MXPA00003246A - Mineral pelletisation - Google Patents

Abstract

The present invention is directed to a process of making fired mineral pellets by mixing particulate mineral material with moisture and binder comprising substantially water soluble organic polymer to form a moist pelletisable mix, pelletising the moist mix by a tumbling process to form green pellets and firing the green pellets, characterised in that the binder comprises alkali metal silicate in a dry weight amount which is either (a) above 0.13%based on moist mix or (b) above 0.08%based on moist mix and at least three times the dry weight of substantially water soluble organic polymer.

Description

FORMATION OF MINERAL PELLETES This invention relates to the production of pellets of iron ore and other minerals, and with the novel pellets themselves. It is a standard practice to make pellets of calcined ore by a process comprising mixing the particulate mineral material with moisture and a binder, to form a mixture that can be formed into wet pellets, pelletizing the wet mixture through a drum mixing process to form crude pellets and calcining the crude pellets to form pellets of calcined ore. Traditionally, the binder was bentonite. This gave raw pellets of adequate strength and calcined pellets that had good strength and other properties, but which were accompanied by certain disadvantages. Therefore, there has been considerable interest in the use of binders, which comprise an organic polymer substantially soluble in water, as a partial or complete replacement of the bentonite. Polymers that have been described for this purpose include natural polymers, such as starches, celluloses and gums, and synthetic polymers such as anionic acrylates, as described, for example, in EP-A-225,171 and in US Pat. WO93 / 03190. When a synthetic polymer is used, it is often accompanied by monomeric inorganic or organic electrolytes, or other additives, of which, sodium carbonate and sodium silicate are generally preferred. For example, such additives are described in EP-A-225, 171, as being used in an amount, which may be up to 150% based on the soluble polymer or below 0.1% and usually below 0.02. % based on the mineral. In practice, the minerals that have been most widely used are mixtures of about 1 to 2 parts by weight of a polymer substantially soluble in water, and 1 part by weight of sodium carbonate. The typical dose of polymer binders substantially soluble in water is generally in the range of 0.01 to 0.1%, but amounts of up to 0.2% have also been mentioned. This is much lower than the typical bentonite dose, which is typically around 0.5 to 1%, often about 0.7%, based on the wet mix. It is desirable that the pellets have a narrow particle size distribution and must acquire a suitable particle size quickly, by conventional pelletizing techniques. One difficulty with existing binders based on organic polymers substantially soluble in water is that the particle size distribution is often very wide. Another problem is that the speed of growth of the pellet and / or the final size of the pellet often tends to be less than what would be desirable. The broad size distribution, which is often obtained, is broader than desired (even, sometimes, when using bentonite as a binder), it is undesirable, due to the inconvenience of having oversized and undersized pellets, and requires, in some cases, crush and / or recycle the pellets of the wrong size. Therefore, an object of the invention is to modify the binder, so as to improve the growth of the pellet, with respect to the size distribution, and / or with respect to the speed of growth and / or final size. Another object is to provide an improved combination of dry strength, wet strength and number of stocks, and in general, improved properties in the final pellets. According to the invention, we use a binder, which comprises the organic polymer substantially soluble in water, and also an alkali metal silicate, and we use this alkali metal silicate in a dry weight amount, which is considerably above the amounts ever used or described in practice, as additives for the organic polymer binders, for example as in EP 225,171. One way to define the amount of the alkali metal silicate is to say that the amount, which is used, is above 0.13% based on the weight of the wet mix. Another way to define the amount of the alkali metal silicate is that it is above 0.08% based on the weight of the wet mixture and is at least three times the weight (on a dry basis) of the organic polymer substantially soluble in water. Thus, the invention includes processes in which the amount of alkali metal silicate is, for example, 0.08 to 0.12%, for example 0.1% or more, and in which is at least three times the dry weight of the organic polymer substantially soluble in water. Thus, in this embodiment, the amount of the alkali metal silicate is at least 300%, based on the polymer, in contrast to the proposed higher amount of 150% in EP-A-225, 171. In general, without However, the amount of alkali metal silicate is above 0.13%, in contrast to the upper limit of 0.1%, and the preferred upper limit of 0.02% proposed in EP-A-225, 171. The amount of the silicate of The alkali metal is, in practice, usually always significantly above the amount of organic polymer and is usually at least twice, and usually at least three times or more, the dry weight of the organic polymer. It is often at least four, and often at least six times, the dry weight of the polymer substantially soluble in water. It can be up to fifteen or twenty times, but usually there is no advantage in using an amount of the alkali metal compound, which is greater than about ten times the dry weight of the polymer substantially soluble in water. The amount of the alkali metal silicate is usually at least 0.15% and preferably at least 0.18% by weight, based on the weight of the wet mix, and is generally at least 0.2%. It can be up to 1% or even 2.