GB2117747A

GB2117747A - A process for pelletising substances of crystalline or crystal- like structure

Info

Publication number: GB2117747A
Application number: GB08307209A
Authority: GB
Inventors: Franz Josef Gattys
Original assignee: GATTYS INGENIEURBURO fur CHEM
Current assignee: GATTYS INGENIEURBURO fur CHEM
Priority date: 1982-03-17
Filing date: 1983-03-16
Publication date: 1983-10-19
Also published as: SE8301317D0; LU84692A1; DE3209619A1; FR2523473A1; IT8320012A0; NL8300917A; DK44983D0; IT8320012A1; GB2117747B; SE8301317L; BE896168A; IT1194157B; GB8307209D0; DK44983A; DD207537A5

Abstract

Crystalline or crystal-like substances, such as quartz sand, waste glass, and kieselguhr are intimately mixed with such a quantity of a mineral substance which reacts with water to form a water-insoluble compound, such as a hydroxide of a metal of Group II of the periodic table, to obtain a microfilm layer on the material grains, and are then agglomerated in known manner using a pelletising liquid. After wetting the crystalline material, the metal oxide is fed in dry form and is intimately mixed with the material to be pelletised, agglomeration then taking place while feeding pelletising liquid. The completion of the reaction can be monitored through the heat of reaction. The pellets can be provided with a coating of the same or another material which is insoluble or difficultly soluble in water.

Description

SPECIFICATION A process for pelletising substances of crystalline or crystal-like structure The invention relates to a process for pelletising substances of crystalline or crystal-like structure.

In industry, large quantities of materials or crystalline or crystal-like structure such as quartz sand, powdered glass, kieselguhr, crystalline salts of all types occur or are produced as raw materials or waste materials. The finely granular structure in which they occur can give rise to considerable difficulties during their further processing, recycling, transportation or depositing, partly on account of the dust produced during transportation and possibly storage, and partly on account of the considerable compacting of the dusty and finely granular materials and the resultant impermeability to air, which often results in difficulty during their further processing. Attempts have therefore been made for some time to agglomerate these materials into lumps, but up to the present time this has been attained only by costly, partly thermal processes.These materials have been completely unsuitable, up to the present time, for simple granulating by the use of water as a pelletising liquid, because of the smoothness of the surface and the crystalline structure of such materials, which exclude mutual physical adhesion or bonding.

According to the invention, there is provided a process for pelletising substances of crystalline or crystal-like structure and comprising mixing intimately the substance to be pelletised with such a quantity of a mineral substance which reacts with water to form a water-insoluble compound, as to obtain a microfilm layer on the material grains, and then agglomerating the mixture so produced by the use of a pelletising liquid.

It has been found, surprisingly, that such mineral substances which react with water to form a water-insoluble compound also cover the smooth fractured surfaces of crystalline materials, such as the crystal surfaces of quartz sand, in the manner of a microfilm and/or form a closed casing of sufficient inherent stability about the grain of the material, which can be made to adhere to the casings of other grains of material. The result is that, on pelletising, the grains quickly agglomerate to ideal "rolling" pellets, which therefore quickly increase in size. The skeleton formed from the casings which adhere together is stable after the reaction of the material, and gives the pellets so produced a durable firmness which is sufficient to allow the pellets to be transported or further processed.The water-insoluble material which represents the skeleton material undergoes no changes due to drying, dissolving and no substantial reaction under atmospheric conditions. In this respect, when mixtures of substances are pelletised, pellets are obtained which contain the individual substances in the original mixture proportions, and the pelletisation produces no segregation or regional concentrations of individual substances.

Hydroxides of metals of main Group II or subsidiary Group II of the periodic table are preferably used as the skeleton material. The choice is preferably made according to the type of material to be pelletised and its further processing. In this respect, inexpensive calcium hydroxide can be used in the majority of cases as the skeleton material, this being useful in nearly all applications, such as in agriculture for neutralising soil acids, and in metallurgical smelting as a basic material for forming lime slags by pyrolysis.

The pelletising is desirable effected by firstly wetting the surface of the material to be pelletised with water, and then feeding the skeleton material in dry form as a metal oxide and intimately mixing it with the material to be pelletised, agglomeration then taking place while pelletising liquid is fed, until the end of the reaction in accordance with the equation MeO + H20 = Me(OH)2, which results in a temperature rise.

In those cases in which the material to be pelletised is aggresive or is of a type which is changed by hygroscopy or by reaction with the components of the atmosphere, the pellets can be provided with a coating of a material which is insoluble or difficultly soluble in water. This can be done by use of a mineral substance which reacts with water to form a water-insoluble compound, and in particular an oxide of one of the metals of main Group II or subsidiary Group II of the periodic table which, by reaction with the pelletising liquid, which either emerges from the pellet or is additionally fed, forms a compound which is insoluble or difficultly soluble in water.

This coating can alternatively be effected by spraying a dissolved or dispersed liquid reagent onto the pellets which by reaction with the pellet material itself forms a compound which is insoluble or difficultly soluble in water, so that a protective casing is formed which hinders diffusion of moisture. Dilute acids or alkalis, or organic solvents such as petrol, benzene, methanol, alcohol, carbon tetrachloride, paraffin or the like can be used as such a reagent, according to the material to be pelletised and the manner of its further processing.

