A PROCESS FOR PRODUCING AGGLOMERATES

Abstract: A process for producing agglomerates of a feed material for a fluidised bed roaster or a fluidised bed combustion furnace is disclosed. The process comprises grinding an input feed which includes the feed material into a particle size distribution which comprises at least a significant portion of fines and, thereafter, compacting the ground input feed to form agglomerates.

Description: A PROCESS FOR PRODUCING AGGLOMERATES BACKGROUND OF THE INVENTION The present invention relates to a process for producing agglomerates of a feed material for a fluidised bed roaster or a fluidised bed combustion furnace. A particular, although by no means exclusive, application of the process of the invention that is of interest to the applicant is in the field of calcining mineral concentrates, such as zinc sulphide concentrates, in a fluidised bed roaster. The prior art belief is that optimum performance of a fluidised bed roaster depends on, amongst other factors, the mineral concentrate feed to the fluidised bed roaster having a preferred particle size distribution. Specifically, the prior art belief is that: i. particles of the mineral concentrate feed that are below a lower limit of a preferred particle size distribution tend to be entrained in the waste gas stream from the fluidised bed roaster; and ii. particles of the mineral concentrate feed that are above an upper limit of the preferred particle size distribution tend to remain in the bed and may lead to defluidisation of the bed. The prior art belief is that the preferred particle size distribution for fluidised bed roasters varies with the type of fluidised bed roaster. For example, 0.1 to 2mm is the preferred particle size distribution for a known type of Lurgi fluidised bed roaster which operates with superficial gas velocities in the bed of approximately 0.67 m/sec at a temperature of 975°C and approximately 0.4 m/sec in the freeboards. The preparation of a mineral concentrate feed for fluid bed roasting is normally done by a process of grinding and froth flotation of an ore containing the mineral. Grinding is necessary to enable liberation of the mineral particles to allow separation of the valuable mineral components of the ore from the other components by froth flotation to form the mineral concentrate. However, grinding often results in a substantial proportion of fines which are considerably smaller in size than a lower limit of a preferred particle size distribution for a fluidised bed roaster. Moreover, froth flotation invariably results in the mineral concentrate being produced as a slurry and, in this form, the mineral concentrate cannot be transported by shipping or handled or stored using conventional solid handling systems, and the mineral concentrate is not a suitable feed for a Lurgi-type fluidised bed roaster. As a consequence, invariably, dewatering and agglomeration of a mineral concentrate are necessary to produce a mineral concentrate in a form that can be readily handled, transported, and stored, and subsequently calcined under optimum operating conditions for a fluidised bed roaster. An object of the present invention is to provide an improved process for producing agglomerates of a mineral concentrate, particularly a finely-ground mineral concentrate. SUMMARY OF THE INVENTION According to the present invention there is provided a process for producing agglomerates comprising the following steps: (a) grinding an input feed which includes the feed material into a particle size distribution which comprises at least a significant proportion of fines; and (b) compacting the ground input feed to form agglomerates. It is preferred that the agglomerates be suitable for use as a feed material for a fluidised bed roaster or a fluidised bed combustion furnace. In this context, the process may include the addition of water to the agglomerates. It is preferred that the compaction step (b) forms agglomerates that are relatively dense with a sufficient "green" strength to enable handling, transportation, and storage without significant breakdown of the agglomerates. The term "relatively dense" is understood herein to mean that the agglomerates be sufficiently compact to withstand handling without degradation to an unsatisfactory size range. It is preferred particularly that the compaction step (b) forms relatively dense agglomerates and/or that the moisture content of the agglomerates be selected so that, when the agglomerates are fed into a fluidised bed roaster or a fluidised bed combustion furnace and are subjected to elevated temperatures, the agglomerates break down into smaller-sized agglomerates that can be processed more efficiently in the fluidised bed roaster or the fluidised combustion furnace. The present invention is based on the realisation that: (i) by appropriate selection of the particle size of the input feed and the compaction of the input feed in step (b) , it is possible to form agglomerates having a preferred pore size; and (ii) by appropriate selection of the moisture content of the agglomerates; when the agglomerates are fed into a fluidised bed roaster or a fluidised bed combustion furnace and are subjected to elevated temperatures, the agglomerates break down into smaller-sized agglomerates that can be processed more efficiently in the fluidised bed roaster or the fluidised combustion furnace. The feed material may be any