Dust Control Handbook for Industrial Minerals Mining and Processing

produce an increase in the penetration rate of the drilling operation [Hustrulid 1982; USBM1995]. Since this is applicable for underground mining, it may also correlate to increasedpenetration rates in surface mining situations. Further testing would be required to definitivelyassert that wet drilling would increase penetration rates of surface percussion drilling operations.Recommendations for Proper Wet DrillingBased on testing results and observational best practices, the following recommendations applyto surface drilling using wet drilling techniques.In order to operate at close to the optimum water flow rate, the operator should slowlyincrease the amount of water just to the point where visible dust emissions are abated.Addition of more water beyond this point will not provide any significant improvementin dust control, but will most likely create operational problems such as bit degradation(when using tri-cone bits) and possible seizing of the drill stem. Using less water willgive poor dust control.It is important that the water be increased slowly to account for the lag time as theair/water/dust mixture travels from the bottom to the top of the hole.Continuous monitoring of the water flow during drilling is necessary to provide optimumdust control and prevent drill steel binding.The water used in the wet drilling system should be filtered to prevent debris fromplugging the drill's wet drilling system.When using a wet suppression system in outside temperatures below freezing, the systemmust be heated when the drill is in operation, and during downtimes the system must bedrained. For most drilling machines the proximity of the water tank and lines to theengine and hydraulic lines is sufficient to prevent freezing during operation (except inextreme cold weather situations). The water tank and lines must be drained when thedrill is not in operation.Disadvantages to Wet DrillingWhile wet drilling may be an advantage when percussion drilling, there is a disadvantage to wetdrilling when rotary drilling. The use of water degrades the tri-cone roller drill bits and shortenstheir lives by 50 percent or more [Page 1991]. This is due to rapid bearing material degradationthrough hydrogen embrittlement and accelerated bit wear. The bit wear is a result of operating inthe abrasive rock dust-water slurry environment.There is a solution to the disadvantage of short bit lives when wet drilling. In order to obtainacceptable drill bit life, the water must not reach the drill bit. This can be achieved througheffective water optimization and water separation. Data gathered by the Bureau of Mines(BOM) from a mine where drilling occurred in monzonite, sandstone, limestone, and iron oreover a 14-year period showed that drill bit life averaged 1,938 ft/bit when wet drilling withoutwater separation. With water separation, the drill bit life increased over 450 percent to anaverage of approximately 9,000 ft/bit [USBM 1988].82 Drilling and Blasting