Mineral pelletisation

Details Mineral pelletisation Mineral pelletisation

2000-03-07

2001-09-25

King, Roy (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Processes

Consolidating metalliferous material by agglomerating,...

C075S773000, C075S321000, C075S322000

Reexamination Certificate

active

06293994

ABSTRACT:

This invention relates to the production of pellets of iron ore and other minerals, and to the novel pellets themselves.
It is standard practice to make fired mineral pellets by a process comprising mixing particulate mineral material with moisture and binder to form a moist pelletisable mix, pelletising the moist mix by a tumbling process to form green pellets and firing the green pellets to form the fired mineral pellets.
Traditionally the binder was bentonite. This gave green pellets of adequate strength and fired pellets having good strength and other properties, but which were accompanied by certain disadvantages. Accordingly there has been considerable interest in the use of binders which comprise substantially water soluble organic polymer, as partial or complete replacement for the bentonite. Polymers which have been described for this purpose include natural polymers such as starches, celluloses and gums, and synthetic polymers such as anionic acrylamides as described in, for instance, EP-A-225,171 and WO93/03190.
When a synthetic polymer is being used, it is often accompanied by inorganic or organic monomeric electrolytes or other additives, of which sodium carbonate and sodium silicate are generally preferred. For instance such additives are described in EP-A-225,171 as being used in an amount which can be up to 150% based on the soluble polymer or below 0.1% and usually below 0.02% based on the ore. In practice the materials that have been most widely used are mixtures of about 1 to 2 parts by weight substantially water soluble polymer and 1 part by weight sodium carbonate.
The typical dosage of substantially water soluble polymeric binders is generally in the range 0.01 to 0.1% but amounts of up to 0.2% have been mentioned. This is much less than the typical dosage of bentonite, which is typically around 0.5 to 1%, often about 0.7%, based on the moist mix.
It is desirable that the pellets should have a narrow particle size distribution and should acquire a suitable size rapidly by conventional pelletising techniques. A difficulty with existing binders based on substantially water soluble organic polymers is that the size distribution is often rather wide. Another problem is that the rate of pellet growth and/or the final pellet size often tends to be less than would be desirable.
The wide size distribution which is often obtained is broader than is desired (even, sometimes, when using bentonite as binder) is undesirable because of the inconvenience of having oversize and undersize pellets and the need, in some instances, to crush and/or recycle pellets of the wrong size. Accordingly one object of the invention is to modify the binder in such a way as to improve pellet growth as regards size distribution, and/or as regards rate of growth and/or final size. Another object is to provide an improved combination of dry strength, wet strength and drop number and, generally, improved properties in the final pellets.
According to the invention, we use a binder which comprises the substantially water soluble organic polymer 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 disclosed in practice as additives for organic polymer binders, for instance as in EP 225,171.
One way of defining 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 moist mix. Another way of defining the amount of the alkali metal silicate is that it is above 0.08% based on the weight of the moist mix and is at least three times the weight (on a dry basis) of the substantially water soluble organic polymer.
Thus the invention includes processes in which the amount of alkali metal silicate is, for instance, 0.08 to 0.12%, for instance 0.1% or more, and in which it is at least three times the dry weight of the substantially water soluble organic polymer. Thus, in this embodiment, the amount of the alkali metal silicate is at least 300% based on the polymer, in contrast to the upper proposed amount of 150% in EP-A-225,171.
Generally, 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 alkali metal silicate is in practice usually always significantly above the amount of organic polymer and is usually at least two times and usually at least three times or more the dry weight of the organic polymer. Often it is at least four, and frequently at least six times the dry weight of the substantially water soluble polymer. 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 substantially water soluble polymer.
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 moist mixture and generally is at least 0.2%. It can be up to 1% or even 2% but usually there is no advantage in using more than 0.7%, and 0.5% is often a convenient upper limit.
If inadequate alkali metal compound is used, there will be inadequate improvement in performance, for instance in pellet growth and/or in the properties of the green pellets or the fired pellets. Additionally, it seems that the alkali metal silicate contributes to the metallurgical properties in a manner somewhat similar to the manner in which bentonite contributes as a result of slag or other bonding during the firing process. It seems that the invention may surprisingly have resulted in achieving inorganic bonding mechanisms within the pellets somewhat similar in quality and quantity to those conventionally achieved using around 0.7% bentonite but without the accompanying disadvantages of bentonite.
The alkali metal silicate is usually a sodium silicate, but other alkali metal silicates can be used. The ratio Na
2
O:SiO
2
can be anywhere in the normal range of 2:1 to 1:5. Accordingly, one preferred type of sodium silicate is sodium metasilicate. Another preferred type of silicate is the material commercially available under the name Waterglass.
Although Waterglass gives very good results, in some instances it is preferred to use a powdered sodium silicate, in which event sodium metasilicate or other powdered silicate, for instance having a ratio Na
2
O:Si
2
2:1 to 1:2.5 or 3 is often preferred. The powdered silicate should either be finely divided or should be in the form of aggregates which degrade during pelletisation to finely divided primary particles.
The substantially water soluble organic polymer can be added as a solution or as a substantially anhydrous emulsion of polymer particles dispersed in a water immiscible liquid or as a hydrous emulsion of polymer particles dispersed in water immiscible liquid. Preferably however the polymer is added as a powder. The powder may consist of primary particles having the desired particle size for use in the moist mix or it may consist of aggregates which disintegrate during admixture with the mineral particles to form the desired primary particle size. Generally the primary particle size is at least 90% by weight below 200 &mgr;m and often below 150 &mgr;m, but it is usually above 20 &mgr;m. Best results are often achieved when at least 90% by weight of the polymer particles are in the range 20 to 100 &mgr;m but a satisfactory product is 98% by weight below 200 &mgr;m and at least 50% by weight below 100 &mgr;m, and generally above 20 &mgr;m.
The amount of polymer must be sufficient for the green 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 0.01 to 0.2%, usually 0.01 to 0.1%, by weight based on the moist mix. Generally the amount is in the range 0.01 or 0.02 up to 0.05%.
The polymer can be a natural polymer (by which we include modified natural polymers) such as cellulosic polymers, starch polymers and

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