GB2117748A - A process for stabilising unstable pellets of pulverent mineral materials - Google Patents

Abstract

A process for stabilising pellets for agriculture and building construction which consist of pulverulent mineral materials such as iron ores, dolomite, or carbonate or phosphate salts and which, after agglomeration, are unstable by virtue of re-drying, hygroscopy or reaction with the atmosphere. After their formation by a known granulation build-up method using pelletising liquid, the pellets are provided with a coating of a mineral material which is either insoluble or difficultly soluble in water.

Description

SPECIFICATION A process for stabilising unstable pellets of pulverent mineral materials The invention relates to a process for stabilising pellets for the chemical industry, the glass and ceramics industry, agriculture, building construction or the like and in which the pellets consist of pulverulent, granular or dusty mineral materials such as ores, dolomite or carbonate or phosphate salts and which, after agglomeration, are unstable by virtue of re-drying, hygroscopy or reaction with the atmosphere.

In industry, pulverulent, granular or dusty materials occur as raw materials or waste materials in large quantities, and their storage, transport and further processing in this form gives rise to considerable difficulties for various reasons. For example, dusts from gas cleaning plants, cement dusts and other similar dusty materials give off a large fraction of dust on loading, transporting and depositing, thus leading to losses and considerable environmental pollution. Hygroscopic materials, such as mineral fertilisers and building materials, according to their degree of hygroscopicity and capacity for binding water, can sometimes form lumps and even, when of high water binding capacity, can sometimes become runny, and are then completely unsuitable for transportation and, where appropriate, further processing.There are also those groups of minerals which react chemically with oxygen or some other component of the air, and thus undergo a change in their material composition on long storage, which can also make them unsuitable for further processing. Finally, there are materials, such as smeltable ores, which are mainly composed of dusts or small material, but which can be further processed in a blast furnace either only or at least partially in the form of pieces.

Various measures have been taken in practice in order to obviate these drawbacks, according to the type of problem, but overall these measures are very costly and obviate the drawbacks only partly or insufficiently.

Thus for example materials which are hygroscopic or react with components of the atmosphere have been stored and transported in lined bags, which on the one hand adds a considerable packing cost and, compared with normal conveying in belt or pipe conveying plants which can be used for dusty or granular materials, considerable transport costs.

However, even this does not lead to a completely satisfactory solution, because, after opening the package, the contents, if not directly used, are again exposed to undesirable air and thus moisture entry. Storage and transport in durable sealable sheet metal containers is not practical for cost reasons. Even the production of nitrate-containing fertilisers with high acid contents, for the purpose of avoiding ammonia losses in mineral fertilisers, does not lead to a satisfactory result. The nitrogen oxides which form in these mineral fertilisers on the one hand likewise represent a loss of nourishing material, and on the other hand are also extremely health-damaging and form such aggressive compounds that after a short time they decompose the package material.

Even agglomerating these materials, by pelletising them by a build-up method using pelletising liquid, normally water, has up to now not given a satisfactory result. This is certainly the case with materials which are hyrroscopic and react chemically with the atmosphere, because pelletisation by buildingup does not eliminate the causes of the change, namely the hygroscopicity and the chemical reactability. These drawbacks are not present in the case of a large number of the other indicated materials, such as ores, coal dusts and most precipitated dusts. However, these materials, in spite of their agglomerability, have not been subjected to pelletising by the build-up method, because after pelletising they lose the pelletising liquid to the atmosphere due to re-drying, after which they disintegrate into their original dust form under the slightest compression stresses.

According to the invention, there is provided a process for stabilising pellets of pulverulent mineral materials which, after agglomeration, are unstable by virtue of redrying, hygroscopy or reaction with the atmosphere, the method comprising, after formation of the pellets by a known granulation build-up method using a pelletising liquid, providing the pellets with a coating of a mineral material which is either insoluble or difficultly soluble in water.

Thus, a process is provided for pelletising materials which are known to be basically accessible to agglomeration with a pelletising liquid, but which heretofore have been considered unsuitable for the formation of durable agglomerates due to the fact that after a short time they change their consistency either by virtue of subsequent drying, or subsequent water absorption due to their hygroscopic nature, or by chemical reaction with substances contained in the surrounding atmosphere, to such an extent that the disintegration of the formed agglomerates results.By coating the surface of the formed pellets or granulates with a layer of a mineral which is insoluble or difficultly soluble in water, the entry of moisture or other components of the atmosphere into the pellet material is reliably prevented, so that no only the physical properties of the pellet, in particular its solidity, but also its chemical stability are reliably assured.

Any material which satisfies the conditions can be used as the coating material. The hydroxides of the metal of the main Group II and subsidiary Group II of the periodic table can be advantageously used. These metal hydroxides, on the one hand, representing mineral materials which either, in most processes, give rise to no disturbing reactions or indeed present advantages when added, and, on the other hand, can be applied to the pellets in a technically extremely simple manner.

For example, they can be applied by firstly processing the material to be agglomerated in known manner by feeding pelletising liquid, and then applying the coating material in the form of a dry metal oxide, this being then hydroxylated by means of the surface moisture or by adding water.

The choices of coating material is determined by the type of pellet material and its further use. Thus, a preferred coating material can be calcium hydroxide, Ca(OH)2, which can be advantageously used in most applications of mineral materials, for example in agriculture for neutralising acid soil, in building construction as a bonding material, and in metallurgical smelting as a basic material for slag formation.

