AU587127B2 - Production of hardened coal agglomerates - Google Patents

Description

PRODUCTION OF HARDENED COAL AGGLOMERATES Field of_ the Invention This invention rel ates to improvements in the production of coal agglomerates which are suited to long- term storage and/or transportation in the agglomerate form. Background of the Invention The formation of coal aggl omerates from aqueous slurries containing particulate coal and oil has been widely practiced for many years. Many agglomeration processes have been proposed requiring varying degrees of energy input and oil consumption. Most processes having acceptable energy input requirements and residence times produce relatively oily or sticky coal agglomerates which, while being suitable as as a feed stock for the immediate production of a coal- oil mixture, have been found to be unsuitable for long-term storage or transportation due to their stickiness and/or poor physical strength. Examples of prior art agglomeration processes may be found in United States Patents Nos.4355999 Masologites, 4302211 Verschun and Austral ian Patent 534563 (AU-B 54496/80) Dudt. In each of the above processes, long residence times and/or multiple agglomeration stages are required to achieve an acceptable coal agglomerate and even then such products are not necessari ly suited to transportation in large bulk carriers of the type which would make transportation of such agglomerates economical ly viable. In the case of AU-B 54496/80, it wil 1 be noted that a four stage process of increasing energy input is required to produce an acceptabl e aggl omerate. Although the agglomerates produced by this process would be acceptably dry (that is, not sticky), the aggl omerates woul d be un l i ke ly to be of suffi ci ent qua l ity to surv i ve transportation without unacceptable production of fines during the transportation process. It is al so wel l known to reduce the oi l iness or stickiness of particulate coal agglomerates by the evaporative de-oiling of such agglomerates. However, such processes have the obvious disadvantage of increasing the energy requirements of the production process since super heated steam must usual ly be produced to provide the necessary energy to cause evaporation of the oil coating the agglomerates. Summary of the In ven i on It is an object of the present invention to provide an improved coal agglomeration method which results in the prodution of better quality agglomerates in lower residece times. The invention therefore provides a process for the production of coal agglomerates comprising agitating an aqueous slurry of coal particles in a first agitating means in the presence of a predetermined quantity of oil to form coal agglomerates, further agitating said agglomerates in the presence of further coal particle bearing slurry to improve the dryness and quality of the agglomerates, character sed by the step of transporting the slurry containing said agglomerates in a pipeline to further improve the strength properties of the agglomerates. The agglomerates produced have been found to be wel l suited for long-term storage and/or transportation in bulk. In a preferred form of the invention, the step of transporting the slurry containing the agglomerates n a pipeline fol lows each of the agitation stages and the transporting is preferably achieved in a pipeline loop. The consolidation which occurs du ing formation and circulation of the agglomerate bearing slurry through the pipeline in combination with the two-step coal addition operation permits the production of strong agglomerates having a relatively dry surface. Agglomerates produced in this way show little tendency for sticking together or for attrition during handling operations. For these reasons, they are eminently suited for long-term storage and/or for transportation in bulk. The results achieved were not predictable and the inventors found the improvement in agglomerate qual ity by the circulation of the slurry in a pipeline quite surprising. The inventors are not yet aware of the physical reasons for the unexpected improvements achieved by the pipeline circulation, but it is clear that the further contact between the agglomerates and the coal particles in the slurry which occurs in the pipeline is most beneficial. In one preferred form of the present invention, the coal particl es contained i n the initia l s l urry are preferably coated with oi l and formed i nto smal l agglomerates by introducing the slurry and the coating oil into the inlet of a turbulent flow slurry pump. This method of oil coating and formation of smal l agglomerates has been described i n our Austral ian Patent No. 529242 (AU-B 56053/80). Of course it wil l be appreciated that acceptable results may be obtained by the simple addition of oi l to the initial agitation stage in accordance with standard practice. However, the use of a turbulent flow slurry pump to achieve the initial oil coating and formation of smal l agglomerates has the advantage of reducing the energy requirements of the agglomeration process. In the present specification the term "pipeline" should be construed^* as a pipe of substantial length, for example, at l east 500m. Si i l arly, the term "oi l" should be construed to include al l suitable hydrophobic liquids such as kerosene, diesel oil, fuel, oil, petroleum residue and heavy aromatic material s such as coke oven tars and bitumen and suitable mixtures thereof. __J_1__£ Description of the Drawi ngs A preferred embodiment of the invention wil l now be described with reference to the accompanying drawing n which: Fig.1 is a schematic diagram showing an arrangement for performing the process according to a preferred embodiment of the invention. Description £f Preferred Embodiment Referring to Fig.1 of the drawings, the arrangement shown for performing the preferred embodiment of the process according to the invention comprises a turbulent flow slurry pump PI into the inl et of which suitabl e oi l and a particulate coal bearing aqueous slurry is introduced in the - 4 -

T manner described in greater detail in our Australian Patent

2 No.529242. While it may be convenient to inject the oil

3 directly into the inlet of the pump, it will be appreciated

4 that the oil may be added at any suitable position upstream

5 of the pump inlet.

