Design Methodology, Testing and Evaluation of a Continuous Miner Cutterhead for Dust Reduction in Underground Coal Mining

The Western Mining Resource Center (WMRC) at the Colorado School of Mines (CSM) received a grant from the National Institute for Occupational Safety and Health (NIOSH) to perform a study that will help reduce the amount of respirable dust generated from the cutting of coal. The study is based upon laboratory testing and field experience. This information is being used to develop a new cutterhead design in conjunction with Joy Mining Machinery. The new cutterhead design is being evaluated with existing computer models developed at CSM. The new continuous miner cutterhead will be tested at the Twenty Mile Coal Company’s Foidal Creek Mine. Baseline testing with the current cutterhead will also be performed. This paper presents laboratory test results, design methodology, and test results performed to date. BACKGROUND A need to reduce the amount of respirable dust exists in the underground mining industry. Crippling and fatal diseases result from retention of coal dust in the lungs. These included coal workers pneumoconiosis (CWP, a.k.a Black Lung), progressive massive fibrosis (PMF), silicosis, and chronic obstructive pulmonary disease. In 1998 mine operators reported 224 cases of CWP and PMF (combined), 138 of which occurred among underground coal miners. In the same year 14 cases of silicosis were reported, 8 being from underground miners. These statistics do not include the occupational health of all coal miners, since the miners participate in the programs at their own discretion. INTRODUCTION In the United States, approximately 1/3 of annual coal production comes from underground mines. Almost all underground coal production is produced by continuous miners and longwall shearers. As mechanical mining technologies advance, more underground non-coal mining operations utilize mechanical methods for mining and development. The majority of respirable dust generated by mechanical miners is generated by material that is crushed directly under the individual bits/cutters on the cutterhead. Figure 1 shows the crush zone underneath a bit, and the resulting fractures in the rock that lead to production. Reducing the amount of dust generated reduces the amount of dust that can become airborne in the working area and pose a health hazard. It is known that cutting geometry affects the amount of dust generated under an individual cutter/bit. These cutting geometry factors include bit tip angle, angle of attack, and bit penetration. Also, reducing the number of cutters engaging the rock, which results in an increase of bit spacing, can reduce the total amount of dust generated. These variables also have a major effect on production. A full scale test program has been performed to help quantify the effects of the different variables on dust generation as well as production. Field tests are planned to confirm the full scale laboratory cutting tests. Figure 1: Crush zone, where dust is generated. LINEAR CUTTING TEST PROGRAM A full scale laboratory test program was performed to help quantify the effect of cutting geometry on the generation of respirable dust. This test program consisted of full scale cutting tests using different bit types at differing cutting geometries in a coal measure rock, a high silica sandstone. The Linear Cutting Machine (LCM) at the Colorado School of Mines was used for conducting the cutting tests. The LCM forces a large rock sample through an actual bit at a preset cutting geometry. After each pass of cutting tests, muck samples were collected to determine the relative percentages of respirable dust. The linear cutting tests also measure forces acting on the cutter to ensure that the bits are operating as they would on an actual excavator while providing an acceptable level of production. This full-scale testing 2002 SME Annual Meeting Feb. 25 27, Phoenix, Arizona 2 Copyright  2002 by SME eliminates the uncertainties of scaling and any unusual rock cutting behaviour not reflected by its physical properties. This is because the cutting action of the LCM very closely simulates the cutting action seen in the field. Linear Cutting Test Equipment and Procedures The LCM features a large stiff reaction frame on which the cutter is mounted. A triaxial load cell, located between the cutter and the frame, monitors forces and a linear variable displacement transducer (LVDT) monitors travel of the rock sample. The rock sample is cast in concrete within a heavy steel box to provide the necessary confinement during testing. A picture and a schematic drawing of the LCM are presented in Figure 2 and Figure 3, respectively. Figure 2: Linear Cutting Test Figure 3: A schematic drawing of the LCM. A servo controlled hydraulic actuator forces the sample through the cutter at a preset depth of penetration, width of spacing and constant velocity. During the cut, the triaxial load cell measures the normal, drag, and side forces acting on the cutter. After each cut the rock box is moved sideways by a preset spacing to duplicate the action of the multiple cutters on a mechanical excavator. A drawing of the three force components acting on a cutter is shown in Figure 4. Figure 4: Drawing of forces acting on a pick cutter. In field excavation, the individual cutters on the machine always operate on a rock surface damaged from the previous cutting action. This scenario is duplicated in the laboratory by thoroughly conditioning the rock surface before testing begins. This is accomplished by making several passes before data is collected. A schematic drawing explaining the nomenclature is presented in Figure 5. Figure 5: Schematic of a LCM sample and nomenclature. At the end of each pass of data cuts, muck samples were taken to determine the amount of dust present in the cuttings. To acheive this, 2 carpentry squares were laid over the undisturbed muck to form a sample area. All of the material within the area was collected, using fine brushes to retrieve the fines, and stored in double plastic bags. After the samples were collected, the material was separated for size distribution analysis, using a series of sieve screens. The material passing the finest screen then had their size distribution analysis performed by hydrometer surveys. Figure 6 shows a muck sample to be collected. 