The application of granulation to fine coal preparation

Transcription

1 University of Wollongong Research Online University of Wollongong Thesis Collection University of Wollongong Thesis Collections 1990 The application of granulation to fine coal preparation Komaruddin Atangsaputra University of Wollongong Recommended Citation Atangsaputra, Komaruddin, The application of granulation to fine coal preparation, Doctor of Philosophy thesis, Department of Mechanical Engineering, University of Wollongong, Research Online is the open access institutional repository for the University of Wollongong. For further information contact Manager Repository Services:

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3 Chapter X 264 CHAPTER X CONCLUSIONS AND SUGGESTIONS FOR FURTHER WORK The conclusions that can be drawn from this investigation can be grouped into five categories: 1. Granulation feed preparation 2. Granulation optimization 3. Handleability 4. Granulation economic assessment 5. General X.1 Granulation Feed Preparation Successful beneficiation of fine coal waste requires the combined use of size classification, froth flotation and agglomeration. In particular waste slurry should be initially classified at 75 pm using a hydrocyclone into underflow and overflow for flotation and agglomeration feeds,respectively. The flotation underflowrejectshould be recycled to an oil agglomeration chamber for additional carbon recovery. This recovery process increases the size modulus in the flotation feed which results in an increased system efficiency. Furthermore the high yield suggests that reject generation is reduced. This reduction is associated, in turn, with reduced real estate and environmental protection costs. Hence by rebeneficiating fine coal refuse with up to 42 % ash, tailings disposal problem can be reduced and a clean coal product of about 11.5% ash recovered with 97 % overall recovery.

4 Chapter X 265 This process is associated with the following particular benefits : 1. Flotation efficiency increases due to the increase in the size modulus of the flotation feed 2. The combined process enhancestotalrecovery of coal fines 3. The combined process generates a useful product from fine coal waste slurry. X.2 Granulation Optimization The significant conclusions that can be drawn from the results of the granulation aspect of this investigation include: 1. Granule motion in granulation drums has a significant effect on granule densification, strength and shape. Notably the cylindrical shape of the drum granulator promotes densification in the longitudinal direction only, whereas, in the cone drum granulator densifying granules travel in all directions because the granule formation zone has two dimensional curvature. 2. Granules made in the cone drum granulator are both more spherical and stronger than those made in the cylindrical drum granulator 3. Granulation constitutes an appropriate method to reduce the problems associated fine coal handling. The major problems include : - Poor flowability - Cohesiveness - Tendency to absorb and retain moisture - Dust emissions andfinelosses 4. Binders such as molasses, guar gum and auby gel produce granules with excellent strength. This strength should be sufficient to withstand crushing and impact during storage, handling and transportation.

5 Chapter X Special low humidity storage conditions are required for granules made with molasses since these granules tend to become mouldy in high humidity atmospheres. 6. Optimum addition of either binders (molasses, guar gum and auby gel) or additives (lime, bentonite and kaolin) effects significant strength improvements. 7. Correct water addition is paramount for optimization of both granule size distribution and bulk density. 8. There is a particular water or liquid binder addition where the optimum granule size is produced. 9. The optimum granule size is the size where the granules have optimum characteristics and granule production is maximal. The optimum granule size which varies between 12.5 and 17.5 mm, is independent on the binder used 10. Correct water addition, by using a premixing or water spraying method, is essential because granule characteristics are extremely sensitive to the water addition rate. 11. The variation in the mean granule size due to water addition is given by d50 = d50,m - (dso,m - dso.o) exp (-k m.am) where dso is an arbitrary dso mean granule size (mm), AM is the water addition (%), dso.o is the initial dso feed size, dso, m is the mean granule size after extensive granulation for a feed material with m% moisture and k m is the rate constant (mm/min.). 12. It is found that the effects of granulator speed, inclination and length on granule compressive strength can be adequately expressed by the equations x c = 80N0-2 * c = i-1.2i 2 x c = L L For design purposes the relationship between granulator length and diameter is adequately described by

6 Chapter X 267 g = ( L - UxlO- 4 L 2 ) Adequate granule strength and low moisture content can be achieved by curing granules for at least 7 days. 15. Lime, bentonite and kaolin improve both granule curing time and residual moisture content. 16. Granules made using the guar gum binder exhibit very good resistance to water absorption. 17. Generally, an improvement in granule resistance to water is attained by coating the granules with heavy oil. However, this effect does not hold true for granules made with guar gum. X.3 Handleability 1. There is a critical d50 particle size of coal, below which the flowability rapidly deteriorates. The critical d50 size is associated with optimumflowability and bulk density. Unfortunately, production offinecoal with particle size smaller than the critical size cannot be avoided in coal preparation. This causes serious problems in the handling, storage and transportation of coal. 2. Granulation produces a coal product with superior flow properties relative to untreated fine coal products. 3. Optimum sized granules produced using optimum granulation feed conditions exhibit significant abrasion resistance. This high abrasion resistance is also associated with high compressive strength, high impact strength, smooth granule surfaces and low porosity. 4. The abrasion resistance exhibited by granulated coal product suggests that significantly reduced dust emissions and fine coal losses during handling, transportation and storage will result