6 The slurry general ly contains 30-50% by weight of

7 solids, including particulate coal which may result from a

8 grinding operation, washery or tailings pond. Any suitable

9 oil, such as a suitable grade of fuel oil, may be used to TO achieve .agglomeration and the quantity of oil introduced T.1 into the inlet of the pump PI is selected according to the T2 nature of the particulate coal contained in the slurry (see

13 above Patent No.529242).

14 The pump PI discharges into a first agitation tank 1 in

15 which the oil coated coal and partially formed agglomerates

16 produced in the pump PI are further agglomerated. A second

17 pump P2 is connected to the tank 1 and recirculates the

18 agglomerate bearing slurry produced in the tank 1 through a

19 pipeline loop LI back into the agitation tank 1. During its

20 passage through the pipeline loop L1, the agglomerates are

21 consolidated to increase their strength and the strengthened

22 agglomerates are recycled into the mixing tank 1 so that

23 further growth can occur by contact ith fresh oi l coated

24 coal particles. The length of the pipe loop LI is selected

25 in conjunction with other operating parameters (such as oil

26 addition level, particle size distribution, residence time

27 in the tank/pipe loop) to achieve the required consolidation

28 and is preferably longer than 500 metres; for example 1600m

29 has been used in some pilot plant trials. It has been

30 surprisingly found that the consolidation which occurs in

31 the pipe loop LI in combination with the agitated tank 1, is

32 not readily achieved in the agitated tank 1 alone, certainly

33 not in the same overal l residence ti e. In addition, the

34 size of the agglomerates can be controlled by adjustment of

35 pipeline velocity and combined residence time in the tank 1

36 and pipe loop LI.

37 If desired the pipeline may include flow disturbing

38 means which increase the mixing of the sl urry in the pipeline as it is transported therethrough. See for example our Australian Patent No. 529242 or United States Patent No.3856668. In an experimental pilot plant constructed to test the viability of the process according to the present invention, the fol lowing parameters have been found to be successful: An agitated tank having a volume of 300 m has been used in conjunction with a 100mm diameter pipe loop. The agitator is fitted with a 21 kW motor. The total length of the pipe loop was 1600m with bypasses fitted to al low use of 400m, 800m or 1600m lengths. When using the 1600m length it was found necessary to use more than one pipe loop pump to provide the necessary head. Three 3/2 high head Warman s l urry pumps were instal led for this purpose. Various combinations of pipe loop length and combined pipe loop-agitated tank residence imes have been used to successful ly produce the desired agglomerates depending on the nature of the feed slurry. A typical set of conditions include a combined mean residence time of three hours using a pipe loop length of 800m. When processing smal l batches of material, (say 2-3 tonnes) a smal ler agitated tank having a vol ume of approximately 2m3 may be used in conjunction with the pipe loop. A third pump P3 continuously transfers agglomerate bearing slurry from the tank 1 to a further tank 2 to which fresh particulate coal bearing slurry is added. The second mixing tank 2 operates in a si ilar manner to the first mixing tank 1 and a fourth pump P4 ci culates agglomerate bearing slurry from the tank through a second pipeloop L2 and back into the tank 2 to further improve the strength of the agglomerates. The addition of fresh slurry to the tank 2 improves the surface condition of the agglomerates reducing their oiliness while the second pipeloop L2 consolidates the agglomerates produced in the tank 2 and improves their strength. A fifth pump P5 transfers the agglomerated product from the tank 2 to a dewatering/cl assifying screen from which any smal l undersize agglomerates are returned to the tank 1 after separation of the waste mineral matter and water. In a modification of the above embodiment, the second mixing tank 2 is eliminated and the agglomerate bearing slurry from the first mixing tank 1 is pumped directly into the second pipe loop L2 for further conditioning in the presence of fresh particulate coal bearing slurry, which may be introduced into the. inlet of pump P4 in any suitable manner. A batch of approximately 2.2 tonnes of agglomerates has been produced using a pilot plant according to the preferred embodiment desc ibed above and the batch sub ected to flowability tests. In the pilot plant, the coal was processed through a hammer mill and ball mil l to generate a size distribution similar to a typical pulverized fuel specification. The fuel oi l used to achieve agglomeration was heated to a temperature of 30-35°C before addition to the slurry and the slurry was circulated through the pipe loop L1 for several hours prior to the oil addition to increase the temperature of the slurry to approximately 25°C. The remainder of the process was as described above and the resultant de-watered agglomerates were found to be strong and well formed with a top size of 2.3mm. The fol lowing Table 1 summarises the results for this run (d.b. = dry basis).