%, but there is usually no advantage in using more than 0.7%, and 0.5% is often a convenient upper limit. If an inadequate alkali metal compound is used, there will be inadequate performance improvement, for example, pellet growth and / or properties of raw pellets or calcined pellets. Furthermore, it appears that the alkali metal silicate contributes to the metallurgical properties in a manner somewhat similar to the manner in which the bentonite contributes, as a result of the slag or other bond during the calcination process. It appears that the invention can, surprisingly, have resulted in the achievement of inorganic binding mechanisms, within the pellets, somewhat similar in quality and quantity to those conventionally achieved using approximately 0.7% bentonite, but without the attendant disadvantages of the bentonite. The alkali metal silicate is usually a sodium silicate, but other alkali metal silicates can be used. The ratio of Na20: SiO2 can be anywhere in the normal range of 2: 1 to 1: 5. Therefore, a preferred type of sodium silicate is sodium metasilicate. Another preferred type of silicate is the material commercially available under the name aterglass. Although Waterglass gives very good results, in some cases it is preferable to use a sodium silicate powder, in which case, the sodium metasilicate or other powder silicate, for example, having a ratio of Na20: SiO2 of 2: 1 to 1: 2.5 or 3 is often preferred. The powder silicate will be finely divided or it will be in the form of an aggregate, which is degraded during the pellet formation to finely divide the primary particles. The organic polymer substantially soluble in water may be added as a solution or as a substantially anhydrous emulsion of dispersed polymer particles in a water-immiscible liquid or as an anhydrous emulsion of dispersed polymer particles in a water-immiscible liquid. Preferably, however, the polymer is added as a powder. The powder may consist of primary particles having a desired particle size, for use in the wet mix or may consist of aggregates, which disintegrate during mixing with the ore particles, to form the desired primary particle size. Generally, the primary particle size is at least 90% by weight below 200 μm and often below 150 μm, but usually it is 200 μm. Frequently, the best results are obtained when at least 90% by weight of the polymer particles in the range of 20 to 100μm, but a satisfactory product is 98% by weight below 200μm and at least 50% by weight below lOOμm, and generally above 20μm. The amount of the polymer should be sufficient for the raw pellets to have suitable properties and, in practice, is usually typical of the amount of organic polymer, which is conventionally used. Thus, the amount is usually in the range of 0.01 to 0.2%, usually 0.01 to 0.1% by weight, based on the wet mixture. Generally, the amount is in the range of 0.01 or 0.02 to 0.05%. The polymer can be a natural polymer (which is why we include modified natural polymers), such as cellulosic polymers, starch polymers and gums. Binders for the formation of pellets based on such materials are known and can be used in the invention. Preferably, however, the polymer is a synthetic polymer formed from a monomer or mixture of ethylenically unsaturated, water-soluble monomer. The polymer can, with some mineral ores, be cationic, for example when it is formed of a monomer mixture containing a cationic monomer, such as the dialkylaminoalkyl (meth) acrylate or acrylamide, usually as quaternary ammonium or acid addition salt. or diallyl diamonium chloride. Usually, however, the polymer is anionic as a result of being formed of an anionic monomer, such as an ethylenically unsaturated sulphonic acid, or more commonly, a carboxylic acid, such as acrylic acid or methacrylic acid. The polymer can, instead, be non-ionic, for example, being a substantial acrylamide homopolymer. Preferably, the polymer is a copolymer of acrylamide with an anionic or cationic monomer, with the preferred polymers being copolymers of 10 to 50% (often 15 to 30%), by weight, of sodium acrylate, with the remainder being acrylamide. Typically, the polymer has an intrinsic viscosity of 3 to 16dl / g, the intrinsic viscosity being measured by a suspended level viscometer in a 1N sodium chloride solution, buffered at a pH of 7 to 25 ° C. The polymer can be substantially linear to maximize solubility, as a result of being made only of the monoethylenically unsaturated monomers. However, it is often preferred that the polymer has some crosslinking in an insufficient amount to significantly insolubilize the polymer. For example, the polymer can be made in the presence of 5 to 50 ppm of a crosslinking agent. Suitable substantially water soluble polymers made in the presence of a crosslinking agent are described in more detail in WO 93/03190 and reference is made to this for further details thereof. Blends of synthetic and natural polymers can be used, for example, mixtures of guar gum and synthetic anionic polymer of the types discussed above. The mixture that can be formed into pellets should contain enough moisture to promote the formation of pellets without being so wet that it inhibits satisfactory results. The total moisture content of the wet mixture is usually in the range of 5 to 15% water, based on the total mixture (as measured by weighing the mixture before and after heating to a constant weight at 105 ° C) . Generally the amount of humidity is at least 8%, but usually it is not more than 12%. The precise optimum in any particular process tends to vary according to the materials that are being used. If the initial mineral is wet and / or if the alkali metal silicate is introduced as a solution (eg Waterglass) and / or if the polymer is introduced as a solution or an aqueous emulsion in a non-aqueous liquid, it is generally unnecessary to add water separated to the mixture to obtain the desired moisture content, although this can be done if desired. The best results are usually obtained based on the moisture naturally present in the particulate mineral and then adding the polymer, and preferably also the silicate, as a powder. The binder may include minor amounts of any of the monomeric electrolytes or other additives conventionally included with organic polymers, such as any of the materials listed in EP-A-225, 171, in the amounts proposed therein. When the additives include silicate, then, for the purposes of the present specification, this silicate is considered as part of the alkali metal silicate in the binder. Thus, the binder may include sodium carbonate in a small amount. Preferred binders of the invention are novel and comprise 1 part of the polymer substantially soluble in water, from zero to 1.3 parts of sodium carbonate and from 2.5 to 15 parts, preferably from 3 to 10 parts, of sodium silicate (all parts by weight). The binder system may additionally contain an insoluble crosslinked polymer, as in EP 225,171. The binder is generally completely or substantially free of bentonite, for example because the alkali metal compound is proposed to give some or all of the effects of the bentonite, without needing to include the bentonite in the mixture. Therefore, the amount of bentonite which is added in the wet mix will always be much lower than the conventional one as a binder, and therefore, it is usually below 0.03% by weight and preferably below 1.0%. and more preferably less than 0.05% by weight of the wet mixture.