7 Chapter X Measurement of granule compressive strength, impact strength and abrasion resistance provides a convenient assessment of handling suitability. 6. The Durham Cone provides a convenient method to assess theflowabilityof untreated and granulated coal products. However, difficulties are experienced when adverse coals are tested. X.4 Granulation Economic Assessment When granulation uses rebeneficiated fine coal waste slurry, the granul economics cannot be separated from the economics of the complete coal preparation process. In particular economic assessment of the complete process indicates that 1. For the combined beneficiation process, related costs have the greatest bearing on the economics of the granulation process. 2. The economic variables having the greatest effect on granulation economics are coal feed cost (CFP), oil cost (OP) and oil consumption (OC). 3. Granulation economics and profitability prediction for granule production based on the aforesaid economic variables can be adequately described by - Production Cost (PC), A$ / tonne product: PC = 74.1 (PR) CFP PC = CFP OP PC = OP.OC where PR is the production rate - Pay Out Time (POT), years: Log POT = (0.04 CFP)+{5.2xl0-3 *OP*10 A (0.03 CFP)} Log POT = xl0-5 OC.OP Discounted Cash Flow Rate of Return on Investment (DCFR), % : DCFR = CFP OP DCFR = OC OP.OC

8 Chapter X This analysis suggests that the process is profitable when applied to a situation where the cost of coal feed and oil is less than A$ 10 and A$ 100 /tonne, respectively, at an oil consumption of less than 20 % and at production rates more than 1500 tpd. 5. Due to the generally low consumption of binders and additives the impact of binder and additive costs on granulation economics is less significant. X.5. General Concern in regard to the disposal of fine coal refuse has resulted from the high cost associated with providing land for the same and protecting the environment. Byrebeneficiatingfinecoal refuse from coal preparation plants, this concern can be reduced and a clean coal product recovered. Furthermore the problems associated with coal handling, such as poor flowability, cohesiveness, dust emission and fine losses, become more serious as the fines and moisture content increase. Unfortunately when dry the converse problems of fine losses and dust emissions occur. These problems can be avoided by granulatingfinecoal products using a granulation process. Granulated fine coal exhibits favourable characteristics compared to those displayed by conventional fine coal products. The granulated fine coal product generated in this investigation are found to be relatively strong, sufficiently durable and to have relatively high abrasion resistance. In fact the strength should be sufficient to withstand crushing and impact during storage, handling and transportation. The granule abrasion resistance should significantly reduce dust emission and fine losses during the same. Furthermore granulated coal exhibits good flowability without dust pollution emissions and fine losses. The handling and high moisture content problems

9 Chapter X 270 associated with fine coal are hence avoided, m addition, granulated fine coal product made using combustible binders is improved in quality and can be tailored in size distribution to suit a particular utilization. In summary granulation can economically enhance fine coal handling, ash, calorific value, environment and utilisation characteristics. X.6 Suggestions For Further Work This investigation revealed that many aspects of coal granulation require further investigation. These aspects include development of mathematical expressions to predict granule size distribution. Notably, a literature survey indicated that no suitable expression exists to describe adequately total granulated product size. Obviously description of granule size is very important for the control and optimization of granule characteristics. The theoretical analysis of granulation conducted in Section JJI.2.3 suggested that the feed particle size distribution determined the granule optimum packing. This optimum packing, in turn, greatly affected granule characteristics and binder consumption. As suggested in this section the optimally packed granules associated with high tensile strength can be attained by feed particles possessing a size distribution with dispersion modulus n < This prediction requires experimental confirmation. Since inconsistent results were experienced during the initial measurement of granule drop number, a more reliable method to measure granule impact strength should be developed. This measurement procedure should take into account that granule impact strength measured using a drop test is dependent on the mass, size, strength, porosity and shape of the typical granules. These factors are very variant for the same size granules. Account of these factors will also require the use of