I_-__li_ li Summary of_ Resu Its for Aggl omerates Produced b^ tjie Pilot Plant Feed Coal Ash %d.b 20.1 Size analysis prior to agglomeration (% passing) Agglomerated Product Size, mm 0.5 98.8 0.25 96.0 0.125 83.7 0.063 61.0 Fuel oil addition (%by weight dry feed coal) 16.7 Ash, %d.b 6.9 It may be concluded from the tests conducted to date (as detailed further below) that the method of the invention produces agglomerates which are stronger, less sticky and have a lower (2%-3%) ash level than agglomerates produced by the prior art methods. While detailed comparative tests have not been conducted, qual itative observations have indicated that the prior art methods would not be capable of producing an agglomerate product of the same quality without unacceptable residence times in stirred tanks. The relatively short term circulation of the partly formed agglomerates in the presence of fresh slurry causes additive consolidation and further release of mineral matter, to a greater extent than would be achieved by further stirred tank processing for an equivalent time. It is not clear why pipeline circulation achieves these results although it is clear that the agitation which occurs in a pipel ine is different in character to stirred tank agitation. The batch of agglomerates produced by the pilot plant was subjected to testing to determine the flow properties of the agg l omerates and thei r abi l ity to withstand transportation in bulk. For the design and performance evaluation of material handling facilities, it is necessary to examine samples of the materials which are likely to produce the most difficult flow conditions. The conditions of moisture content, temperature and storage time relevant to the material under actual operating conditions need to be duplicated in the tests. However, since the main aim of the tests is to obtai n the characteristics of the aggl omerates for prel iminary assessment of handl ing characteristics of the aggl omerate, particul arl y under sea transportation conditions, oi led agglomerates having a moisture l evel equivalent to that expected from a stock pi le of the material were tested. Table 2 lists the properties of the oiled agglomerates used in the tests (d.b. = dry basis a.d.b. = air dry basis).

