WO2002064846A1

WO2002064846A1 - Silicate binders including calcium bearing curing agent

Info

Publication number: WO2002064846A1
Application number: PCT/GB2002/000430
Authority: WO
Inventors: Jennifer Emma Nicholls; David Peter Carter; Robert Macdonnell Hunter; Mfanafuthi Shadrack Nkutha
Original assignee: Ineos Silicas Ltd
Current assignee: Ineos Silicas Ltd
Priority date: 2001-02-12
Filing date: 2002-01-31
Publication date: 2002-08-22
Anticipated expiration: 2003-08-12
Also published as: EP1360339A1; ZA200305964B; GB0103278D0

Abstract

A method of binding the particles of a particulate material comprises preparing a composition comprising the particulate material, an alkali metal silicate and a curing agent, forming said composition into a shaped body and allowing said mixture to cure so as to form a rigid shaped body, wherein said curing agent is a composition comprising an intimate mixture of calcium carbonate and calcium carbonate and calcium oxide in which the calcium oxide comprises from 10 to 90 per cent by weight of the intimate mixture. The method is useful in agglomerating fine materials, particularly for use in extractive industries, a typical example being fine particulate chromite ore.

Description

SILICATE BINDERS INCLUDING CALCIUM BEARING CURING AGENT

This invention relates to the use of alkali metal silicates for binding or agglomerating particulate material and, in particular, to the use of alkali metal silicates in combination with a curing agent to bind particulate material.

The need to agglomerate particulate materials to assist in the processing or handling of such materials exists in many industries and one such industry is the mining and extractive industry.

During the mining and processing of ores large amounts of fine material are generated. In the reduction of chrome based ores to produce ferrochrome the smelting furnaces currently in operation cannot handle large concentrations of fine material so there is a need to agglomerate this material. Agglomeration processes currently in use include processes which produce pellets, briquettes and blocks with a variety of shapes using a variety of binder systems such as sodium silicate, bentonite, cement or lime combined with molasses.

Commercial methods for agglomerating chrome ore fines utilising bentonite as a binder include the Outokumpu process (South African patent 78/4945) and Showa Denko process (US Patent 3 993 471 and South African Patent 83/7277) and a commercial method using sodium silicate as a binder in a pelletisation process is disclosed in a patent by J.J. Marsh (South African Patent 95/10532). The capital investment required for these process routes is very large and high temperatures are required for the curing processes.

An alternative to the above process routes is a block making process which requires much less capital investment, the blocks being cured at temperatures between

20° C and 50° C as compared to 1400° C for both the Outokumpu and Showa Denko processes and 300° C to 400° C for the agglomerating process that utilises sodium silicate. Thus the block making process is much less expensive to operate.

The block making process comprises mixing the mineral fines with a binder, forming the mixture produced into a block, compacting under pressure and/or consolidating by vibrating in a mould, demoulding and then allowing the blocks to harden at ambient temperature so that the blocks are hard and do not break easily after 24 hours.

The most commonly used binder for the blocking of chrome ore fines is cement because it allows the blocks to be shaped before they set. The use of cement is, however, undesirable because the amount of cement used leads to a decrease in the productivity of a smelting furnace and impurities introduced with the cement (such as sulphur and phosphorus) can affect the quality of metal eventually produced from the ore.

Sodium silicate has been evaluated as an alternative binder in place of cement in the blocking of chrome fines. The result has been unsatisfactory due to the poor green strength of the blocks produced and the extended time period required for the curing of the blocks at temperatures between 20° C to 50° C. A method wherein a silicate can be employed in an efficient agglomerating process at ambient temperatures or just above has now been devised. The process can provide a delayed setting time, enabling bodies to be readily shaped and the bodies produced have a good green strength. According to the invention a method of binding the particles of a particulate material comprises preparing a composition comprising the particulate material, an alkali metal silicate and a curing agent, forming said composition into a shaped body and allowing said mixture to cure so as to form a rigid shaped body, wherein said curing agent is a composition comprising an intimate mixture of calcium carbonate and calcium oxide in which the calcium oxide comprises from 10 to 90 per cent by weight of the intimate mixture.

The calcium oxide and calcium carbonate are present as an intimate mixture and the curing agent is believed to be more effective in controlling the reaction rate of the alkali metal silicate if the curing agent comprises particles which contain both calcium oxide and calcium carbonate rather than consisting of a mixture of particles of calcium oxide and particles of calcium carbonate.