10 Chapter X 271 suitable sample sizes and the use of appropriate statistical methods to predict granule impact strength. The economic analysis suggests that an investigation should be conducted to evaluate the energy requirement for granule production.this investigation should also identify the efficiency of energy utilization and techniques to improve the same. Furthermore the economic analysis identified that the use of binders and the binder addition rate have major effects on granulation economics. Therefore an investigation of autogenous granulation should be conducted. Obviously significant benefits could result if effective autogenous granulation can be developed. Alternatively the use of new less costly effective binders should be investigated. These new binders include proprietary binders, corn starch, dust cote, resin etc. Further coal granulation studies should also examine the use, characteristics and economic merits of chemical binders. The chemical binders examined should include humic acid. In addition, further investigation of binder use must be continued to seek more economic binders without sacrificing granule characteristics. For example, the use of domestic or industrial waste, for instance used paper, should investigated. Further organic material extracted from sewage may also be a suitable low cost binder. Such investigations should also identify the full environmental benefits resulting from the use of these waste products as binders. This current investigation should be extended to examine the granulation characteristics of different feed materials, notably, that obtained from different coal preparation plants with differing processes. Likewise the granulation of different blends of coal including coals of different rank, should also be investigated. Obviously this work should be extended to investigate the granulation characteristics of fine demineralized coal. Due to the current interest in this product priority should

11 Chapter X 272 be granted to this particular investigation. In all cases the investigations should be conducted initially at laboratory scale. For situations where favourable results are obtained the investigation should be scaled up to pilot plant scale and actual plant investigations. It is also the author's intention to continue this investigation in regard to the suitability and response of Indonesian coals and coal blends especially low rank coals to granulation. Where initial laboratory results are promising, this investigation will be scaled up to pilot plant and actual plant investigations. The fact that product produced from the Jameson high intensity froth flotation cell is veryfine,typically down to 30 pm average size, suggests an investigation be conducted to examine the use of granulation on such material. Obviously such fine material if untreated will create serious handling problems when wet and extreme dust problems if dry. Obviously this investigation should be initiated with laboratory tests and continued into pilot and full plant scale. In regard to the effect of granulator internal surface and shape, this initial investigation identified that there was a difference in granule size distribution, growth and strength between granules produced in cylindrical and cone drum granulators. Obviously any investigation initiated, to examine this dependence should be extended to examine the effect of granulator design, on the mechanics of granule formation. In particular granulation experiments should be conducted to identify the effect of the granulator length to diameter ratio on granule formation and growth. A comparative study should also be conducted to identify the characteristics and benefits of different types of granulators. This study should be extended to examine drum, disk, cone drum and proprietary granulators. The effect of adding water and binder by premixing, including the method of mixing should be investigated further. This work should confirm and quantify the

12 Chapter X 273 initial observation that granule formation kinetics and strength are dependent on water and binder addition techniques. This investigation should also identify whether granules formed by coalescence differ in strength to those formed by layering. The use of binders at optimal addition rates should be scaled up to actual operation plant. Such an investigation would confirm the binder consumptionratefor optimum granule production under actual plant conditions. The investigation should be extended to different kinds of raw material and different feed material. Obviously the economic and environmental benefits of granulation should also be assessed in this scaled up investigation. As revealed in this initial investigation granule formation requires a particular feed moisture level which differs for different binders. This implies that additional dewatering and controlled drying is necessarily. Attainment of theserequirementsat full plant size requires technical and economic evaluation. It should be noted that the required granulator feed moisture content implies the large scale use of thermal dryingtechniques. The latter suggests that a full economic and technical investigation is required to seek further improvement in dewatering and thermal drying. Obviously fine coal particle size enlargement by the use of granulation cannot be conducted in isolation. That is, the use of other size enlargement processes including the use of elevated temperature granulation should be examined. Such an investigation should include a fulltechnical and economic evaluation especially where heated processing is involved. The suitability of granulated product in the various methods used to handle, store and transport coal products should be investigated fully. In particular, the response and possible degradation of granulated coal product during handling, storage and conveying should be examined. This examination should also monitor product dustiness and weatheringresistance during storage and handling. Obviously

13 Chapter X 274 this investigation should be extended to examine the effects of time consolidation as which occurs in large stockpiles and in the holds of ocean going coal carriers. A larger scale Durham Cone flowability tester consistent with actual plant scale, should be developed. The suggested outlet dimension should be at least 450 mm and the hopper wall slope 25. Preferably the hopper should be constructed from 304-2BSS to facilitate testing of a wide range of coals. This development should eliminate the flow difficulties experienced with adverse coals, particularly wet fine coals, using the current small outletflat hopper tester. Alternatively the use of a newly developed proprietary flowability tester [137] could be evaluated. This tester appears promising for assessing the flowability of products in the actual plant situation. All subsequent investigations should incorporate thorough scale up and economic analysis to identify the application, environmental and economic merits of coal granulation.