material. The feed material may be an organic material, such as coal washery slimes. The feed material may also be an inorganic material, such as a mineral or a mineral concentrate. By way of example, in one particular application of interest to the applicants, it is preferred that the feed material be a zinc sulphide concentrate for calcining in a fluidised bed roaster. In situations where the feed material is a mineral concentrate, it is preferred that the input feed for step (a) comprises an ore containing the mineral. In that event, it is preferred that the process further comprises an additional step of treating the ground mineral-containing ore to separate the mineral from other components of the ore and to form a slurry of the mineral. It is preferred that the particle size distribution of the ground mineral-containing ore produced in step (a) be 100% less than 100 micron. It is preferred particularly that the particle size of the ground mineral-containing ore produced in step (a) be less than 40 micron. It is preferred more particularly that the particle size of the ground mineral-containing ore produced in step (a) comprises at least 50% of the particles being less than 10 micron. It is preferred more particularly that the particle size of the ground mineral-containing ore produced in step (a) comprises at least 10% of the particles being less than 2 micron. It is preferred that the size of the agglomerates produced in step (b) be 1 to 25mm. It is particularly preferred that size of the agglomerates produced in step (b) be 4.5 to 15 mm. It is preferred that the moisture content of the agglomerates produced in step (b) be less than 15% at the time the agglomerates are fed to a fluidised bed roaster or a fluidised bed combustion furnace. It is particularly preferred that the moisture content of the agglomerates produced in step (b) be less than 13% at the time the agglomerates are fed to a fluidised bed roaster or a fluidised bed combustion furnace. It is preferred that the moisture content of the agglomerates produced in step (b) be greater than 1% at the time the agglomerates are fed to a fluidised bed roaster or a fluidised bed combustion furnace. It is preferred that the moisture content of the agglomerates produced in step (b) be greater than 5% at the time the agglomerates are fed to a fluidised bed roaster or a fluidised bed combustion furnace. It is preferred that the compaction step (b) be carried out in a high pressure filtration apparatus. According to the present invention there is also provided a process for producing agglomerates of a mineral concentrate comprising the following steps: (a) grinding an ore containing the mineral into a particle size distribution which comprises at least a significant proportion of fines; (b) treating the ground ore to separate the mineral from other components of the ore and to form a slurry of the mineral; and (c) dewatering and compacting the slurry of the mineral to form relatively dense agglomerates of the mineral concentrate that have a particle size distribution and/or a moisture content which are selected so that, when the agglomerates are fed into a fluidised bed roaster or a fluidised bed combustion furnace and are subjected to elevated temperatures, the agglomerates break down into smaller-sized agglomerates that can be processed more efficiently in the fluidised bed roaster or the combustion furnace. According to the present invention there is also provided a process for calcining a mineral concentrate comprising, feeding agglomerates of the mineral concentrate formed by the process described in the preceding paragraphs into a fluidised bed roaster or a fluidised bed combustion furnace, and calcining the agglomerates therein. PREFERRED EMBODIMENT OF THE INVENTION The present invention is described further by way of example in relation to experimental work carried out by the applicants on zinc sulphide concentrate from the Century deposit in North Queensland, Australia. The results of the experimental work are summarised in part in the accompanying figure and also in the following discussion. The figure is a graph which illustrates the particle size distribution of calcine produced from zinc sulphide concentrate in fluidised bed reactors from agglomerates formed by a range of different compaction processes, as summarised below. Sample a - high pressure filtration in a Denver tube press to form agglomerates which were dried to a moisture content of 1% and prepared with a particle size distribution of 9.5 to 16mm. 5 Sample b - i. high pressure filtration in a Denver tube press to form ' agglomerates having a moisture content of 12% and a particle size distribution of 9.5 to 10 16mm; and ii. high pressure filtration in a Denver tube press to form agglomerates having a moisture content of 12%, drying the 15 agglomerates to a moisture content of 1%, re-wetting the agglomerates to a moisture content of 12% and a particle size distribution of 9.5 to 20 16mm; Sample c - low pressure filtration and mixing in an Eirich agglomerator to form agglomerates having a moisture content of 11% and a particle size 25 distribution of less than 2mm. Sample d - low pressure filtration and thereafter hot air drying in a rotary drier to form agglomerates having a moisture content of 11% and a 30 particle size distribution of less than 10mm. Sample e - shredding a cake produced in a Denver tube press and