Generally, bentonite is not added to the wet mix as a binder. Generally, the particulate mineral material that is used to form the wet mixture is substantially free of bentonite clays. The particulate mineral which is used in the invention is usually a metal mineral and is preferably iron ore. The mineral particles from which the pellets are formed are usually at least 90% by weight, and preferably at least 98% by weight below 500μm and generally below 250μm and preferably below 150μm, by weight. example 80 or 90% being less than 50μm, or in some cases, lOOμm. The minimum particle size is usually at least 90% by weight above 10 μm. The wet mixture is formed by mixing the particulate mineral with the polymer, the alkali metal compound and other additives in a conventional manner, for example by mixing with a paddle mixer. The wet process is then formed into pellets by a drum-mixing process, which is why we mean that the pellets are not formed by extrusion or other compression molding processes, but instead are formed by mixing Typically, the formation of pellets is conducted using a drum to form balls or a rotating inclined disc The pelletizing processes for forming pellets of ore by these and other conventional drum mixing processes are well known and in the invention, are usually operated to give a range of particle size, which is relatively narrow and generally falls somewhere Rang of 9-16mp interval ?. For example, as much as, for example above 25% by weight and preferably above 50%, it is preferably within a range of 2 or 3mm. The crude pellets are subjected to calcination in a conventional manner, for example at a temperature of up to above 1000 ° C and typically not more than 1700 ° C, for example 1200 to 1500 ° C, or preferably, of 1250 ° C. 1400 ° C. By means of the invention, it is possible to improve the growth of the pellet and the size distribution. Instead of, or in addition to this, it is possible to obtain improved physical properties of the crude pellets or of the pellets burned, for example, as indicated by improving the dry strength and / or the wet strength and / or number of broths. , compared with the corresponding pellets made with the same binder, but with the amount of sodium carbonate and / or silicate being below 150% by weight of the polymer. Pellets made by the processes of the invention may have superior properties with respect to wet strength and / or dry strength or number of falls (when tested by conventional laboratory tests to determine such values), compared to pellets made in the absence of a large amount of alkali metal silicate.

The invention includes the mineral pellets, raw, novel, which are made in the process of the invention. These pellets are formed of mineral particles bonded together by a binder comprising an organic polymer substantially soluble in water and an alkali metal silicate, in an amount as defined above. The invention also includes mineral pellets, calcined, which are obtained by the defined process. The following examples are examples of the invention.