Tab ] e 2 Propert i es of 0_ile_^ Agg l omerate s Mo i s ture 7- (a. d. b. ) approx i mate l y 5 O i l % (d. b. ) " 1 7 Ash % (d. b. ) " 6.9 Size Analysis: Size, mm % Passing 2.0 68 1.0 4 0.5 0.5 Agglomerate Handling Character!^' sties The ability of a bulk material to flow is dependent on the strength developed by the material due to consolidation and weathering. As a result of this strength, the material may be able to form a stable arch or pipe. Free flowing bulk materials have no cohesion and hence no strength. Tests have indicated that agglomerates manufactured in accordance with the present invention, and at 5% moisture level, behave essential ly as a free flowing material. Although this free flow characteristic is slightly different from a perfectly free flowing material such as dry sand whose unconfined yield strength is always zero, energy coal showed much greater strength than that of the agglomerates. This means that the flowability of the agglomerates is much better than that of typical Australian export coals. Although there is some breakage of the agglomerates under high stress conditions, tests have also shown that the handl ing characteristics of the agg l omerates are satisfactory for ship loading transportation and unloading, and for hopper storage. ili£ Transportation Tests Degradation of particles due to stress and to vibration of the ship is not a serious problem for the majority of bulk solids as the particles are intrinsical ly strong. However consolidation (or compaction) of the bulk solids can create serious material handling problems if the bulk solids contain a large proportion of fines or the bulk solids have a cohesive characteristic. The difficulty in handling compacted bulk solids is dependent on the degree of strength developed in the compacted material and this important characteristic depends on the proportion of fines, moisture content, consolidation pressure, storage time and i n the case of oi l ed agglomerates the oil content. Tests have been conducted to evaluate the effect of st on the compaction and degradation of our oiled agglomerate predict the degree of degradation and compaction of the agglomerates which could be expected in the cargo hold of a 100,000 DWT bulk carrier. These tests have shown: (1) During loading into the cargo hold of a ship, agglomerates are compacted, by the weight of the material during loading, but further compaction due to storage time and ship vibration is smal l. (2) At a stress above lOOkPa the agglomerates formed into a consolidated cake although blocks of the cake were easily broken into separate agglomerates. This means that the compacted agglomerates near the bottom of the ship hold would not come crumbling down easily when the agglomerates are unloaded. However this is not essential if the material is unloaded using a grab. (3) At a stress of 150kPa (estimated stress for a material depth of 20m in the cargo hold) the degradation of agglomerates would be expected to result in an increase in the 0.5mm size particles of about 5%. At the half depth of the hold (10m deep) the figure would be less than 1%. These fines do not appear as discrete dry particles but are attached to the surround ng l arger parti cl es of agglomerates. Hence they do not form a dust problem in handling. Further Example of_ th^ Invention To check the vi bi lity of the process embodying the invention for lower grade feed material, further tests were conducted using the pilot plant described above and are detai 1 ed below. Five tankers containing a waste thickener underflow from a high volatile energy coal preparation plant were transported to the pilot plant for testing. Fol lowing transfer of the slurry to a surge tank, the solids concentration was adjusted to approximately 20% (by weight) prior to desliming. Desliming was undertaken by pumping the slurry through two 100mm KRT 2118 IV cyclones. An initial test was done using a 20mm apex stopper diameter. However this was subsequently enlarged to 25 mm diameter to give a cyclone underflow sol ids concentration of approximately 40%. A cyclone inlet pressure of 250 kPa was used and the flowrate to each cyclone was approximately was 17m3/h. The underflow was stored in temporary tanks during processing and returned to the surge tank on completion of the desliming. Some additional water was used to rinse the larger particles from these tanks. The deslimed slurry was circulated through a bal l mil l closed circuit to grind the sol ids to a size distri ution close to that of typical pul verised coal (99% passing 300 microns). Approximately 10m3 of this slurry was reserved for secondary addition to. the agglomerates after initial agglomeration. After grinding, agglomerating oil was added to the slurry at the inlet to the bal l mi l l sump pump whilst the slurry was circulated through a 1 km long 100mm diameter pipeloop and surge tank circuit. The o l was added in several steps to ensure that excessive oil was not used. A low sulphur furnace oi l (0.4% sulphur) was used to permit the production of agglomerates having a low sulphur specification. Circulation of the slurry was continued until the agglomerates had reached 2-3 mm in diameter. At this stage additional finely ground slurry was added to the surge tank to absorb excessive oi l on the agglomerate surface and produce a "non sticky" transportable product. Circulation was continued for a further two hours prior to dewatering the agglomerates on a 0.5mm wedge wire vibrating screen. A series of water sprays were used on the screen to rinse off excessive clays and other mineral matter prior to discharge of the agglomerates into the storage hopper. Details of the various slurry and solids balances are summarised below. These figures are approximate and are based on best estimates from tank vol umes and flows. Note that there are some variations in volumes due to additional water used for rinsing and pump gland seals. Thickener Underf 1 ow total volume = 104m3 solids = 35t after unloading to total volume -= 154m3 surge tank slurry density = 1.108t/m3 solids concentration = 20.6% sol ids = 35t Size analysis and Ash distribution Size fraction W Weeiigghhtt Ash mm F Frraaccttiioonn % % %d.b. +0.5 3 3..99 22.6

-0.5 + 0.25 1 155..88 31.6 -0.25 +0.125 1166..00 47.7 -0.125 +0.063 1 122..00 37.5 -0.063 5522..33 58.9 total 48.8 Cyclone Underf1 ow total volume = 47m³ slurry density = 1.199t/m³ solids concentration = 40.5% sol ids = 23t solids recovery yield from cyclone fee, d.b. = 65.7%

*coal matter recovery from cyclone feed, d.b. = 72.8%

*coal matter •= solids - mineral matter; assuming mineral matter = 1.1 x ash.

Size analysis and Ash distribution

Size fraction Weight Ash mm Fraction % %d.b.

+0.5 7.0 21.7

-0.5 +0.25 22.3 29.5

-0.25 +0.125 22.3 40.3

-0.125 +0.063 16.2 33.4 -0.063 32.2 67.8 Total 44.3 Cyc1one Overf1o tota1 vol ume = 122m³ slurry density = 1.048t/m³ solids concentration = 9.55% sol ids = 12t Size analysis and Ash distribution S Siizzee ffrraaccttiioonn Weight Ash mmmm Fraction % %d. b.

