In the eastern U.S., where a majority of the coal production is washed, primary crushers are usually reducing a 20- to 24-inch feed to a more easily conveyed 4- to 6-inch top size. In the West, especially at large surface mines such as those found in the Powder River Basin (PRB), the raw feed to the primary crusher can be as high as 60 to 70 inches. Because the RoM characteristics, these mines do not wash coal and they are looking to create a direct-ship, 2-inch product. Employing some of the largest primary coal crushers, they are reducing 20- to 70-inch RoM coal to 2 inches in a single pass.
Another concern that surrounds the crushing process is the generation of fines. More than a dust control nuisance, excessive fines can be a safety issue. More mines are becoming cognizant of the fact that fines only create more headaches downstream, according to Mike Hamby, vice president-sales, Gundlach Equipment Corp. 'In the past, the mines did not worry about generating fines,' Hamby said. 'Fines are now viewed as wasted material and the mines are taking accountability all the way to the face. Fines represent product that is lost the moment it's extracted.' Gundlach manufactures a single-stage, two-roll and double-stage, four-roll crusher. Editor's note: Although feederbreakers and sizers could be considered primary crushers, this article defines primary crushers as single roll crushers and two stage crushers.
Real estate comes at a premium in coal country and another noticeable trend is that the mines want one piece of equipment that could do the job without multiple stages of handling in the smallest footprint possible, Hamby explained. Today's crusher manufacturers are seeing more larger applications, 500 to 800 tons per hour (tph), rather than the traditional smaller installations (150 to 200 tph)
As far as improvements, Hamby sees a number of refinements being implemented to make today's machines better. Gundlach has a coupling-mounted roll that allows quick removal and replacement. The company also offers a gear box that allows for the timing of the rolls to eliminate or dramatically grinding and reduce fines generation.
Wear materials is another area that Hamby says crusher manufacturers have made great improvements. 'We just replaced a set of rolls in the PRB that had 50 million tons of life,' Hamby said. 'As little as 15 years ago, we were very happy to get 8 million tons out of that same application.'
Crusher Selection & Application
When selecting a primary crusher, a mine needs to consider several parameters, explained George Edmiston, mineral division sales manager, McLanahan Corp.
'Some of the factors that dictate the selection of the crushing equipment include the desired product size, capacity, the HGI [Hardgrove Grindability Index ], percentage of rock in the feed and the hardness of that rock,' Edmiston said. 'Since many of the available types of crushers cross over application lines, other considerations such as space restrictions, mining methods and cost may also figure into the final decision of crusher selection.' Edmiston presented a paper at Coal Prep 2009 on the application of crusher types.
'The single roll crusher with its 6:1 ratio of reduction is ideal for reducing large feed lumps to a medium size product while producing a considerably lower percentage of fines,' Edmiston said.
Minimum product sizing of a single roll crusher is generally limited to 2 to 3 inches. The crushing is carried out along the full width of the extra long, curved crushing plate and a low speed crushing roll.
The curvature of the crushing plate provides an ample throat opening to capture large irregular feed lumps, Edmiston explained. 'The replaceable crushing plate tips or liners are slotted to intermesh with the roll teeth to produce a cubical product and effectively reduce slabbing,' Edmiston said.
Single roll crushers are V-belt driven and employ a large diameter flywheel with gear and pinion set to reduce roll speed. With the assistance of the inertia generated by the flywheel this crusher is operated with relatively low horsepower and requires lower headroom in comparison to other crushers used for primary reduction. The roll diameter and width of the crusher will ultimately be dictated by the feed size, product size and capacity.
Generally belt driven crushers employ some form of tramp relief mechanism to allow momentary movement of the crushing plate or one of the crushing rolls, Edmiston expalined. This can be accomplished with mechanical toggles or gas cylinders. 'Single roll crushers, while ideal for many applications, are limited in throughput capacity and are, therefore, not applicable for high tonnage requirements,' Edmiston said.
Two stage crushers, also belt driven, perform primary and secondary crushing, or secondary and tertiary crushing in a single pass in one machine. McLanahan's Triple and Quadroll crushers reduce RoM feed material to marketable sizes. with capacities exceeding 7,000 tph.
Depending on the design, the top stage of the crusher is comprised of either a single roll or double roll layout. The primary stage receives the feed material and accomplishes the initial reduction prior to the bottom stage. The bottom stage of either model is made up of a double roll configuration which receives the material from the top stage and performs the final size reduction. The primary stage of a two stage crusher is typically driven from the secondary stage through chain and sprocket or interstage V-belts. Independent drives may be required for each crushing roll in higher capacity applications.
'The two stage design combined with the proper tooth configuration produces a uniform cubical product with minimum fines,' Edmiston said. 'Higher roll speeds made possible by utilizing belt drives allow for tremendous throughput capacity.'
Generally, the Quadroll is recommended for a medium size feed material, while the Triple Roll, with its single roll top stage, is effective in handling larger size feeds, Edmiston explained. Typical feed sizes range from 18 to 72 inches with final product size of 2 to 3 inches.
The Impact of Crusher Type on Fines Generation
Fine coal particles create dust problems in material handling and are more costly to clean in the preparation plant, so minimizing the generation of fines in the crushing process can provide tangible benefits to the mining company, explained Kip Alderman, principal, Advanced Coal Technology, Inc. 'The hardness of the coal probably has the greatest impact upon product fineness, but the type of crusher employed and the crusher settings also have significant impacts upon the ultimate product fineness,' Alderman said.
Alderman also presented a paper that covered this subject at Coal Prep 2009. He performed a study to evaluate the difference in crushers on fines generation. In addition, tests were performed to compare two types of crushers, and to evaluate the impact of pre-screening ahead of the crusher on fines generation.
A large surface mine in southwest Wyoming was experiencing dust accumulations at the numerous transfer points throughout the material handling system. The mined coal has an HGI of 60. Baghouses were installed at the transfer points with the dust discharged down-flow from the transfer point. Coal with a 24-inch top-size was fed from a truck dump to an inclined vibrating screen. The plus 1-1/2-inch oversize coal reported to a Jeffrey Flextooth crusher.
The mining company approached Alderman with two primary crushing questions:
Are there crushers that could produce less fines and dust than the existing crusher?
Does pre-screening the feed reduce the percentage of fines generated by crushing?
Research suggested that two-stage double-roll crushers would produce a lower percentage of fines at the same top-size than other crushers. Pre-screening feed to remove most of the undersize material was also identified as a way to reduce the percentage of fines in the final product. A study on liberation and crusher type provided useful information on a variety of crushers and settings when crushing identical coal feeds.
Figure 1 shows summary data and a chart comparing the results for the different crushers. The blue bars in Figure 1 show crushers/settings that generated product top-sizes as large, or larger, than that of the single roll crusher. In every case, those crushers produced more fines than the single roll crusher.
Based upon this information, it was decided to collect large samples to evaluate the existing screen and crusher, and perform small scale crushing tests at Gundlach's facilities near Belleville, Ill.
Crushing tests were performed using a Gundlach two-stage double-roll crusher. Table 1 shows the results from crushing the unscreened and the screened feed.
Crushing screened feed generated a product with the fewest fines. Compared to the screen/ Flextooth system, the two stage double roll crusher product with screened feed contained nearly 30% less fine coal. Based upon the results of the study, the system was replaced with a screen/two stage double-roll crusher system. While not totally eliminated, dust accumulations were reduced with the new system.