Example 1 The tests were conducted in a mixture of particulate mineral, which contains magnetite and, as the main component, hematite. In a test suite, a wet mix was formed into pellets in a conventional manner, using approximately 0.6% bentonite as the binder. The particle size distribution was broad, with only 15.6% by weight of the pellets falling within the preferred size range of 11.2 to 13.2 mm (this value is the average of four tests). The process was repeated using, as a binder, a mixture of 0.0129% sodium carbonate and 0.03% of a polymer of 80% by weight of acrylamide and 20% by weight of sodium acrylate, made in the presence of a small amount of methylene bis acrylamide and having an intrinsic viscosity of approximately 6dl / g, as described in WO93 / 03190. The moisture content of the mixture was 8%. The particle size distribution was again wide, the average of five tests showed 16.3% by weight of the pellets falling within the defined size range. When this test was repeated, but with the addition of 0.46% (dry weight) of Waterglass, the particle size distribution was much narrower, with the average of four tests showing 49.4% by weight of the particles within the range of defined size.

Example 2 A series of tests was conducted using the same 0.0129% sodium carbonate, and 0.03% polymer binder as in Example 1, and a particulate magnetite concentrate having a moisture content of 9.65%. The effect of adding different amounts of sodium silicate (in the form of powdered metasilicate) in the proportion of the pellets in the fraction of defined size, and the wet and dry strengths and the number of falls of the resulting pellets, It is shown in the following table.

Tests marked with a * were conducted in the absence of a binder.

Example 3 Two tests were conducted on a magnetite concentrate containing 9.5% moisture. The magnetite was formed into pellets using, as a binder, 0.0129% sodium carbonate, 0.09% sodium silicate and 0.03% of a polymer of 80% by weight of acrylamide and 20% by weight of sodium acrylate made in the presence of a small amount of methylene bis acrylamide and having an intrinsic viscosity of about 7 dl / g, as described in WO 93/03190. The second test of the process was repeated, but in the absence of sodium silicate. The effect on the particle size distribution, the resistance characteristics and the average number of falls of the pellet are shown in the following table.

Claims (13)

1. A process for making pellets of calcined ore, by mixing the particulate mineral material with moisture and a binder comprising an organic polymer substantially soluble in water, to form a mixture that can be formed into pellets moisture, pelletizing the wet mixture by a process of mixing in a drum, to form crude pellets and calcining the crude pellets, characterized in that the binder comprises an alkali metal silicate in a dry weight amount, which is either (a) above 0.13% based on the wet mix or, (b) above 0.08% based on the wet mixture and at least three times the dry weight of the organic polymer substantially insoluble in water.

2. The process according to claim 1, characterized in that the amount of the alkali metal silicate is at least 0.18%.

3. The process according to claim 1, characterized in that the amount of the alkali metal silicate is 0.2 to 0.5%.

4. The process according to any of the preceding claims, characterized in that the amount of the alkali metal silicate is four to fifteen times the dry weight of the polymer.

5. The process according to any of the preceding claims, characterized in that the alkali metal silicate is selected from sodium metasilicate and Waterglass.

6. The process according to any of the preceding claims, characterized in that the alkali metal silicate is added as a powder.

The process according to any of the preceding claims, characterized in that the polymer is added as a powder and the moisture content of the wet mixture is 5 to 15%.

The process according to any of the preceding claims, characterized in that the polymer is selected from cellulosic polymers, starch polymers, gums and anionic or cationic polymers of a monomer or mixture of unsaturated monomer ethylenically soluble in water.

The process according to any of the preceding claims, characterized in that the polymer is formed from 10 to 50% by weight of sodium acrylate or methacrylate and from 50 to 90% by weight of acrylamide.

10. The process according to any of the preceding claims, characterized in that the particulate mineral is particulate iron ore.

11. The process according to any of the preceding claims, characterized in that the particulate mineral material has a particle size of at least 90% by weight below 250μm.

12. Crude ore pellets, formed of mineral particles bonded together by a binder, comprising an organic polymer substantially soluble in water and an alkali metal silicate in a dry weight, which is either (a), above 0.13 % based on the pellets or, (b) above 0.08% by weight, based on the pellets and at least three times the dry weight of the organic polymer substantially insoluble in water.

13. The calcined ore pellets obtainable by a process according to any of claims 1 to 12.