thereafter reagglomerating the shredded cake in an Eirich agglomerator to form agglomerates having a moisture content of 11% and a particle size distribution of 0.1 to 1mm. The grain size distribution of the unagglomerated concentrate used to produce the samples was as follows: 98% < 20 micron 85% < 10 micron 63% < 5 micron 30% < 2 micron The samples were calcined in either a 300mm diameter pilot plant fluidised bed roaster or a 70mm diameter bench scale fluidised bed roaster. The fluidised bed roasters were found by the applicants to have similar fluidising conditions to the commercial Lurgi-type fluidised bed roasters discussed above and thus the results of the experimental work are indicative of the results that would be achieved in those Lurgi-type fluidised bed roasters. With reference to the figure, the particle size distribution for samples b, c, and d and, to a lesser extent, sample e, are within the preferred particle size distribution of 0.1 to 2mm for optimum operation of a Lurgi-type fluidised bed roaster. This is a significant and, in many respects, an unexpected outcome, because the input feed which produced the samples comprised agglomerates which were formed, without added binder, from slurries of finely ground zinc sulphide concentrate and, on the basis of known prior art, it was expected that such agglomerates, i.e. agglomerates formed from fines, would disintegrate quickly into fines of less than 0.1mm in the fluidised bed roasters and would be blown from the fluidised bed roasters. The result of sample b is particularly significant, and unexpected, because the particle size distribution of the input feed of agglomerates was in the range of 9.5 to 16mm, which is considerably larger than the upper limit of the preferred prior art particle size distribution for Lurgi-type fluidised bed roasters, and it was expected that, under normal circumstances, such large- sized agglomerates would not be fluidised and hence render the roasters inoperable. It is believed by the applicants that the unexpected result with sample b is due to the agglomerates being relatively dense. Whilst not wishing to be limited to a particular theory, it is thought by the applicants that when the relatively dense agglomerates were subjected to elevated temperatures in the fluidised bed roaster: (i) there was a sudden increase in pressure within the pores of the agglomerates due to the evolution of steam and, (ii) as a consequence of the relatively small pore size, the agglomerates were not able to accommodate the increased internal pressure and therefore fractured into smaller components. The sample e was prepared in view of concern that agglomerates of zinc sulphide concentrate may break-down into fines during transportation from an ore resource to a commercial fluidised bed roaster. The result for sample e shows that re-agglomeration of shredded agglomerates produced agglomerates that had a satisfactory particle size distribution when calcined in a fluidised bed roaster. The result for sample b ii also illustrates the effect of moisture content on the agglomerates. Specifically, as is noted above, sample b ii was formed by re-wetting the dried agglomerate which had been used to produce sample a and thereafter re-forming this agglomerate to a moisture content of 12%. The results for sample b ii were the same as for sample b i and indicate that agglomerates can be re-wetted without loss of performance should the agglomerates be over-dried. The experimental work carried out by the applicants has also shown that, above a critical value of 4.5mm (and up to 25mm), the lump size of agglomerates produced by high pressure filtration, as described for sample b i above, did not affect significantly the particle size distribution of the calcined product of the fluidised bed roasters. Thus, the top size of the agglomerates is not a significant factor since agglomerates up to 25mm will break-down in fluidised bed roasters into smaller components having a particle size distribution predominantly in the range of 0.1 to 2mm. In summary, the experimental work established that it is possible to form agglomerates from fine ground zinc sulphide concentrate which have a "green" strength that should withstand handling, transportation and storage and calcining in a fluidised bed roaster. As a consequence, it is possible to obtain the benefits of fine- grinding zinc bearing ore, such as liberating impurities in such ores, in producing zinc sulphide concentrates without having an adverse effect on subsequent calcining of the zinc sulphide concentrate. A particular outcome of the experimental work is that relatively dense agglomerates having a relatively large particle size distribution, i.e. in excess of 4mm, were broken-down into smaller agglomerates when subjected to elevated temperatures in a fluidised bed roaster and the smaller agglomerates were in the preferred particle size distribution for optimum performance of a Lurgi-type fluidised bed roaster. Many modifications may be made to the process of the invention as described with reference to the experimental work carried out by the applicants without departing from the spirit and scope of the invention. In particular, whilst the experimental work relates to zinc sulphide concentrate, it can readily be appreciated that the invention is not so limited and extends to any suitable material such as, by way of example, coal washery slimes.

