Document 0qKgeGgZqVRkeQygaw9nyz1J

HIOH-OENSfTY BURUNDUM in lho lorm of small cylinders proves to be efficient grinding medium to reduce eoal temples . in for onotyiis. Tests show Insignificant eontamfooiton o sample and real time sovlng In length of. grinding ^ High-Density Burundum Speeds Up Co Crushing for Laboratory Analyses Effective testing of cool characteristics coll* for pulverising samples to o given fineness. Here is a report on effectiveness of the crushing medium in ball mills, to speed up the job of pre paring coal sompfes--possible savings in both time and money By WAITER W PERKINS, Chief Cvremin, US, Stoiwvtrt Co Coal is one of. tho most totted ma teriel* used by, industry- The cool uter want* to be assured he's getting all the fuel he*s paid for, ond the producer hat to know whether he (a meeting the cus tomer's specifications. Because of this two-way Interest, numerous laboratories re kept busy just testing coal. Tool Methods. Many standard test procedures have evolved that the dif ferent laboratories uae with little or no local variations. Descriptions of theso tests appear in Pan V of the 1949 Amer ican Society for Testing Materials Standards, pp 575 to 664, and in Bul letin 492 of the Bureau of Mines, Meth ods af Analysing Coat ond Coke. Accuracy ond speed of making the analysis are important for all labnra- lories, whether run by (he coal-using or coal-producing businesses, or pri vately owned. An important siep in the analysis concerns pulverising of the coal sample. In present practice the coal sample received by the laboratory must be given a preliminary drying to bring its moisture in balance with oimosphcric conditions. After drying, the coal is crushed quickly hy rolls to 20-mesh site and reduced fry a riffle sampler to about 200 grams. Tho 200-gram sample is then crushed to sub 60-mesh particles. Both the Bureau of Mines and the ASTM sug gest using l-gallon ball mills, about half filled with 9^-in- diameter pebbles for this reducilon process. The Bureau mentions rotational speeds of 45 to 50 rpm, while the ASTM Standards lions speeds of 60 to 75 rpm. According to these authorities 10 to 20 minutes are needed te v down bituminoiia coals, while sbetit hour is needed for anthracites.- T ogree that from 0.2 to 0.49b of the a pie usually remains on the screen *L the milling period. Since these panicles are likely to be the ask ing substances, they are reduced hand to pass the screen, and then in with the bulk sample. The me'" ush, volatile matter, elements! tea nenta, calorific value and fusibibiJ ash are found from this sub ' mareriaJ, Boll milling, as a coat-red# method, has certain advent*!**- ENGINEERING AND MANAGEMENT SECTION possible some savings in space and In moneylost Comparison. Two porcelain mill For the larger jar, speed* from SS to 70 rpm aeemed equally effective with- 0.4% of the sample remaining on 60 jars were used for comparative testing. The 0.48-gallon jar had a volume of about 1600 cc, and the 14-gallon jar a volume of about 5000 cc. With covers mesh. Rotation time effects Were found as follows for a speed el 1)1 rpm with amall jars: and hardware (hey have an empty weight of 8 and 16 lb, respectively. Two samples of bituminous coal were tested in each site jar. The coaU were crushed to a nominal 20 mesh by the laboratory of a large coal user. Size analysis of these two coals when dry, were: Time IS min 20 min 30 min 45 min 60 min % retained on 60 mesh Flint \ 2.6 Burundum 2.0 0.4 0.3 . 0.3 04 04 Sample / On 6 mesh............. 0.7% On 8 mesh............ 5.6% On 12 mesh.......... 15.1% On 14 mesh.......... 10.8% On 16 mesh.......... 8.6% On 40 mesh...........344% On 50 mesh........ . 5.4% On 60 mash........ . 2.0% On 100 mesh___ . 