GB1574719A - Agglomeration of finely divided metalliferous materials - Google Patents

Description

(54) THE AGGLOMERATION OF FINELY DIVIDED METALLIFEROUS, MATERIALS (71) We, NATIONAL RESEARCH DEVELOPMENT CORPORATION, a British Corporation established by Statute, of Kingsgate House, 66 - 74 Victoria Street, London, S.W.1, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to the agglomeration of finely divided metalliferous materials comprised by ores, ore concentrates and metallurgical waste materials produced as by-products of metallurgical processes, and in particular to the pelletisation of these materials using a cold bonding technique.

In recent years pelletisation techniques have found large-scale application, for instance in pig iron production where finely divided iron ore concentrates are pelletised for ease of handling. Most of these pellets are produced by thermal processing in which green pellets are indurated at temperatures of between 12500 and 13 500C to develop the requisite mechanical strength. These thermal processes however require considerable expenditure of energy.

More recently cold bonding techniques have been introduced using Portland cement to bond the ore, but the pellets obtained are initially very sticky and must be separated from one another, usually by moist ore concentrate, until the cement has set sufficiently to allow the pellets to be handled. Also the use of Portland cement is undesirable as the cement itself must be manufactured consuming valuable energy resources.

An inexpensive alternative to Portland cement has now been found which gives improved products.

According to the present invention a process for the agglomeration of finely divided metalliferous materials, comprised by ores, ore concentrates or metallurgical waste materials produced as by-products of metallurgical processes, comprises fabricating a mixture, comprising the metalliferous material, pulverised fuel ash (PFA), an alkaline earth metal oxide or hydroxide and water, into shaped bodies and permitting the bodies to harden as a result of the hydration reaction of the mixture. Usually the alkaline earth metal component of the mixture is provided by lime and the process comprises fabricating a mixture, comprising metalliferous material, PFA, lime and water, into shaped bodies and permitting the bodies to harden as a result of the hydration reaction of the mixture. Other alkaline earth metal compounds which may be used include magnesia or mixtures of both lime and magnesia such as dolomite.

The invention also includes products in the form of shaped bodies produced by these processes. These products may be in any suitable form ind include pellets, briquettes and similar shapes.

The metalliferous materials which may be agglomerated by the process of the invention are characteristically in finely divided form, typically contain recoverable metal values, and agglomeration advantageously renders these materials more easy to handle and transport.

The metalliferous materials may be comprised by ores or ore concentrates of both ferrous and non-ferrous origin. and especially may be comprised by ore concentrates derived from ores in which the metal containing and gangue components are finely divided.

Preferably, however, the present process is applicable to the agglomeration of metallurgical waste materials produced as by-products of metallurgical processes. These materials, often containing recoverable metal values, are usually dumped wasting valuable resources and creating a toxic environmental hazard, and thus advantageously the present invention assists in the recycling of these materials. For example, processes such as smelting and refining give rise to dust and fume which may be agglomerated by the present process.

Furthermore the invention is particularly applicable to the agglomeration of steelmaking dusts such as those which derive from processes in which low grade scrap e.g. auto-shredder scrap, has been used, and also, generally, the present process is especially beneficial for the agglomeration of metallurgical waste material having significant non-ferrous metal content.

Other metallurgical waste materials, such as sludges and slimes produced as by-products of hydrometallurgical processes, may also be agglomerated by the process of the present invention.

It will be appreciated that the ore, ore concentrate or metallurgical waste material may itself comprise a significant amount of alkaline earth metal oxide or hydroxide e.g. lime, in particular when the metalliferous material is comprised by a dust or fume from a smelting or refining process. For instance, steel-making processes in which powdered lime is injected into the melt during refining give rise to dust containing a high proportion of lime. Thus the ore, ore concentrate or metallurgical waste material itself may provide all or part of the alkaline earth metal component e.g. lime, of the mixture, though more usually lime or similar alkali is added to the ore, ore concentrate or metallurgical waste material together with PFA to give the desired mixture.

Thus in a further embodiment according to the present invention a process for the agglomeration of finely divided ore, ore concentrate or metallurgical waste material comprises intimately mixing the ore, ore concentrate or metallurgical waste material with a bonding agent comprising PFA, lime and water, fabricating the resultant mixture into shaped bodies and permitting the bodies to harden as a result of the hydration reaction of the mixture.