The feed quantity is so determined, according to the type of material and its reaction products, that either a complete conversion reaction takes place or only a partial conversion occurs.

The coating is advantageously effected in a ploughshare mixer with a peripheral drum speed of 0.120 to 0. 180, and preferably 0.165 m/sec, and an operating temperature of 30"C. The temperature rise is limited to a maximum of 10"C during the coating stage by controlling the liquid feed. In this respect, the quantity of wetting liquid to be fed is 2 to 5% by weight and preferably 3.5 to 4,5% by weight with respect to the weight of the material to be pelletised, and the quantity of bonding agent to be fed is three to four times, and preferably three times the weight of the wetting liquid used.

The invention will now be described hereinafter with reference to an example.

EXAMPLE Glass is manufactured basically by fusing sand, soda and lime in quantities of about 12% Na20 and CaO by weight, and about 76% of SiO2 by weight. The production of pellets with a uniformly distributed quantity of the component materials by simple build-up agglomeration with water has not heretofore been possible. Instead, the quantities of the materials distributed in the pellets is nonuniform, and in addition a residual quantity of non-pelletisable sand remains, which can be between 10 and 25% of the total quantity of the component material.

In accordance with the process of the invention, about 150.0 kg of washed sand and 6.0 kg of water are fed to a pelletising ploughshare mixture of V = 300 litres, and are mixed intensively until the surfaces of the sand grains are completely wetted. About 18.0 kg of quicklime, CaO, are then mixed in until the water adhereing to the surfaces of the grains has reacted in accordance with the equation CaO + H20 = Ca(OH)2. As this reaction is exothermic, the end of the reaction is indicated by the absence of any further production of heat.

On further feeding of water as pelletising liquid, pelletising takes place together with simultaneous reaction of the quicklime which has remaimed unreacted in the first stage of the process, and if necessary with the feed further flux materials. In preparing a raw material for glass, further flux materials in the form of about 26.0 kg of soda and an additional 16.0 kg of quicklime are added, together with a total of 26.0 kg of water as pelletising liquid. The result is table pellets which do not disintegrate even under storage pressure or transportation and which show no exposed sand grains even under a microscope.

Claims

1. A process for pelletising substances of crystalline or crystal-like structure and comprising mixing intimately the substance to be pelletised with such a quantity of a mineral substance which reacts with water to form a water-insoluble compound, as to obtain a microfilm layer on the material grains, and then agglomerating the mixture so produced by the use of a pelletising liquid.

2. A process as claimed in claim 1, wherein the mineral substance for forming the microfilm layer is a hydroxide of a metal of main Group II or subsidiary Group II of the periodic table.

3. A process as claimed in claim 2, wherein the surface of the material to be pelletised is firstly wetted with water, and then the metal oxide coating material is fed in dry form and intimately mixed with the substance to be pelletised, agglomeration then taking place during the feed of pelletising liquid, until the consequent reaction terminates.

4. A process as claimed in any one of claims 1 to 3, wherein the pellets are provided with a coating of a material which is insoluble or difficultly soluble in water.

5. A process as claimed in claim 4, wherein the coating is of a mineral substance which reacts with the pelletising liquid which either emerges from the pellet or is additionally fed, from a compound which is insoluble or difficultly soluble in water.

6. A process as claimed in claim 5, wherein the coating is of an oxide of a metal of main Group II or subsidiary Group II of the periodic table.

7. A process as claimed in claim 4, wherein the coating is made by spraying a dissolved or dispersed liquid reagent which, by reaction with the pellet material, forms a compound which is insoluble or difficulty soluble in water, so as to form a protective casing which hinders diffusion of moisture.

8. A process as claimed in claim 7, wherein a dilute acid or alkali, or an organic solvent is used as the reagent.

9. A process as claimed in claim 8, wherein the reagent is petrol or benzene or methanol or alcohol or carbon tetrachloride or paraffin.

10. A process as claimed in any one of claims 7 to 9, wherein the reagent is sprayed in a quantity which results in reaction on the pellet surface only.

11. A process as claimed in any one of claims 1 to 8, wherein the coating is carried out in a ploughshare mixer with a peripheral drum speed of 0.120 to 0.180 m/sec and an operating temperature of 30"C or substantially 30"C.

1 2. A process as claimed in claim 11, wherein the temperature rise during the coating stage is limited to a maxim of 10"C by controlling the application of wetting water or feed of the coating substance, the coating stage lasting for five minutes.

1 3. A process as claimed in any one of claims 1 to 12, wherein the quantity of wett ing water is 2 to 5% by weight of the substance to be pelletised.

14. A process as claimed in claim 13, wherein the coating material is fed in a quan tity which is three to four times the weight of the wetting liquid used.

1 5. A process for pelletising substances of crystalline or crystal-like structure substantially as hereinbefore described with respect to the foregoing Example.

1 6. A pelletised substance of crystalline or crystal-like structure when produced by the method of any one of claims 1 to

1 5.