6.6% Through 100 mesh 10.9% II 0.05% 0.4% 5.7% 7.7% 8.6% 404% 8.0% 3.1% 10.8% 15.2% Up to 15 minutes, the sample is still relatively coarae with Burundum, while after 20 minutes there la little further gain. A time of 20 minutes was also critical with the large jar. As a sidelight, grinding sample If up to an hour with both amall and large jars did not completely pulverize the coal to pas* through 60 mesh. Sample 1 broke up more readily. The 60-mesh retention or passage la Comparison tests were made to find the most critical point in milling testa. the effect on particle alxe reduction 'of Despite comparable size 60-mesh reduc (1) different ratios of coal to media charges (2) rotational speed (3) time of run. Tests were run with fUnt-pebbles tion, the larger jar produces leu resi due to be caught on a 100-mesb screen. For instance, in the small jara, 2.5 lb and hlgh-dehsity cylindrlcal-shaped Bur undum as the crushing media. -All the milling tesla were run' on a 3-lier var iable-speed mill similar to the one in the of flint with 100 grams b! coal at 111 rpm left a total residue of about 29% on 100-mesh screen. On the other hand, 3.5 lb of Burundum under the some con photo above, left. In the smaller jar, 34 lb of Bumn- dum or 2.5 lb of flint with an 80-gram ditions left a total residue of 9.7%, while the large jar rotating at 67 rpm produced a residue of only 2,4% on the coal charge was best. It was decided, however, to work with 100-gram eharges since ft was more practical for the lab 100-mesh screen. Had only 80 grams of eoal been used In the smaller jars, a better showing would Have resulted, but oratory. The targer jar was most effi this would be outside the limits Imposed cient with eoal eharges between 150 and on this test project. 200 grams, and a Burundum charge of Results. Our work showed optimum 10-12 Ih. Leadings of over 200 grams tond'rtlont that will save time In prepar of coal and 12 lb of media caused the ing coal aamples. Some of the condl- effectiveness to fall off. No trials were tions, such as media to eoal charge made with flint In the larger jar. ratios, when found, may be considered Rotating speed proved an important as constants, while that of speed may be Allround the material becomes wellJjhsd- Moisture staying in the jar keep* jb cmJ more nearly representative of ;b Woced equilibrium elate.. The .*b**isa causes no significant containhsdoo of the sample. Ball milling is trbihelj elean operation. After a run Nr can be emptied quickly, brushed si residual particles, and be ready R iaother sample. 'J*t mills can bo mode even more de***** by sabstituting smaller sizes for Presently accepted gallon ones. This Jj*-ihe handling problem. Shorter on<^ considerable grosareduction make It possible for *r physically handicapped perS? handle the lore more eerily, r l*rs, which are less costly, moke factor In time needed for pulverizing with the small jars. Speed should be at least 110 rpm. Nature of the coal may havo some bearing on the moil effective speed. Sample I broke down more quickly and thoroughly with Increasing speeds up to the machine Unit of 124 rpm. Sample IT reached a mealmum be tween 110 and 115 rpm, then fell off in effectiveness, though not seriously. Typical results, using 100 grams of coal sample II with 2.5 lb of flint and 3.5 lb of Burundum over a 20-minute milling period, were: Speed 90 rpm 111 rpm 124 rpm % retained on 60 mesh Flint Burundum 3.0 0-9 2.6 0.4 2.9 0.5 varied as the eoal differs. There is sim ilar particle-size reduction through a 60mesh screen of eoal ground in either the 0.46- or 14-gallon jars. Wo can conclude that for comparable results, a jar weighing no more than 11 lb, complete with charge, will do ao ac ceptable a job as one weighing about 25-27 lb. The smaller physical dimen sions expand the capacity of the mill it self. Unloading of jars is more convenient. Thcie are fewer pieces of crushing media in the charge ond less chanee of contamination from residual matter. Variable-speed drives prove desirable because optimum speeds can be used as needed. High-density media prove bet ter in rapid pulverization than flint. ENGINEERING AND MANAGEMENT SECTION II