+ +00..006633 2.2 11.1

--00..006633 ++00..004455 1.6 6.1

--00..004455 ++00..003388 1.6 8.2

--00..003388 94.6 58.7

TToottaall 56.0 Crushed S 1 urry P r o r to Agglomeration total volume - 71m³ slurry density - 1.136t/m³ . solids concentration = 28.8% solids =^• 23t Size Analysis Size, mm %passing 0.5 99.9 0.25 99.3 0.125 94.3 0.063 75.1 Ash, % d.b. = 44.3 Agglomeration volume of initial slurry used for agglomeration = 61m³ sol ids m 20t total oil added - 3666kg (assuming oil density ■ 0.94t/m³) oil added on initial solids (d.b.) = 18.3% (by weight) Additional solids added = 3.3t during secondary agglomeration oil added on total - 15.7% solids (d.b.) Product Agglomerates Estimated agglomerated = 12.2t product, dry, oil free Estimated agglomerated - 17t product including oil and 10% moisture Product ash, %dry oil free = 5.9 Estimated product yield = 52.2 from ground desli ed slurry, %d.b. (for tailings ash = 86.5% d.b.) Estimated product yield = 34.9 from original thickener underflow, %d.b. Estimated coal matter yield = 97 from ground slurry, %d.b. Estimated coal matter yield =• 70.6 from original thickener underflow, %d.b. Estimated oil based on = 21.6% dewatered product basis (including oil and 10% moisture) Chemical Anal sis: (expressed on moisture free basis, incl uding oi 1 ) Ash 5.0% Vo l ati l e Matter 46.8% F i xed Ca rbon 48.2% Tota l S u l ph ur 0.42% Speci fi c E nergy 33.9M<J/kg The moisture of the agglomerates from the product hopper after overnight drainage was approximately 12.5%. Agglomeration and mineral matter separation of the deslimed product was. readily achieved in the above example. The product ash was significantly lower than that achieved i,n bench scale tests using conventional stirred agglomeration techniques; this results from the higher l evels of consol idation and excl usion of mineral matter achieved in the ipeline and agitated tank circulation system. Petrographic examination of bench scale products showed that the mineral matter in those agglomerates contained approximately 25% pyrite. It is presumed that use of the present invention results in elimination of the majority of this pyrite, as long as it is ground to a size which al lows separation over the dewatering screen.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. A process for production of coal aggl omerates comprising agitating an aqueous slurry of coal particles in a first agitating means in the presence of a predeter ined quantity of oil to form coal agglomerates, further agitating said agglomerates in the presence of further coal particle bearing slurry to improve the dryness and quality of the agglomerates, characterised by the step of transporting the slurry containing said agglomerates in a pipeline to further improve the strength properties of the agglomerates. 2. The process of cl aim 1, wherein the step of transporting the s l urry in a pipel ine fol l ows the firstmentioned agitation stage. 3. The process of claim 1 or 2, further comprising the step of introducing further coal bearing aqueous slurry to the slurry being transported in said pipeline. 4. The process of claim 3 wherein said transportation of the slurry comprises recirculation in a pipeline loop. 5. The process of claim 2, further comprising delivering said slurry containing said agglomerates from said pipeline to a second agitating means and agitating said agglomerates in the presence of fresh coal bearing slurry. 6. The process of cl aim 5, further comprising transporting the agglomerates and slurry from said second agitating means through a further pipel ine to further improve the quality of said agglomerates. 7. The process of cla m 6 where n said transportation of agglomerates and slurry from the second agitating means comprises recirculation in a pipeline loop. 8. An apparatus for producing coal agglomerates comprising a first agitating means, means for introducing an aqueous slurry of coal particles into said first agitating means together with a predeterm ned quantity of oil, means for withdrawing toe slurry and agglomerates produced by said first agitating means, a pipel ine connected to said wi hdrawing means, and means for col lecting said agglomerates from said pipeline for further processing. 9. The apparatus of claim 8, further comprising a second agitating means, means for introducing said agglomerates from said pipeline into said second agitating means, means for introducing fresh coal bearing aqueous slurry into said second agitating means, second pipeline means attached to said second agitating means and means for circulating said agglomerate bearing slurry through said second pipeline. 10. The apparatus of claim 8, further comprising means for introducing fresh coal bearing aqueous slurry into said pipeline. 11. A process for the production of coal agglomerates substantial ly as hereinbefore described ith reference to the accompanying drawing. 12. An apparatus for the production of coal agglomerates substantial ly as hereinbefore described with reference to the accompanying drawing. 13. Coal agglomerates when produced by the process of any one of claims 1-7 or 11.