The coating can be made by spraying a dissolved or dispersed liquid reagent which by reaction with the pellet material forms a compound which is insoluble or difficultly soluble in water, so that a protective casing is formed which hinders moisture diffusion. Dilute acid or alkali, or organic solvents such as petrol, benzene, methanol, alcohol, carbon tetrachloride, paraffin or the like can be used as a reagent of this kind, depending on the pellet material and the method of its further processing.

The reagent which reacts with the pellet material is preferably fed in such a quantity that only a partial conversion of the pellet material takes place during the conversion reaction. An example of the use of this embodiment of the process can be the pelletising of natural phosphates, such as phosphorite and apatite used in agriculture, or the pelletising of dolomite (calcium magnesium carbonate). In such cases, the coating is preferably carried out by spraying dilute sulphuric acid on to the pellets or by the direct use of dilute sulphuric acid as the pelletising liquid in the final stage of the pelletising operation, the quantity of sulphuric acid used being such that the conversion of the tertiary phosphate with the simultaneous formation of anhydrite (CaSO4. H20) proceeds only as far as calcium hydrogen phosphate (CaHPO4).The casing which is formed in this manner consists of calcium hydrogen phosphate which is soluble only in mineral acids, and insoluble anhydrite, so that the entry of moisture into the core of the pellet and any diffusion of moisture into the atmosphere are prevented, whereas because of the solubility of the calcium hydrogen phosphate in the mineral acids of the soil, the granule can be decomposed.

The coating is preferably carried out in a ploughshare mixer having a drum peripheral speed of 0.1 20 to 0.180 m/sec. and preferably 0.165 m/sec. and an operating temperature of 30C. The temperature rise during the coating being limited to a maximum of 10"C by controlling the liquid feed. In this respect, the amount of bonding agents such as CaO or CaSO4 used for bonding 1 mole of water per mole of substance maybe 2 to 5% by weight, and preferably 2 to 3% by weight with respect to the total weight of finished pellets.

The invention will now be described hereinafter with reference to some examples.

EXAMPLE 1 About 40.0 kg of dolocat (a mixture of dolomite and calcium carbonate) are weighed into a pelletising ploughshare mixer of V = 300 litres, and are processed into pellets after feeding 7.0 kg of water as the pelletising liquid. After a mixing time of 15-20 minutes, 3.0 kg of quicklime are fed, and are intensively mixed with the pellets until there is no further rise in temperature. Hydroxylation in accordance with the reaction CaO + H20 = Ca(OH)2 has then terminated, and the resultant pellets are durably firm, storable and transportable, for use as fertiliser or in the fermentation industry.

EXAMPLE 2 About 50 kg of calcium phosphate and 11.25 kg of water (as the pelletising liquid) are weighed into a pelletising ploughshare mixer of V = 300 litres, and processed into pellets. 2.75 kg of concentrated sulphuric acid (98.0-99.5% by weight) are now fed, and are uniformly distributed over the pellet surface by mixing at a temperature of 50-60"C. The formation of the casing proceeds on the surface in accordance with the reaction CA3(PO4)2 + H2S04 = CaSO4 + 2CaHPO4, followed by CaSO4 + H2O = CaSO4. H20. The resultant pellets are mechanically firm, stable, storable and transportable, and can be used in practice in the normal manner.

Claims (12)

1. A process for stbabilising pellets of pulverulent mineral materials which, after agglomeration, are unstable by virtue of redrying, hygroscopy or reaction with the atmosphere, the method comprising, after formation of the pellets by a known granulation build-up method using a pelletising liquid, providing the pellets with a coating of a mineral material which is either insoluble or difficultly soluble in water.

2. A process as claimed in claim 1, wherein the coating mineral material is a hydroxide of a metal of main Group ll or of subsidiary Group II of the periodic table.

3. A process as claimed in claim 2, wherein the coating material is applied to the pellets in the form of a dry metal oxide by rolling or by sprinkling-on, and is then hydroxylated by water absorption from the pellet or by feeding further pelletising liquid.

4. A process as claimed in claim 1, wherein the coating is by spraying a dissolved or dispersed liquid reagent which, by reaction with the pellet material, forms a compound which is either water-insoluble or difficultly soluble in water, so that a protective casing is formed which prevents diffusion of moisture.

5. A process as claimed in claim 4, wherein the reagent is a dilute acid or alkali, or is an organic solvent, the reagent being chosen in accordance with the pellet material and the manner in which it is to be further used.

6. A process as claimed in claim 5, wherein the reagent is petrol or benzene, or methanol or alcohol or carbon tetrachloride or paraffin.

7. A process as claimed in any one of claims 4 to 6, wherein the reagent is fed in such a quantity that, during the conversion reaction, only a partial chemical conversion of the pellet takes place, so that the properties of the initial pellet substance are not substantially changed or impaired.

8. A process as claimed in claim 1 to 7, wherein the coating is carried out in a ploughshare mixer with a peripheral drum speed of 0.1 20 to 0.180 m/sec. and an operating temperature of 30"C or substantially 30"C.

9. A process according to claim 8 wherein the peripheral seed of the drum of the ploughshare mixer is 0.165 m/sec.

10. A process as claimed in claim 8 or claim 9 wherein the temperature rise during the coating is limited to a maximum of 10"C by controlling the feed of liquid and wherein the coating step is performed for five minutes.

11. A process as claimed in any one of claims 1 to 3, wherein the coating material is fed in a quantity of 2 to 5% by weight of the finished pellets for bonding 1 mole of water per mole of pulverent mineral material.

1 2. A process for stabilising pellets substantially as hereinbefore described with reference to either of the foregoing Examples.

1 3. A stabilised pellet when made by the process of any one of claims 1 to

1 2.