Pulverised fuel ash (PFA), sometimes known as "fly ash", arises as a by-product of the combustion of pulverised fuels, for instance in power stations. This material exhibits pozzolannic properties, combining with lime at ordinary temperatures in the presence of water to form stable insoluble products which possess cementing properties, and thus the present invention conveniently makes use of a waste material which might otherwise present a disposal problem.

The amount of PFA which is added to the mixture may be varied widely, and as little as 1 % by weight of PFA may be sufficient though much greater amounts of PFA may be added, such as up to 30%by weight or more. Usually, however, satisfactory products are obtained using less than 10%by weight of PFA and the preferred PFA content is in the range from 2.5 to 5 % by weight. The amount of lime, usually in the form of hydrated lime (Ca(OH)2), which is added may be varied widely, though this is generally dependent upon the amount of PFA added and its silica content and the indigenous lime content of the ore, ore concentrate or metallurgical waste material. For example, usually from 0.1 up to 1 part by weight of hydrated lime, to every one part by weight of PFA is required, and preferably the molar ratio of lime to the silica present in the PFA is about 1:1, e.g. from 0.8 to 1.2 moles of lime to every one mole of silica.

Other substances may also be added to the mixture as desired. For instance, carbon may be added, especially when the material is to undergo further reductive processing, and for this purpose a carbon addition of from 5 % to 10% by weight is generally satisfactory. Also in this latter respect the use of PFA is particularly apt because of its own inherent carbon content.

The PFA. lime and other substances, if any, may be intimately mixed with the metalliferous material by any suitable means such as those which are known and used in the art.

Water is added to the mixture usually either at the same time as the PFA and lime or subsequently. Addition of water may conveniently assist in the fabrication of shaped bodies from the mixture and is also required for the hydration reaction to take place.

The mixture is fabricated into shaped bodies as desired, and may be cast or moulded into shapes such as briquettes, slabs or tablets. Preferably however the moist mixture is pelletised, for instance using a rotating disc type pelletiser. The shaped bodies, with further additions of water if required. may then be left to harden, conveniently without any special precautions being taken to separate the bodies one from another during the initial stages of hardening.

The hardening reaction of the mixture is believed to be primarily the reaction of the PFA/lime mixture though the metalliferous materials may contain components which participate in the reaction.

Pellets and other shaped bodies produced by the process of the present invention exhibit outstanding properties. Generally the crushing strengths and abrasion and weathering resistances of products of the present invention are far in excess of similar products made with water alone and no bonding agent. Also in some cases, particularly when the metalliferous material has a high non-ferrous metal content, PFA/lime bonded pellets may be stronger and more resistant than pellets bonded with Portland cement, and moreover the amount of PFA required to obtain products of acceptable strengths is appreciably less than the amount of Portland cement required. Particularly advantageous. however, is the fact that products of the present invention do not suffer from the stickiness associated with similar products made with Portland cement.

Subsequent to production, the shaped products of the present invention may be further treated as desired. They may be subjected to further processing for recovery of metal values, for instance, by hydrometallurgical processing or more preferably by pyrometallurgical processing such as reductive or chloridised distillation.

The invention is further illustrated, with reference to the accompanying diagram in the following examples which relate to the pelletisation of steelmaking dusts. It will be appreciated however that the invention is applicable to the agglomeration of other metalliferous materials besides these comprised by steelmaking dusts.

Example 1 A typical electric arc steelmaking dust, in which 60%of the particles are less than 5 microns in size, is pelletised using a PFA/lime bonding agent and the green strengths of pellets are measured over a period of about 30 days. For the sake of comparison pellets are also produced with water alone and with a Portland cement bonding agent, and similarly tested.

The three batches of dust are prepared adding in the first batch 5 % by weight of PFA and 2.65% by weight of hydrated lime (this corresponding to a 1:1 lime to silica molar ratio); in the second batch no additives are used, and in the third batch 10% by weight of Portland cement is added. 7% by weight of carbon, in the form of ground coke 75 to 150,um size, is added to all three batches of dust. Each batch of dust is then separately pelletised by careful addition of dust and water to the inside of a rotating tyre giving pellets of diameter from about 12 mm to about 18 mm.

