Minneapolis, MN: U.S. Department of the Interior, Bureau of Mines, TN 376, 1991 Mar; :1-3
Objective: Increase the productivity of domestic mining operations by improving the performance of rock drilling systems. Approach: Investigate the use of chemical solutions at zero surface charge concentrations as drilling fluid additives to enhance penetration rates and extend bit life. Background: Bureau researchers have investigated the use of chemical additives to enhance drilling performance and have found that the most efficient drilling occurred when the drilling fluid was able to maintain a zero surface charge (ZSC) on the rock surface. Laboratory drilling studies using 5/8-inch diameter diamond-impregnated coring bits on rock types such as quartzite, granite, taconite, and marble. Cationic compounds were used as the surface charge neutralizing additives during these tests. The drilling studies demonstrated enhanced drilling performance, however, only at the singular additive concentration that produced the ZSC condition. Bureau researchers made a breakthrough when they found that polyethylene oxide, although a nonionic polymer, was able to maintain the ZSC condition over a wide range of concentrations. Success using polyethylene oxide as the drilling fluid additive during subsequent laboratory tests prompted testing under field conditions. How It Works: The mechanism underlying the enhanced drilling performance is hypothesized to be activated cracking. Scientists know that the rock surface is under maximum tension when under zero surface charge conditions. Under these conditions, the likelihood of increased crack formation at or near the rock surface is increased since the rock surface is weakened under the tension. When the increased density of cracks is propagated, faster rock breakage occurs and results in faster drilling penetration rates. Less bit wear is also observed. The wear force on the bit is lessened since a rock surface with a higher density of cracks provides less resistance to the advancing bit than a rock surface with a lower density of cracks. The activated cracking hypothesis has been validated by Bureau researchers in the drilling of transparent, synthetic quartz with water alone and with ZSC concentration solutions of PEO and other chemical additives. High speed photography of synthetic quartz drilling showed a zone of cracking in front of the advancing bit when drilling with ZSC concentration solutions of PEO and each of the chemical additives tested. But this cracking did not occur when drilling with water alone. To best take advantage of this chemically-induced rock weakening, drillers need to ensure that the mechanical energy introduced for drilling is not so high that it swamps out the chemically-induced effect. Drillers may have to use less rotational speed and/or thrust than they are accustomed to using when drilling with water alone. Field Test Results: In controlled field tests conducted in Ohio and Pennsylvania, PEO was shown to improve the drilling penetration rate of NQ (3") diamond core bits in several rock types. When tests were conducted in a side-by-side pattern using holes alternately drilled with and without PEO, a 25-30 pct penetration rate improvement was gained in drilling shales and limestone. A similar 35-50 pct improvement was obtained in claystone rock. A 50 pct increase represents a cost savings of about 30 pct for contract drilling. Drillers noticed reduced drill string chatter and vibration, increased flocculation and settling of effluent particles, and reduced fluid loss in moderately fractured rock formations. In another controlled full-scale test, use of PEO improved the drilling penetration rate of AWG (2") diamond core bits by 11 pct and bit life by 9 percent in blocks of hard Sioux quartzite. These tests were conducted with a 2100 pound thrust and a rotation speed of 500 rpm. An initial field test of the PEO additive for drilling blastholes in taconite yielded a 70 pct increase in penetration rate with a 26 pct increase in bit life. The holes were drilled with 15-inch diameter rotary tricone bits to the depth of 40- to 45-ft on adjacent benches at a producing iron mine in Minnesota. Hole flushing was done using high-pressure air and water mist. The rust bench (165 holes) was drilled entirely with water mist as were the initial 40 holes of the second bench. The remaining 100 holes of the second bench were drilled using a mist of a 15 ppm PEO solution. The average penetration rate for water-drilled holes on both benches was about 0.55 ft/min. By contrast, the average penetration rate for the PEO drilled holes on the second bench was 0.93 ft/min, a 70 pct increase in penetration rate. The average bit life was 2745 +/- 12 ft (2 bits) when drilling with water mist alone, while the average bit life was 3468 +/- 40 ft (2 bits) when drilling with a PEO solution mist. This was an increase in bit life of about 26 pct. With the cost of PEO at about $0.73 per 1000 gal for a 15 ppm solution, potential yearly cost savings per drill for this drilling operation is about $95,000 assuming a bit life increase of 26 pct. Drillers reported that dust reduction was better drilling with a mist of PEO solution compared to a mist of water alone. A field test was also conducted at the Nevada Test Site when an 8-ft diameter shaft was drilled through volcanic welded tuff. Penetration rates were increased from 11 to 17 pct when using only 5 ppm of PEO polymer. With 400 gpm full-water-flushing of the hole, the reagent consumption at 15 ppm is still only 75 lbs (about $420) per day. Potential cost savings for this drilling operation range from $1,500 to $3,000 per day if PEO was used on a continual basis. The use of the PEO additive with percussive bits was also tested. 76-mm button bits drilled into granite showed no penetration rate improvement, but the rotational power requirement was reduced by 15 pct. 51-mm button bits and cross bits showed 2.3 times less wear and decreased corrosion in drilling of quartzite and schist. Potential applications of PEO in other fragmentation processes in mining and mineral processing include rock sawing with diamond saw blades, and comminution. Health, Safety, and Environmental Considerations: Polyethylene oxide (PEO) is a non-toxic, high-molecular weight, nonionic polymer. The U.S. Environmental Protection Agency, has given approval to a wide variety of applications of PEO in solution including agricultural uses as soil stabilizers, crop sprays, and seed coatings. It is not hazardous to the environment; it breaks down in nature to other inert substances over a time span of a few months. It evaporates into the air leaving little or no residue at all The U.S. Food and Drug Administration has also given wide approval for the commercial uses of PEO including use as ingredients in denture adhesives, cosmetics, soaps, and detergents, use as a coating of or ingredient in pharmaceutical tablets, and use as a foam stabilizer for beer. A Mine Safety and Health Administration (MSHA) inspection determined that there were no measurable toxic substances or carcinogens associated with the usage of PEO in mining operations including preparation, drilling, loading of ANFO, and after blasting.
Minneapolis, MN: U.S. Department of the Interior, Bureau of Mines, TN 376