AU2380292A - Ore pelletisation - Google Patents

Description

Ore Pelletisation

This invention relates to ore pelletisation processes which comprise forming an intimate mixture of particulate ore and particulate binder in the presence of moisture, forming green pellets by agitation of the mixture (for instance by rolling or tumbling) and firing the green pellets to produce ore pellets.

Bentonite has been a widely used particulate binder but numerous proposals have been made to use synthetic or natural organic polymers.

One class of natural polymers that has been used are various soluble starches. Another class are soluble cellulose derivatives which are usually esters (especially carboxymethyl cellulose) or ethers (especially hydroxyethyl cellulose) . Another class are soluble gums such as xanthan gum or guar gum. It has been proposed to use mixtures of binder clay (bentonite) with the polymers. For instance Clum et al in Mining Engineers 1978 (NY) 30(1), page 53 show the results obtained using binders comprising guar gum, hydroxyethyl cellulose, polyoxyethylene oxide, and also bentonite.

There have been numerous proposals to use various soluble particulate synthetic polymers. Thus the particulate binder may comprise synthetic polymer particles often having a size up to 300μm formed by polymerisation of water soluble, ionic, ethylenically unsaturated monomer or monomer blend to form water soluble polymer particles. The monomer blend is free of cross linking agent, so as to avoid cross linking with the consequential risk of insolubility.

For example we describe in EP-A-225171 the use, as particulate binder, of water soluble synthetic polymer that has intrinsic viscosity 3 to I6dl/g and that is an anionic polymer and we describe in EP 0288150 the use of cationic polymers. The use as pelletisation binder of soluble anionic synthetic polymer has several advantages over the use of bentonite, but it can suffer from one disadvantage in that it is difficult to achieve adequate dry strength in the ore pellets at economic dosages. Even if the dosage is increased in order to improve dry strength, there may then be other disadvantages, such as stickiness and aggregation of pellets in the drum and instability during the pelletising process. Similarly, the use of natural polymers alone has not proved entirely satisfactory since they may not lead to the optimum combination of green strength, dry strength and drop number.

In an ore pelletisation process according to the invention, particulate ore is mixed with particulate binder in the presence of moisture and the mixture is pelletised, and the particulate binder comprises a blend of 1 part ionic synthetic water soluble polymer with from 2 to 30 parts of a soluble natural polymer selected from soluble gums, soluble starches and soluble cellulose derivatives. Throughout this specification, parts are parts by weight. The natural polymer can be, for instance, a cellulose ether such as hydroxyethyl cellulose, a cellulose ester such as carboxymethyl cellulose, a soluble starch, or xanthan gum, but it is preferably soluble guar gum. The gum may have been treated in known manner to increase its solubility, for instance it may be a phosphated guar gum. The amount of synthetic polymer is generally at least 0.005% and usually at least 0.01% (by weight of the total mix) but the amount is generally not more than 0.1% and is frequently less, for instance below 0.06%. Amounts of 0.01 to 0.04% are often suitable.

The total amount of water soluble synthetic and water soluble natural polymer used in the invention is usually at least 0.03% and often at least 0.05%. It is generally undesirable for it to be more than 0.3% and it is usually below 0.2%. Amounts of 0.05 to 0.1 or 0.15% are often suitable.

The amount of the natural polymer is usually at least 0.02% and generally at least 0.04%. Although the amount can be, for instance, 0.2% or even more it is preferably below 0.15% and generally below 0.1%. It is very surprising that these low amounts of natural polymer give a beneficial effect, since it is usually necessary to use relatively large amounts, typically 0.4% or more, to obtain beneficial binding results when using natural polymer.

The amount of the natural polymer is generally (per part by weight of the soluble synthetic polymer) at least 3 parts and frequently at least 5 or 6 parts. It is normally below 15 parts, and is generally below 10 parts. The binder preferably also includes sodium carbonate or other water soluble monomeric additive of the type described in EP 225171. The amount of this is generally from 0.2 to 2 parts, often around 0.7 to 1.5 parts, per part by weight of the synthetic polymer. Preferred binders comprise 1 part by weight soluble synthetic polymer, 0.7 to 1.3 parts by weight sodium carbonate and 2 to 12 parts by weight guar gum.

The components of the binder may be premixed or they may be supplied to the pelletising process separately but preferably substantially simultaneously.

The total amount of binder (water soluble synthetic polymer plus natural polymer plus sodium carbonate or other salt) is typically in the range 0.03 to 0.3%, often around 0.05 to 0.2%. The polymer can be cationic, for instance as described in EP 0288150, but is generally anionic as in EP 225171. The amount by weight of sodium acrylate or other anionic monomer is generally in the range 5 to 90% by weight, with the balance preferably being acrylamide. It is normally preferred for the polymer to be a copolymer of acrylamide with 10 to 40%, often 15 to 30%, sodium acrylate. However it can be desirable to use larger amounts of sodium acrylate, e.g., 50 to 80%, typcially around 70%.

