This invention relates to a process and an installation for the production of sintered magnesite from magnesium hydroxide in which a magnesium hydroxide suspension is first freed from water by filtration, the filter cake thus obtained is dried in a drying zone and heated and dehydrated in another heat-treatment zone, after which the powder-form dehydrated product is briquetted under pressure and the briquettes thus obtained are sintered in a sintering zone at a temperature in the range from 1800.degree. to 2000.degree. C.
The starting material used in a process of this type is a precipitated product which consists of very fine-grained Mg(OH).sub.2 crystallites having a particle size of less than 1.mu. and in many cases as little as 0.1.mu.. The removal of water from the magnesium hydroxide suspension is carried out in settling tanks and then in suction or pressure filtration units. In suction filtration units, the water content may be reduced to around 50%, whereas in cases where pressure filtration is applied residual water contents of the order of 40% are obtained with frame filter presses, and residual water contents of around 25% are obtained where a special tube filter press is used.
In conventional processes, the filter cake thus obtained is then dried in a storey furnace and dehydrated. Dehydration in the storey furnace takes place at around 850.degree. to 950.degree. C. During its heat treatment for drying and dehydration in the storey furnace, the filter cake disintegrates into very fine particles most of which have a primary particle size of less than 1.mu.. This extremely fine powder is then compressed under high pressure in roller-type compacting presses into almond- or pillow-shaped briquettes in either two stages or in a single stage (in that case, however, with a high proportion of "returns" from sieving of the briquettes). Where fluted rollers are used, the powder is compressed into sticks. During the compressing or compacting operation, the dehydrated powder generally has a temperature of only 200.degree. C. The briquettes are then sintered in rotary furnaces or shaft furnaces at temperatures in the range from 1800.degree. to 2000.degree. C., depending on the purity of the dehydrated powder. Sinter densities of 3.2 to 3.4 g/cc are obtained.
These known processes are attended by some serious disadvantages. The consumption of heat amounts to between about 3500 and 4000 kcal/kg of sintered MgO and is thus very high. Since the filter cake disintegrates substantially to primary particle size (i.e., crystalite size) during its drying and dehydration in the storey furnace, briquetting is very difficult and is characterized by a high energy consumption.
Another disadvantage lies in the lack of control over the dehydration process which adversely affects sinterability. This is because, through an excessively long residence time at temperatures above 800.degree. C., crystallites begin to undergo fusion into relatively large agglomerates which impairs the vitrification properties. Another adverse effect on sinterability arises out of the charging of the sintering zone with almost cold briquettes or sticks because the slow heating of the agglomerates to the sintering temperature involves a loss of reactivity. Finally, the storey furances used for drying and dehydration are fairly unreliable in operation and necessitate high maintenance costs.
USPTO Backgrounds
USPTO Backgrounds