2002 SME Annual Meeting Feb. 25 27, Phoenix, Arizona 3 Copyright  2002 by SME Figure 6: Cuttings sample collection. Linear Cutting Test Parameters The tests performed for this program consisted of four major variables: cutter type, line spacing between cuts, penetration of cuts, and attack angle. The dependent (measured and calculated) variables were average cutter forces (normal, drag and side), specific energy and muck size distribution for determination of percentages of repirable dust. The constant variables were rock type, cutting sequence (single scroll pattern), cutting speed (10 in./sec., 254 mm/s), skew angle (0°) and tilt angle (0°). The two different standard point attack bits tested in this program were the U-92 and the U-94 (Figures 7 and 8, respectively), both produced by Kennametal. Both of these bits would commonly be used on continuous miners for producing coal where harder coal measure rocks in the floor or roof are encountered. The U-92 had a 16 mm (0.63”) diameter tip and the U-94 had a 19 mm (0.75”) tip. Both of the commercially available conical bits had a tip angle of 75 degrees. Figure 7: U-92, with 16 mm tip. Three different spacings, 13, 19 and 25 mm (0.5”, 0.75” and 1”) were tested. The two different tested penetrations were 2.5 and 5.1 mm (0.1” and 0.2”). The tested angles of attack were 47.5 and 52 degrees. Angle of attack is defined as the angle between the bit axis and the tangent of the cut surface (Figure 9). These parameters were chosen to represent the geometries that may be used on a continuous miner designed to operate in a difficult roof rock, such as a hard sandstone. Figure 8: U-94, with 19 mm tip. Figure 9: Angle of attack description. The rock used for the cutting test was Lyons Sandstone. This was a hard abrasive sandstone that is similar to roof rock in many underground coal mines. It is representative of relatively difficult cutting conditions. The Lyons Sandstone had a compressive strength of 120 MPa (17,350 psi) and a tensile strength of 6.1 MPa (890 psi). It should be noted that the Lyons Sandstone is a very abrasive rock, as can be seen by its measured Cerchar Abrasivity Index of 3.3. Linear Cutting Test Results The LCM tests have successfully provided data to begin the quantification of the generation of fines based on different cutting geometries. During the cutting tests, forces in all 3 dimensions (normal, drag & side) acting on the bits were recorded to ensure that the cutting action is representative of cutting in the field. The normal force results are a function of thrust required for the cutters to penetrate rock. Cutterhead torque requirements are derived from the drag force results. Cutting coefficient, the ratio of the drag force over the normal force, provides a measure of the direction that the bit is being loaded. And, the specific energy results provide insight to the efficiency of the different cutting geometries. Figure 10 shows a sandstone sample during linear cutting testing. A summary of the force results is presented in Table 1 and descriptions of the tests results follow. 2002 SME Annual Meeting Feb. 25 27, Phoenix, Arizona 4 Copyright  2002 by SME Table 1: Linear cutting force results. Average Forces Specific Tool Attac k Spaci ng Pene. Norm al Drag Side Energy I.D. Angle (mm.) (mm.) (kN) (kN) (kN) (kWhr/m ) U94 48 deg. 12.70 2.54 9.3 5.4 2.8 47.4 U94 48 deg. 19.05 2.54 13.6 7.5 2.5 44.5 U94 52 deg. 12.70 2.54 6.4 3.7 1.3 32.9 U94 52 deg. 19.05 2.54 11.3 6.0 1.0 35.6 U92 KHD 52 deg. 12.70 2.54 4.2 2.9 0.4 25.9 U92 KHD 52 deg. 19.05 2.54 7.1 4.9 1.1 29.0 U92 KHD 52 deg. 25.40 2.54 11.1 7.6 -1.2 33.5 U92 KHD 52 deg. 25.40 5.08 8.9 6.6 -1.3 14.6 Figure 10: Sample during for linear cutting tests. Normal Force Results Normal force requirements ranged from 4.2 to 13.7 kN (950 to 3,070 lbf). The wider tip width generated higher normal forces compared to the smaller tip. Wider spacings also generated larger normal forces, while other variables were held constant. The increase in attack angle, from 48 to 52 degrees, created a reduction in the normal forces. This was due to the reduced vertical profile generated by increasing the angle of the bit’s axis. And the increase in penetration reduced the normal force, due to more efficient fracture propagation. Chart 1 shows a plot of the normal force results. 0 2 4 6 8 10 12 14 16 10 12 14 16 18 20 22 24 26 Spacing (mm.) N or m al F or ce (k N ) U-94, 48 deg, P=2.54 mm U-94, 52 deg, P=2.54 mm U-92, 52 deg, P=2.54 mm U-92, 52 deg, P=5.08 mm Chart 1: Normal force results. Drag Force Results Drag forces ranged from 2.9 to 7.6 kN (660 to 1,700 lbf). As with normal force, the drag forces increased with increases in tip diameter and spacings while the other variables were held constant. The increase in attack angle created an increase in the drag force requirements, which is opposite of the trend for the normal force requirements. And the increase in penetration resulted in a decrease in the drag force, illustrating more efficient fracture propagation. Chart 2 shows a plot of the drag force results.