One particularly useful form of curing agent which can be employed in this invention is the by-product dust formed during calcination of limestone. During the calcination process a proportion of the lime becomes entrained in the exhaust gases and must be removed therefrom, typically by means of bag filters, wet scrubbers, electrostatic precipitators or gravel bed filters. The composition of this dust depends upon the conditions under which it was produced and the composition of the limestone used in the kiln. The major component is usually calcium oxide (lime) but, in view of exposure of the dust to carbon dioxide in the exhaust gases, a further significant component is calcium carbonate. Generally, sulphate, usually in the form of calcium sulphate is present and the major non-calcareous contaminants are compounds of silicon and aluminium, probably derived from clay which is present in the limestone.

Preferably, the curing agent contains from 20 to 80 per cent by weight calcium oxide and, more preferably, from 30 to 70 per cent by weight calcium oxide. Preferably, the curing agent contains at least 5 per cent by weight calcium carbonate and more preferably at least 15 per cent calcium carbonate. Generally, not more than 40 per cent by weight calcium carbonate is present.

The curing agent is preferably a finely divided solid and preferably it has a weight mean particle size below 100 μm, as determined by Malvern Mastersizer, using the method detailed hereinafter. More preferably, the curing agent has a weight mean particle size below 50 μm and a weight mean particle size below 25 μm is even more preferable. Usually, the curing agent will have a weight mean particle size above 5 μm. In general, any alkali metal silicate can be used in the process of the invention but sodium or potassium silicates are preferred and, generally, sodium silicate is more economical to use. Alkali metal silicates are available with a range of silicate to alkali metal ratios and, in general, silicates having any ratio are suitable for use in the invention. However, a silicate having a molar ratio of Si0₂ to M₂0, where M is an alkali metal, above 1.6 is preferred since such silicates are generally cheaper and are less corrosive than those having a lower Si0₂ to M₂0 molar ratio. Preferably, the molar ratio, Si0₂ to M₂0 is in the range 1.6 to 3.3. More preferably, the ratio is in the range 1.6 to 2.6 and, even more preferably, is in the range 1.8 to 2.3. The alkali metal silicate can be employed in any suitable form. Aqueous solutions having a range of concentrations and a range of Si0₂ : M₂0 ratios are readily available commercially and these are convenient for use in this invention. Generally, these solutions will have a solids content in the range 35 to 55 per cent by weight, expressed as weight of Si0₂ plus M₂0. More preferably, the solids content is in the range 40 to 50 per cent by weight, expressed as Si0₂ plus M₂0.

A wide range of proportions of alkali metal silicate to curing agent can be used. Generally, the ratio of silicate to curing agent is in the range 1 : 1 to 10 : 1 by weight and, preferably, the ratio of silicate to curing agent is in the range 1.5 : 1 to 7 : 1 , the silicate being expressed as weight of Si0₂ plus M₂0 present. There is no restriction on the nature of the particulate material which can be agglomerated using the silicate and curing agent. Typical particulate materials include powdered coal, crushed ores, particulate waste products, ferrochrome, silicon carbide, tailings from precious metal extraction and waste from the steel industry. One particularly useful embodiment of the method of the invention is the formation of agglomerated masses of ore particles which, in the non-agglomerated form, are too small to process effectively. The method of the invention has been found to be particularly useful in the recovery of small particles of chrome-bearing ores, particularly chromite ores, which would normally severely affect the efficiency and safe operation of the smelting furnace during processing, because of their size. In the method of this invention, when applied to such small particles, the particles which are agglomerated using alkali metal silicate and curing agent usually have a very wide range of particle size. The method is suitable for agglomerating material containing particles ranging in size from about 0.5 μm to about 30 mm.

It is preferable for the particulate material to have a low water content and preferred particulate material contains less than 5 per cent water by weight.

The rigid shaped bodies which are formed using the method of the invention may take any suitable form. The method can, for example, be used to produce granules or pellets in a granulator or pelletiser or to produce moulded or extruded bodies of any desired shape.

In a particular embodiment of the method of the invention the rigid bodies are blocks or bricks and these blocks are particularly preferred in the recovery of small particles of chrome ores.

