Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
1999-07-12
2002-01-01
Marcheschi, Michael (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S416000, C501S145000, C501S150000
Reexamination Certificate
active
06334894
ABSTRACT:
The present invention relates to the treatment of mineral particles. In particular, it relates to the heat treatment by flash calcining of mineral particles suitable to be employed as coating, filler and extender materials in products such as papers, polymers, paints, cements, concretes and the like. Such minerals include, for example, clays especially kaolins processed or produced by various processes such as grinding or precipitation.
Calcination is a known process wherein particles of a material, eg mineral, are heated. In flash calcining the particles are heated extremely quickly, almost instantaneously. In contrast, in the alternative process of soak calcining heating is carried out at a more gentle rate.
Prior art methods of flash calcining have one or more disadvantages. For example, in fluid bed type furnaces it is necessary, in order to treat certain feed particulate materials, to establish an inert coarse particle bed in the furnace before introduction of the feed material to ensure efficient heating of the feed material. Also, if the feed material contains fluxing agents there is a likelihood above a certain temperature, eg 900° C., that fluxed material will build up on the resident bed causing efficient operation of the system to be hindered or, eventually, the system to stall.
In other cases, the particles come into direct contact with combusting fuel. Particles passing through the hottest part of the flame tend to be overheated which can cause the product to be abrasive. Also it is difficult to ensure that the particles are all treated at a uniform temperature and product quality is difficult to control.
In still further prior art methods, the flash calcination treats only the exterior parts of the particles and a further calcination step, with considerable expense, is required to give suitably complete flash calcination of the particles.
In any case, it is desirable to improve certain product properties compared with those obtained using the prior art methods.
It is an object of the present invention to carry out flash calcining of mineral particles by a method which reduces or eliminates the above disadvantages and which provides improved product properties.
According to the present invention in a first aspect there is provided a method of heat treating kaolin particles which includes the steps of establishing in a furnace a heating zone suitable to calcine the particles and passing the particles rapidly through the heating zone whereby the particles are flash calcined in the heating zone, wherein the furnace is of the kind in which a toroidal fluid flow heating zone is established and the particles are heated in the said heating zone to a temperature of at least 550° C. by application to the particles of a temperature increase rate greater than 10
3
Celsius degrees per second to give a rapid blistering of the particles caused by rapid dehydroxylation of the kaolin and the particles have a residence time of less than 1 second in the said toroidal fluid flow heating zone.
Furnaces of the toroidal fluid flow kind are known per se. Such furnaces are described for example in U.S. Pat. No. 4,479,920 and WO96/03256. Usually, a hot gas is passed through gaps between angled blades or vanes in a ring of blades or vanes provided in the operational chamber or reactor part of the furnace. The blade ring is formed in an annular gap between the wall of the chamber and a central block, eg an upwardly pointing conical portion, located on the axis of the chamber. Gas flow is caused to follow a rotary path in a doughnut shaped region around the block and in individual swirls within the rotary path. This provides heat transfer to material, eg particulate material, to be heated in the gas flow.
Although heating of particulate materials in a toroidal fluid flow heating device is well known and use of such a device has been mentioned for heat treatment of kaolin in WO98/03256, such devices are not used industrially in the kaolin industry for kaolin treatment. Use of such a device to provide flash calcining of kaolin in accordance with the invention and the benefits to be obtained have not previously been suggested.
The transformations of kaolinite (the principal constituent of kaolin) on heating are known. A significant peak in the differential thermal analysis at 550-650° C. denotes the formation of metakaolin from hydrous kaolin by a hydroxylation reaction:
This is a strongly endothermic reaction and it is the evolution of water of which use is made to give flash calcined kaolin unique properties as follows.
During flash calcination, kaolin particles are subject to extreme temperature gradients, eg from ambient to 550° C. or higher in fractions of a second. This causes the water vapour generated from the reaction to expand extremely rapidly, in fact faster than it can diffuse through the crystal structure of the particle. The pressures generated are sufficient to produce sealed voids as the interlayer hydroxyl groups are driven off, and it is these swollen interlayer spaces, or voids or blisters, between the kaolin platelets, which typify flash calcined kaolins and give them their characteristic properties, as described later.
Thus, in the method of the present invention the kaolin particles are heated to a temperature greater than 550° C., preferably a temperature in the range 800° C. to 1100° C., more preferably 900° C. to 1050° C., preferably 950° C. to 1000° C., by application to the particles of a temperature increase rate greater than 10
3
Celsius degrees per second, eg at least 5×10
3
Celsius degrees per second, in some cases at least 10
4
Celsius degrees per second, whereby rapid dehydroxylation to give the required blistering effect described above.
The delivery of mineral particles into the toroidal fluid flow heating zone in the method according to the first aspect of the present invention is desirably carried out by injection of the particles dispersed from one another in a carrier fluid. This ensures that the particles are not in contact with one another when they are rapidly heated in the toroidal fluid flow. Such contact can cause fluxing and fusing of particles and leads to a product which can be abrasive. Such abrasiveness is undesirable. For example, where the product is to be employed in paper making, eg as a coating or filler material, it can harm the paper making machinery.
The carrier fluid may comprise a gas such as air or inert gas which may be initially blown over a source of the particles to be suspended in it or to which the particles may be added to carry them into the furnace. The carrier fluid may be at ambient temperature eg a temperature of from 15° C. to 30° C. although it could if required be heated.
The rate at which heat is transferred from carrier fluid to entrained particles is largely dependent on the boundary layer which surrounds each particle. This boundary layer is an envelope of carrier fluid which is stationary relative to the particle and which acts as an insulator, thus limiting heat transfer rate. If this layer can be disrupted, faster heat transfer will occur. Relative motion between the air and particle causes an increase in heat transfer due to convection and a decrease in the depth of the stationary air layer. For particle Reynolds numbers in the range 20-2,000 the following equation is applicable:
Nu=2+0.69Re
0.5
Pr
0.33
where:
the Nusselt number, Nu=hd/k
the Reynolds number, Re=pud/&mgr;
and the Prandtl number, Pr=c
p
&mgr;/k
This means that as the relative velocity (u) between the carrier gas and the particle increases, the heat transfer coefficient (h) rises due to a decrease in thickness of the boundary layer. The required relative velocity can be obtained by use of the toroidal fluid flow heating device.
By suspending particles in jets of hot air, which may be formed in the fluid flow heating zone, where much of the high velocity component is in the horizontal plane, it is possible to subject particles to impact velocities in excess of their terminal velocity. This allows very e
Imerys Minerals Limited
Marcheschi Michael
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