making, the outlook for fused magnesia compared with some other refractory raw materials is relatively bright, with more fused magnesia being used in refractory brick formulations. The recent upturn in demand for fused magnesia can at least in part be attributed to the recovery of world steel markets.

With the current strength of global steel markets, increased prices for Chinese fused grades as a result of rising Chinese electricity costs and disruptions to supply from China, a window of opportunity exists for new and existing non Chinese refractory magnesia producers.

Higher electricity costs in China are expected to add a premium of US$30-50 per tonne to the cost of fused magnesia. Uncertainties over energy costs have resulted in shortages of material, with producers either temporarily shutting down production or simply not selling the material. Fused magnesia sales from QMAG, for example, increased by 35 per cent in the June quarter 2000 over the March quarter partly in response to this disruption to Chinese supply.

The price of fused magnesia depends significantly on its quality.

At the lower end of the scale, Chinese fused magnesia (97.5% MgO) is being sold at around US$300 per tonne into Europe, whereas 98.5% material would sell at around US$450-550 per tonne. At the other end of the scale, highest quality fused magnesia with MgO contents of greater than 99%, would sell for around US$1200-1400 per tonne (April 2001).

As indicated by the variation in price, one type of fused magnesia based on its quality and properties is not necessarily in competition with another type of fused magnesia of different quality and with different market applications. For example, Tateho Dead Sea Fused Magnesia, which manufactures an extremely high purity product, does not compete against lower quality Chinese fused magnesia. Customers choosing the Tateho product require a product with extremely low impurities for use in very heat intensive refractory applications.

Primary author: William Hughes

Source: NSW Department of Mineral Resources