Introduction & History
Diamond core drilling, or more commonly called diamond drilling, has always been a vital part of subterrranean exploration. Enormous financial commitments are made based on the results of hundreds of thousands of meters of core samples worldwide. Without this information, no mines could be explored for or worked, nor could many of our largest civil engineering structures, such as buildings, bridges or dams be safely situated. Most core drilling in Canada is concerned with mineral exploration–and that will be the focus of this article.
Canada is full of stories of mineral discoveries substantiated through diamond drilling. The development of towns like Bathurst N.B., Chibougamau Que, Rouyn-Noranda Que, Sudbury Ont., Timmins Ont., Flin Flon Man., Yellowknife NWT and Kimberly B.C. are but a few examples of early prospects brought to fruition with the help of diamond drilling.
Today, the job of exploring for new mineral deposits (prospecting) is tougher than ever, mainly because most shallow “high grade” deposits have been looked at or mined out. High-tech exploration techniques such as geochemistry and geophysics yield new areas of interest otherwise hidden by soil or thick layers of barren rock. But the area of interest, or “anomaly” can only be investigated with the help of core drilling.
Before one looks at current diamond drilling technology, it is interesting to look where the technique originated.
History of diamond drilling
Indications of diamond drilling can be traced back to early Egypt, where it appears diamonds helped bore holes into rock used to build the pyramids. Modern diamond drilling can be traced to a Swiss engineer, J.R. Leschot in 1862. The industrial revolution needed to find a way to drill holes and blast enormous amounts of rock, quickly. Tunnels, railways, dams, mines and other structures were required. Percussive drills were used, but the steel of the day could not withstand the high impacts required to break rock. Leschot designed and patented a drill which utilized a new concept-the placement of diamonds set into a tube of steel. This “perforator” could out drill by a factor of 3:1 any percussive drill of its day.
It soon became apparent that the smooth cutting action of Leschot’s drill was ideally suited to cut core samples. For the first time, economical core samples were available to engineers and geologists. This information opened up a new era in building, design and confidence.
The first hand set drill bits were simple, yet required great expertise to fabricate. The diamonds were large multi-crystalline “carbonados” (or black diamonds), typically 2 or 3 to the carat. The setter (and usually foreman of the drill job) would manually drill a small hole into the end of a steel blank, insert the diamond, and peen it in with copper. The diamonds had to be placed so that not only would they cut a precise O.D. and I.d. core sample, but had a sharp edge exposed to the rock. Typically, 30 feet (9.1 m) in hard rock was the life of an edge. After that, the diamond had to be turned and reset.
The drill was a simple, hand cranked machine, later improved by advent of steam power. Today holes can be drilled to a depth of 3000 m or more from surface, and drill bits last hundreds, sometimes a thousand meters.
History of diamond drill bits
The first hand set drill bits were simple, yet required great expertise to make. The diamonds were large multi-crystalline “carbonado’s” (or black diamonds), typically 2 or 3 to the carat. The setter (usually the foreman of a drill job) would manually drill a small hole into the end of a steel blank, insert the diamond, and peen it in with copper. The diamonds had to be placed so that not only would they cut a precise O.D. and I.D. core sample, but the diamonds had to be placed with a sharp side exposed to the rock. Typically, 30′ (9.1 m) in hard rock was the life of an edge, after that the diamond had to be turned and reset.
The price of carbonados increased dramatically, and by the late 1930’s approached $100.00 per carat. Many smaller, cheaper diamonds were available, but they were very time consuming and difficult to hand peen into blanks.
The solution to this problem was the forerunner of all drill bits currently in use today, the cast bit.
Cast bits and new materials in the 1950’s
Graphite was a material increasingly being used for high temperature casting. Diamond tool designers turned to graphite for the next advance in tool fabrication.
A graphite mold was turned on a lathe, many small holes drilled into it and the small diamonds inserted into the holes. Secondly, powdered metal was introduced around the diamonds. The steel blank was fitted, and the entire assembly was put into a furnace, melting the brazing metal into the powdered metal, brazing the diamonds, the metal powder and the steel into a solid tool.
While casting solved the diamond cost problem to some degree, the problem of wear had to be attended to. The powdered metal had to be chosen to hold the diamonds firmly in place, and not allowed to erode too quickly so that the diamonds would fall out and destroy the tool. Powdered steel mixed with braze filler was not as tough as the steel blanks used in the hand set days.