Intrinsic viscosity can be in the range 2 or 3 to 16dl/g, often in the range 5 to 12dl/g, but in some instances can be higher, for instance up to 25dl/g.

It is generally desired that the soluble synthetic polymer should be wholly linear in which event it will normally have been polymerised in the absence of any added cross linking agent. However it can be advantageous for the synthetic polymer to be a water soluble, partly cross linked polymer. The amount of cross linking agent should be selected so that it is insufficient to render the polymer particles predominantly water insoluble but sufficient to give a useful benefit, particularly an increase in the dry strength of the ore pellets, provided that the amount is such that the particles still behave predominantly as water soluble polymer particles, for instance as regards their film-forming and rheological characteristics. The amount of cross linking agent typically is 5 to 50ppm, preferably 7 to 20ppm when the IV is 2 to 7 l/g and 2 to 30ppm, preferably 5 to 15ppm, when IV is 7 to 16dl/g. These IV's are measured on the polymer in the absence of cross linking agent and the amounts of cross linking agent are calculated as methylene bis acrylamide. Different, generally larger, amounts will be required to obtain the same rheology and solubility characteristics using other cross linkers. Generally the amount of cross linking agent is below I8ppm, measured as methylene bis acrylamide. In this specification, IV values are determined by conventional single point IV measurement in dl/g at 20°C. Some or all of the components of the particulate binder used in the invention can be supplied as a dispersion of particles in oil, but it is generally preferred for them to be supplied as a dry powdered particulate composition. The particles may be aggregates, for instance as described in EP 0326382. The size of the binder particles is normally below 300μm, generally below 200μm and preferably below 150μm, but is generally above 20μm.

The particulate ore is preferably an iron ore but can 5 be any other mineral ore that is capable of being pelletised, for instance a zinc ore. The materials and process conditions can be broadly as described in EP 225171, except that the binder must include the defined large amount of guar gum. Bentonite can be used as part 10 of the binder.

In Examples 1 and 2 below, pelletisation processes were conducted as in the examples of EP 225171 using various combinations of guar gum and anionic polymer formed as in EP 225171. The results were as follows. 15 Example 1

Product A - a 20% anionic polyacrylamide blended

50/50 with sodium carbonate Product B - a guar gum

Product C - a 2/7 active polymer blend of A and B 20 Green

Strengt /Kg 0.09% C 1.00 0.10% B 1.31 Example 2 25 Product D - a 1/5 active polymer blend of A and B

Green Dry Drop %,

Strength/Kg Strength/Kg Number Moisture 0.12% D 1.18 8.15 21.3 10.5

0.12% B 1.30 6.08 45.0 10.0

30 A combination of an anionic polyacrylamide blend with sodium carbonate and guar gum gives acceptable green properties whilst increasing the dry strength over that obtained with guar gum on its own.

In the two examples described, the dry strength has 35 been increased by 24.0 and 34.0% respectively as a result of blending the guar gum with the synthetic polymer. k Thus, even though dry strength tends to be a problem with binders based on synthetic polymer, the addition of the synthetic polymer to the guar gum increases the dry strength attainable using a similar amount of the natural polymer in the absence of the synthetic polymer.

Claims (7)

1. An ore pelletisation process comprising mixing particulate ore with particulate polymeric binder in the presence of moisture and pelletising the mixture, characterised in that the particulate polymeric binder comprises a blend of one part ionic synthetic water soluble polymer with 2 to 30 parts of a soluble natural polymer selected from soluble gums, soluble starches and soluble cellulose derivatives.

2. A process according to claim 1 in which the synthetic polymer is an anionic polymer and the natural polymer is guar gum.

3. A process according to claim 1 or claim 2 in which the amount of synthetic polymer is 0.005 to 0.1% and the amount of the natural polymer is from 0.05% to 0.2%, by weight of the total mixture.

4. A process according to any preceding claim in which the amount of natural polymer is 3 to 10 parts by weight per part by weight synthetic polymer.

5. A process according to any preceding claim in which the synthetic polymer is formed of a blend of 10 to 90% acrylamide and 90 to 10% sodium acrylate and has intrinsic viscosity 2 to I6dl/g.

6. A process according to any preceding claim in which the synthetic polymer is formed of a blend of 60 to 90% by weight acrylamide and 40 to 10% by weight sodium acrylate and has IV 5 to 9dl/g.

7. A process according to any preceding claim in which the ore is iron ore in the form of particles mainly below 250μm.