Production of high-purity magnesia

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Published: Monday, 22 April 2013

The spray roaster process (pictured) was originally developed by J. Aman in 1958 for the decomposition (pyrohydrolysis) of MgCl2 brines. Consequently, it is also well known as the Aman Process. However, its most common application is the regeneration of spent hydrochloric pickle liquors in the steel industry.

By Heinz Krivanec*

The spray roaster process (pictured) was originally developed by J. Aman in 1958 for the decomposition (pyrohydrolysis) of MgCl2 brines. Consequently, it is also well known as the Aman Process. However, its most common application is the regeneration of spent hydrochloric pickle liquors in the steel industry.

Pyrohydrolysis is a combination of the evaporation of water in chloride solutions and the chemical reaction between metal chlorides and water at elevated temperatures to form the corresponding metal oxides and hydrogen chloride. The CMI spray roaster is a cylindrical, refractory lined steel vessel with conical top and bottom sections. Several burners are fitted tangentially to the circumference of the cylindrical section at its base. These burners (fired with liquid fuel or natural gas) inject hot gases into the roaster to create a vortex specifically designed for the size of the plant and the application.

Since the evaporation and chemical reactions at high temperatures are very fast processes the resulting oxide is formed as a very fine solid. The fineness of the oxide product is significantly increased when processing dilute metal chloride solutions, resulting in a very fine, chemically reactive oxide powder. Depending on the type and process conditions, the oxide powder has a bulk density between 0.2 - 0.8 kg/l and a specific surface area between 2 - 20 m²/g (B.E.T.).

The oxide primary particles are generally irregular in shape with a mean equivalent spherical diameter of approx. 0.1µm, while the diameter of the agglomerates ranges up to 500 µm.

Depending on its application, the spray roaster oxide may be subjected to further treatment. MgO for refractories for instance is hydrated to Mg(OH)2 in order to wash out any chlorides of metals which do not hydrolyse and which are contained in the oxide from the CMI Spray Roaster.

Subsequently the Mg(OH)2 is filtered to separate the chlorides. Further steps include calcination, usually in ‘Nichols-Herreshoff’Multiple Hearth Furnaces (MHF), producing CCM with >98% w/w MgO and sintering in Vertical Shaft Kilns (VSK) or rotary kilns producing sintered magnesia with >99% w/w MgO.

The CMI NESA¨ MHF consists of a series of circular hearths, placed one above the other and enclosed in a refractory-lined steel shell.  A vertical rotating shaft through the centre of the furnace carries arms with rabble blades, which stir the charge and move it in a spiral path across each hearth.ÊMaterial is fed to the top hearth, and rabbled across it to pass through drop holes to the hearth below. It passes in this way over and across each hearth to the bottom where the product is discharged through one or more ports.

 
Heated gases flow counter-currently to heat the charge to reaction temperature and to carry on the desired reaction. The CMI NESA¨ VSK consists of a cylindrical shell made of special steel, which is lined with high temperature refractory.

A very specific crystal structure is needed if the Mg(OH)2 is to be used as an additive, for example as a halogen-free flame retardant filler for plastics. This is achieved by hydration of MgO under carefully controlled conditions.

Magnesia from natural MgCI2 brines

Spray roasting can be used to produce magnesia from natural MgCl2 brines. Such brines are found in arid countries, most often they are the products of solar evaporation ponds. Such ponds are used to produce raw materials for the chemical industry, for example sodium chloride (NaCl), potassium chloride (KCl) or bromine (Br2). These brines typically contain 400 - 420 g/l MgCl2, 30 - 40 g/l CaCl2, NaCl, KCl and can be used as a feedstock for the production of magnesia by spray roasting.

In the early 1970s such an application with a capacity of 50,000 tpa of MgO sinter was built in connection with the potash production from the Dead Sea waters in Israel, by ICL. The plant is still in operation and has been extended to three spray roaster units, with a combined capacity of 70,000 tpa, which feed a common washing, calcination (supplied by CMI NESA¨), briquetting, and sintering line. About 700,000 tpa of HCl (18-20% w/w) is produced as a by-product which is used in the production of phosphoric acid and other phosphates from phosphate ores. A similar installation was built in Germany which processed MgCl2 brines produced by leaching an underground bischofite deposit.

 
Magnesia from synthetic MgCl2 solutions

Subsequently, processes have been developed for the preparation and purification of synthetic magnesium chloride solutions made from HCl and magnesite ores or ore tailings. These hydrometallurgical processes allow the use of low grade raw materials which usually cannot be treated by conventional methods or which would yield products of insufficient quality. As an example a 25,000 tpa MgO sinter installation was operated in Serbia until 1990 using magnesite and flue-dust as raw materials.

These were leached in a semi continuous leaching process using 18% w/w regenerated HCl and giving a solution of not more than 300 g/l MgCl2, therefore pre evaporation was necessary. Further developments included an acidulation instead of a pre- evaporation in order to reduce energy consumption. The first application using acidulation was located in Hačava (Slovakia), a 25,000 tpa installation for the production of +99% w/w MgO sinter using locally mined magnesite and flue dust. The use of acidulation achieved the most economic MgCl2 concentration, of about 420g/l.

The isotropic concentration limit of the HCl solution may be avoided by feeding not simply water, but also magnesite ore fines suspended in water to the adiabatic absorption column (acidulation). Thus, HCl reacts in the absorber with the magnesite producing magnesium chloride, enabling the solution to absorb additional hydrogen chloride. The concentrations of HCl and MgCl2 in the solution are therefore only dependent on their solubilities in water at actual operating conditions. The acidulation process is controlled at a concentration of about 390 g/l of MgCl2 in the acidic liquor. In this way, not only the production of pure magnesium chloride brines but also the thermal decomposition to magnesia is achieved in a single continuous process. HCl is needed only for the initial start-up of the plant and to make up chloride losses arising from the presence of metals which do not hydrolyse in the Spray Roaster.

Current applications and future trends

Spray roaster magnesia applications are focused on the relatively small chemical grade market of about 150,000 tpa which represents 7% of the current world magnesia production. Due to its purity it is used for the production of high purity chemicals as well as for specialities such as food additives and pharmaceuticals. It should also be mentioned that high purity MgO is a truly recyclable neutralising agent when used in chloride systems. New applications in the field of organic chemistry, such as the production of biodegradable plastics, are likely to emerge and boost future demand.