The compressive strengths of samples of each batch of pellets are then measured at various stages over a period of about 30 days, the quantity measured being the force required to crush a single pellet. For each measurement a sample of ten pellets is used and the average is calculated. The results obtained are given in the accompanying diagram which shows curves for the compressive strength of pellets in Kilogrammes - force against the age of the pellets in days. In this diagram (A) is the curve for pellets made with 5% PFA, 2.65 Ca(OH)2 and 7% carbon, (B) the curve for pellets made with 10%Portland cement and 7%carbon, and (C) the curve for pellets made with dust, water and carbon. As can clearly be seen from this diagram PFA/lime bonded pellets exhibit improved properties over Portland cement bonded pellets or pellets made with water alone.

Example 2 Steelmaking dust pellets are prepared as in Example 1. Separate batches are prepared with various levels of different additives or without additive, both with or without the addition of carbon. Pellets are prepared using 2.5, 5 and 10% by weight of Portland cement, or 1.5, 2.5 and 5 % by weight of PFA together with the corresponding amounts of lime (i.e. 0.8, 1.32 and 2.65 % by weight Ca(OH)2 respectively), and also with no additive besides water, all of these both with and without 7% by weight of carbon (ground coke 75-150,um) giving a total of 14 batches of pellets. In addition two further batches of pellets are prepared, one with 2.5%by weight of PFA 1.32% by weight of Ca(OH)2 and 7%by weight of carbon of particle size less then 350,am, and the other with 33% by weight of PFA and 17.5% by weight of Ca(OH)2 (33% PFA being sufficient of itself to contribute all the carbon required).

The pellets are kept for a period of about two months, at which time the compression strength of samples from each batch are measured as in Example 1. The results obtained are given in Table I bellow, and show that the PFA/lime pellets are comparatively stronger than Portland cement pellets containing higher levels of additives.

Table I Compressive Strength of Steelmaking Dust Pellets Test Addition Addition Addition Addition Water Age of Compressive Variation of of of of con- pellets strength in pellet Portland Pulverised Calcium Carbon tent Kgf per diameter cement Fuel Ash Hydroxide pellet mm % % % % % days 1 2.5 - - - 9.2 54 32.9 11 - 17 2 5 - - - 9.2 54 33.6 13 - 16 3 10 - - - 10.2 53 28.75 13 - 15 4 2.5 - - 7 9.4 59 20.75 13 - 16 5 5 - - 7 8.8 59 31.25 13 - 15 6 10 - - 7 9.1 59 42.2 13 - 16 7 - 1.5 0.8 - 9.3 61 33.45 14 - 17 8 - 1.5 0.8 7 8.7 61 36.95 13 - 16 9 - 2.5 1.32 - 9.3 60 38.85 12 - 15 10 - 2.5 1.32 7* 8.5 61 30.6 12 - 15 11 - 2.5 1.32 7 12.5 55 42.6 14-17 12 - 5 2.65 - 10.9 63 42.5 12 - 18 13 - 5 2.65 7 9.3 24 55.7 17 - 20 14 - 33 17.5 - 12.4 63 46.1 14 - 18 15 - - - - 7.7 54 21.95 12 - 14 16 - - - 7 8.0 56 24.85 3 - 15 *Carbon particle size minus 350 m All other carbon containing tests 75 - 150 m.

Example 3 Resistance to Abrasion Steelmaking dust pellets prepared as in previous Examples are tested for abrasion resistance. Samples of the pellets are placed in a wire mesh cylinder which is rotated at constant speed for various numbers of revolutions or lengths of time, the material abrading from the pellets and passing through the mesh giving a measure of the abrasion resistance of the pellets. The mesh cylinder has an aperture size 1.676 mm, in 15 cm in diameter and 25 cm long. 0.2 kg samples of pellets are used, and the cylinder is rotated at speed of from 50 to 150 rpm for up to 10 minutes.

Pellets without additives except carbon and water, and also pellets bonded with either PFA or Portland cement are used. The results obtained and further information concerning the pellets and conditions of testing are given in Table II below, which clearly show the superior abrasion resistance of PFA bonded pellets.

Table Ii Resistance to Abrasion Additives to steel Water Age of RPM % wt. loss of pellets through 1.676 mm dust content pellets mesh after various periods of rotation % % days 50 revs 100 revs 150 revs 10 mins - 7.66 13 45 0.0023 0.0032 0.0040 - 7.66 13 155 0.0023 0.0058 0.0123 - 7.66 13 148 2.3075 Carbon 7 8.0 27 130 0.823 P.F.A. 5 ) Ca(OH)2 2.65 ) 9.34 15 153 0.29 Carbon 7 ) Portland cement 10 ) 9.09 19 50 0.055 0.066 0.075 Carbon 7 ) 9.09 19 150 0.76 Example 4 Resistance to Weathering Similarly as in previous Examples pellets are tested for compressive strength and abrasion resistance. However prior to testing the pellets are soaked in water overnight.