In this particular embodiment, a pre-mix of the dry, particulate ore and the curing agent is first prepared. To this is added the alkali metal silicate solution. The components are usually mixed in proportions which provide from 1.0 to 4.5 parts, preferably from 1.5 to 3.5 parts, by weight of an alkali metal silicate expressed as weight of Si0₂ plus M₂0 present and from 0.25 to 2.0 parts, preferably from 0.5 to 1.5 parts by weight of the curing agent with respect to 100 parts by weight of the particulate ore, expressed as dry weight. After the components have been thoroughly mixed, the resultant mixture is transferred to a mould, compacted, demoulded and allowed to cure to form the rigid body. Usually, the mixture will cure at ambient temperature, but slight heating can accelerate the process. The mixture is usually cured at a temperature in the range 15 to 65° C. Generally, the blocks will be sufficiently hard to use after a curing period in the range 20 to 24 hours. The cured blocks preferably have an unconfined compressive strength of at least 1 MPa. Usually, the blocks have an unconfined compressive strength up to 3 MPa. These blocks can be used to satisfactorily process chrome ores which would normally be too finely divided to process satisfactorily.

Test procedures

Weight Mean Particle Size The weight mean particle size of materials used in this invention is determined using a Malvern Mastersizer model S, with a lens range up to 300 mm RF and MS1 sample presentation unit. This instrument, made by Malvern Instruments, Malvern, Worcestershire, uses the principle of Mie scattering, utilising a low power He/Ne laser. Before measurement the sample is dispersed in isopropanol for 1 minute using a 3000 rpm stirrer to form a suspension. This suspension is stirred before and whilst it is subjected to the measurement procedure outlined in the instruction manual for the instrument, utilising the 300 mm RF lens range in the detector system. The Malvern Mastersizer measures the particle size distribution of the inorganic material based on the volume of the particles. The weight mean particle size (d₅₀) or 50 percentile is readily obtained from the data generated by the instrument. Green Strength

This test is performed on moulded blocks which have been allowed to cure for 30 minutes after demoulding. A block is weighed and placed on a sieve shaker which is operated for 30 seconds at 50 cycles per second. The material is then separated into pieces larger than 15 mm across and fines (less than 15 mm across). The proportion of fines is determined by weighing.

The method of the invention provides an economical means of binding together particulate material to form agglomerates which are non-dusty and can be easily handled. The rate of cure of the agglomerates is such that the initial mixture can be prepared and formed into an appropriate shape before setting, but the body has an adequate green strength to allow demoulding after a short period and a sufficiently strong rigid body is produced in a reasonable time.

The invention is illustrated by the following non-limiting examples.

EXAMPLES

Example 1

In order to assess the performance of various silicate/curing agent combinations these systems were tested in the absence of particulate material, since it is easier to observe the curing in such model systems.

The curing agent used in this example was a by-product from a lime calcination process. It was recovered from the electrostatic precipitators used to remove dust from the exhaust gases on a South African lime calciner. The principal components of this material were as follows (weight percentages); the calcium content is a measure of the CaO, Ca(OH)₂ and CaC0₃ present : calcium (as CaO) 66.5% silicon (as Si0₂) 13.0% aluminium (as Al₂0₃) 7.4% iron (as Fe₂0₃) 1.9% magnesium (as MgO) 1.8% sulphur (as S0₃) 0.5% titanium (as Ti0₂) 0.4% manganese (as MnO) 0.3%

The loss on ignition (1250° C to constant weight) was 7.4% by weight. This sample was estimated to contain 22.2 % CaC0₃ by weight.

45 g of sodium silicate solution having an Si0₂ to Na_∑O molar ratio of 2.6 and a solids content of 43.6% by weight (Si0₂ plus M₂0) was placed in a 500 cm³ beaker. In a 200 cm³ beaker a slurry was prepared from a measured amount of curing agent and 24 g of water. This slurry was added to the silicate with gentle stirring (about 200 rpm). The stirring was continued until the mixture became very thick, after which the mixture was tested with a spatula for gelling. The gel point was taken to be the point at which the mixture ceased to flow and maintained a semi-solid structure. Results are given in Table 1 below.