Claims: I CLAIM : 1. A process for producing agglomerates of a feed material comprising the following steps: (a) grinding an input feed which includes the feed material into a particle size distribution which comprises at least a significant proportion of fines; and (b) compacting the ground input feed to form agglomerates. 2. The process defined in claim 1 comprising compacting the ground input feed in step (b) to form agglomerates that are relatively dense with a sufficient "green" strength to enable handling, transportation, and storage without significant breakdown of the agglomerates. 3. The process defined in claim 1 or claim 2 comprising compacting the ground input feed in step (b) to form relatively dense agglomerates which are selected so that, when the agglomerates are fed into a fluidised bed roaster or a fluidised bed combustion furnace and are subjected to elevated temperatures, the agglomerates break down into smaller-sized agglomerates that can be processed more efficiently in the fluidised bed roaster or the fluidised bed combustion furnace. 4. The process defined in any one of the preceding claims wherein the feed material comprises an organic material. 5. The process defined in claim 4, wherein the organic material comprises coal washery slimes. 6. The process defined in any one of claims 1 to 3 wherein the feed material comprises an inorganic material. 7. The process defined in claim 6 wherein the inorganic material comprises a mineral or a concentrate of the mineral. 8. The process defined in any one of the preceding claims wherein the feed material comprises a zinc sulphide concentrate for calcining in a fluidised bed roaster. 9. The process defined in claim 7 wherein the input feed for the step (a) comprises an ore containing the mineral. 10. The process defined in claim 9 further comprising the step of treating the ground mineral- containing ore to separate the mineral from other components of the ore and to form a slurry of the mineral. 11. The process defined in claim 9 wherein the particle size of the ground mineral-containing ore produced in step (a) is less than 100 micron. 12. The process defined in claim 11 wherein the particle size of the ground mineral-containing ore produced in step (a) is less than 40 micron. 13. The process defined in claim 9 wherein the particle size of the ground mineral-containing ore produced in step (a) comprises at least 50% of the particles less than 10 micron. 14. The process defined in claim 13 wherein the particle size of the ground mineral-containing ore produced in step (a) comprises at least 10% of the particles less than 2 micron. 15. The process defined in claim 9 wherein the size of the agglomerates produced in step (b) is 1 to 25mm. 16. The process defined in claim 15 wherein the size of the agglomerates produced in step (b) is 4.5 to 15 mm. 17. The process defined in any one of the preceding claims wherein the moisture content of the agglomerates is less than 15%. 18. The process defined in claim 17 wherein the moisture content of the agglomerates is less than 13%. 19. The process defined in any one of the preceding claims wherein the moisture content of the agglomerates is greater than 1%. 20. The process defined in claim 19 wherein the moisture content of the agglomerates is greater than 5%. 21. The process defined in claim 1 wherein the compaction step (b) is carried out in a high pressure filtration apparatus. 22. A process for producing agglomerates of a mineral concentrate comprising the following steps: (a) grinding an ore containing the mineral into a particle size distribution which comprises at least a significant proportion of fines; (b) treating the ground ore to separate the mineral from other components of the ore and to form a slurry of the mineral; and (c) dewatering and compacting the slurry of the mineral to form relatively dense agglomerates of the mineral concentrate that have a particle size distribution which is selected so that, when the agglomerates are feed into a fluidised bed roaster or a fluidised bed combustion furnace and are subjected to elevated temperature, the agglomerates break down into smaller-sized agglomerates that can be processed more efficiently in the fluidised bed roaster or the combustion furnace. 23. The process defined in claim 22 wherein the mineral concentrate is a zinc sulphide concentrate. 24. The process defined in claim 22 or claim 23 comprising compacting the slurry of the mineral without the addition of a binder. 25. The process defined in any one of claims 22 to 24 wherein the particle size of the ground mineral- containing ore produced in step (a) is less than 40 micron. 26. The process defined in any one of claims 22 to 25 wherein the particle size of the ground mineral- containing ore produced in step (a) comprises at least 50% of the particles less than 10 micron. 27. The process defined in any one of the claims 22 to 26 wherein the agglomerates produced in step (c) are 4.5 to 15 mm. 28. The process defined in any of claims 22 to 27 wherein the moisture content of the agglomerates produced in step (c) is less than 15%.

Inventors: BOX TERENCE, BROWN PETER JOHN, MCCARTHY DAVID JOHN, TAKOS JOHN, BOX, TERENCE, BROWN, PETER, JOHN, MCCARTHY, DAVID, JOHN, TAKOS, JOHN
Applicants: BOX TERENCE, BROWN PETER JOHN, MCCARTHY DAVID JOHN, PASMINCO AUSTRALIA LTD, TAKOS JOHN, TECH RESOURCES PTY LTD, BOX, TERENCE, BROWN, PETER, JOHN, MCCARTHY, DAVID, JOHN, PASMINCO LIMITED, TAKOS, JOHN, TECHNOLOGICAL RESOURCES PTY. LIMITED
Application year: 1995
Main classification: B01J2
IPC codes: B01J2, C22B1