PFA bonded and Portland cement bonded pellets and pellets without bonding agent are employed and the results obtained are given below in Table III, showing the effect on crushing strength and Table IV showing the effect on abrasion resistance. The Tables also include further information about the composition and age of the pellets and the conditions of the tests. The results obtained clearly show the superior resistance to weathering of PFA/lime bonded pellets as compared with the other types of pellets tested.

Table III Resistance to Weathering Effect on Crushing Strength Additions to steel Age of Strength Strength Loss in strength dust pellets before soaking after soaking after soadking % days Kgf per pellet Kgf per pellet % None 15 18.5 1.5 91.9 Carbon 7 27 20.5 2.5 87.8 P.F.A. 1.5 ) Ca(OH)2 0.8 ) 21 29.9 5.0 83.3 Carbon 7 ) P.F.A. 5 ) Ca(OH)2 2.65 ) 16 39.8 14.5 63.5 Carbon 7 ) Portland cement 10 ) Carbon 7 ) 21 31.2 6.2 80.1 Table IV Resistance to Weathering Effect on Resistance to Abrasion Additions to steel Age of RPM % wt loss of % wt loss of % increase dust pellets Green Pellets soaked pellets in abrasion % days through 1.676 mm shrough 1.676 mm loss on mesh after 10 mesh after 10 soaking minutes rotation minutes rotation None 13 148 2.3075 " 15 69 0.09 " 15 124 48.39 2090 Carbon 7 27 130 0.823 16.0 1940 P.F.A. 1.5 ) 19 68 0.290 Ca(OH)2 0.8 ) 19 120 20.68 Carbon 7 ) P.F.A. ) 15 153 0.29 Ca(OH)2 2.65 ) 15 69 0.305 Carbon 7 15 118 5.63 1940 Portland cement 10 ) 19 150 0.768 Carbon 7 ) 19 56 0.125 127 12.56 1630

Claims (15)

WHAT WE CLAIM IS:

1. A process for the agglomeration of finely divided metalliferous material, comprised by ores, ore concentrates or metallurgical waste materials produced as by-products of metallurgical processes, in which a mixture, comprising the metalliferous material, pulverised fuel ash (PFA), an alkaline earth metal oxide or hydroxide and water, is fabricated into shaped bodies which are permitted to harden as a result of the hydration reaction of the mixture.

2. A process according to Claim 1, in which the mixture comprises metalliferous material, PFA, water and lime.

3. A process according to Claim 1 or 2, in which the metalliferous material is a steelmaking dust.

4. A process according to any of the preceding claims, in which the metalliferous material itself provides all or part of the alkaline earth metal oxide or hydroxide required for the mixture.

5. A process according to any of the preceding claims, comprising intimately mixing the ore, ore concentrate or metallurgical waste material with a bonding agent which comprises PFA, lime and water, fabricating the resultant mixture into shaped bodies and permitting the bodies to harden as a result of the hydration reaction of the mixture.

6. A process according to any of the preceding claims, in which the mixture contains from 1% up to 30% by weight of PFA.

7. A process according to Claim 6, in which the mixture contains less than 10% by weight of PFA.

8. A process according to Claim 6, in which the mixture contains from 2.5 to 5% by weight of PFA.

9. A process according to any of the preceding claims, in which the mixture contains from 0.1 to 1 part by weight of hydrated lime to each 1 part by weight of PFA.

10. A process according to Claim 9, in which the mixture contains from 0.8 to 1.2 moles of lime to every one mole of silica present in the PFA.

11. A process according to any of the preceding claims, in which carbon is added to the metalliferous material.

12. A process according to any of the preceding claims, in which moist mixture is pelletised using a rotating disc pelletiser.

13. A process according to any of the preceding claims, in which subsequent to fabrication and hardening the shaped bodies are subjected to further processing for recovery of metal values.

14. A process according to Claim 1 substantially as hereinbefore described with particular reference to examples 1 to 4.

15. A shaped body produced by a process according to any of claims 1 to 12.