Table 1

Example 2 Small scale agglomerated blocks were prepared as follows, using fine particulate chromite ore as used in current block-making processes employed in South Africa. This ore contained 3% moisture by weight. A dry mixture of 100 parts chromite ore and 1.5 parts of the curing agent used in Example 1 was prepared by blending in a beaker with a spatula. 5 parts by weight of sodium silicate solution, molar ratio of Si0₂ to Na₂0, 2.1 , solids content, 48.2 per cent by weight, was added to this mixture and quickly stirred in with a spatula. This resultant mixture was transferred to a cylindrical mould 32 mm diameter which was standing on a base plate. The mixture was consolidated within the mould using an appropriately sized ram and a light mallet. The moulded cylindrical block (32 mm diameter x approximately 35 mm high) was then removed from the mould and allowed to cure overnight at room temperature.

The unconfined compressive strength (UCS) was measured using a Zwick Universal Testing Machine Type No. Z030, 24 hours after the block was prepared. Two blocks were tested and found to have a mean UCS of 1.33 MPa with a standard deviation of 0.32 MPa.

Example 3

Cylindrical blocks were prepared in a similar manner to that described in Example 2, except that the chromite ore was dried in an oven at 105° C until it contained an undetectable amount of water and the blocks were cured with 5 weight parts of 2.5 molar ratio silicate solution having a solids content of 46J per cent by weight and 0.5 weight parts of the same curing agent as used in Examples 1 and 2 per 100 weight parts chromite ore.

The mean UCS after 24 hours was 4.36 MPa with a standard deviation of 0.64 MPa on 4 samples.

Example 4

Fine chromite ore, as used in Example 2, was mixed with the curing agent used in Example 1 in the ratio 100 parts ore to 0J5 parts curing agent by weight. 3.5 parts by weight sodium silicate solution, having a molar ratio of 2.5 : 1 , Si0₂to Na₂0, was added to the ore/curing agent mix with stirring and the resultant mixture formed into blocks as described in Example 2 above. After demoulding and standing at room temperature for 30 mins, the Green Strength was measured using the test described hereinbefore. The percentage of fines after testing was 28 per cent for blocks made according to this example and 70 per cent for blocks made without any curing agent.

Claims

1. A method of binding the particles of a particulate material comprising preparing a composition comprising the particulate material, an alkali metal silicate and a curing agent, forming said composition into a shaped body and allowing said mixture to cure so as to form a rigid shaped body, wherein said curing agent is a composition comprising an intimate mixture of calcium carbonate and calcium oxide in which the calcium oxide comprises from 10 to 90 per cent by weight of the intimate mixture.

2. A method according to claim 1 characterised in that the curing agent comprises particles which contain both calcium oxide and calcium carbonate.

3. A method according to claim 1 or 2 characterised in that the curing agent comprises by-product dust formed in the calcination of limestone.

4. A method according to any one of the preceding claims characterised in that the curing agent comprises from 30 to 70 per cent by weight calcium oxide.

5. A method according to any one of the preceding claims characterised in that the curing agent comprises at least 5 per cent by weight calcium carbonate.

6. A method according to any one of the preceding claims characterised in that the curing agent has a weight mean particle size below 100 μm.

7. A method according to any one of the preceding claims characterised in that the alkali metal silicate is sodium silicate.

8. A method according to any one of the preceding claims characterised in that alkali metal silicate has a molar ratio of silicate to alkali metal, expressed as Si0₂ : M₂0, where M is an alkali metal, in the range 1.6 to 3.3.

9. A method according to any one of the preceding claims characterised in that the silicate is in the form of an aqueous solution having a solids content in the range 35 to 55 per cent by weight, expressed as percentage Si0₂ plus M₂0 where M is an alkali metal.

10. A method according to any one of the preceding claims characterised in that the alkali metal silicate and curing agent are present in a ratio between 1 : 1 and 10 : 1 by weight, alkali metal silicate to curing agent, the alkali metal silicate being expressed as weight of Si0₂ plus M₂0 present, where M is an alkali metal.

11. A method according to any one of the preceding claims characterised in that the particulate material contains less than 5 per cent water by weight.

12. A method according to any one of the preceding claims characterised in that the amount of alkali metal silicate mixed with the particulate material is such that the shaped body comprises from 1.0 to 4.5 per cent by weight alkali metal silicate expressed as weight of Si0₂ plus M₂0 present, where M is an alkali metal.

13. A method according to any one of the preceding claims characterised in that the amount of curing agent mixed with the particulate material is such that the shaped body comprises from 0.25 to 2.0 per cent by weight curing agent.

14. A method according to any one of the preceding claims characterised in that the shaped body is a block having an unconfined compressive strength in the range 1 MPa to 3 MPa.