In the 1950’s, the production of metal powders used in other industries was becoming sophisticated so that even extremely hard and wear resistant metals, such as tungsten carbide, was now available in powdered form. Now a powdered tungsten carbide metal powder could be used to surround the diamonds. Wear was greatly minimized, tool life was dictated by the size of diamond, the grade of diamond (characterized by crystal shape, and crystal homogenity) and the exposure of the diamond built into the tool.
Surface set bits in use today use this same method of construction, and are still economic to drill soft sedimentary rocks such as mudstone, siltstone and certain soft sandstones. Designs range from the full crown style which offers the lowest manufacturing cost, to the step bit, offering higher penetration rates in harder rock formations.
Typically, surface set bits use 25 to 100 stones per carat. Raw (as mined) diamonds start as larger material, and are crushed and tumbled to yield drill material. The problems with surface set diamond bits were still to be found in harder formations. Just like the hand set bits in prior use, hard rock quickly forms wear flats on the diamonds, and wear flats increase the load required to make the bit cut. These large loads caused equipment to wear prematurely, reduced production, and consumed much fuel.
Manufactured diamonds become available
In the late 1950’s, General Electric and De Beers decided to investigate the fabrication of diamonds. Diamonds are a very dense form of pure carbon. By compressing micro-crystalline carbon enough, one can force carbon to change to macro-crystalline graphite–continuing the process yields diamond. The problem is that a pressure of approximately 75,000 kg/cm2 is required (>1,000,000 psi) at temperatures approaching 1500 degrees C. (2700 deg. F) Eventually, a breakthrough in diamond fabrication was found–a catalyst could be used to enhance diamond formation. The most common catalyst used today is cobalt, but iron and nickel have been used also. It was found that considerable carbon could be dissolved into these metals–similar to dissolving salt into water. If one was to put enough carbon into molten metal, then cool the metal at very high confining pressures, the carbon would crystallize out of the solution as diamond crystals instead of graphite.
Synthetic diamonds can be grown to yield a consistent size and shape. It is an advantage to use whole, strong, “roundish” (actually cubo-octahedral) crystals instead of crushed crystals of random shapes. However, economics dictates that the largest synthetic diamonds available range 500 stones per carat, far too small to hand set into molds. Borrowing technology from another industry, diamond tools were fabricated similar to a grinding wheel. But for the majority of drilling applications, surface set tools were the preferred choice.
New powdered metal technologies needed to match synthetic diamond potential
Starting in about 1975, synthetic diamonds were large enough (50 mesh), tough enough, and cheap enough to be considered for use in diamond drill bits. However, the metallurgy that was so finely developed for the standard surface set bits, was completely unsuitable for the “grinding wheel style”, or impregnated, core bit. Metal powders had to produce two properties:
- Heat removal. A tremendous amount of heat is generated at the diamond/rock interface, and the cooling water alone is not sufficient to remove it. Without heat removal, the metal holding the diamonds becomes soft, and “smearing” happens. The metal holding the diamonds has to wear slightly faster than the diamond, so that there is always an “exposure” of diamond. When a diamond falls out, the metal must keep wearing to allow another diamond to become exposed.
- Metal powder technology has been borrowed from the aerospace and electrical industries in search of thermally conductive, high temperature alloys. Although research continues, appropriate alloys have been discovered, and impregnated tools are now the standard for the industry.
The newest developments in diamonds
There are some drawbacks to using impregnated tools compared to surface set bits. First of all, a fairly high rotational speed is required, up to 5 meters/second. (1700 rpm for “B” sized tools). Precision tools are required to spin at that speed, straight rods, corebarrels, and powerful drills. Secondly, small diamond crystals will not cut soft rock as quickly as larger crystals, therefore they tend not to be used in soft (e.g.. sandstones, mudstones) rock. The cost of producing large diamonds becomes prohibitively expensive.
The challenge has been met by developing an agglomerate of small “micron” sized (1 micron =1 x 10-6 m) particles into larger, engineered shapes. The large agglomerates, called polycrystalline compact diamond, or PCD, are produced in various shapes and sizes. PCD’s are ideal for drilling soft rock. And because of their inherent polycrystalline design, they will not cleave or form wear flats like a whole crystal diamond will. Without wear flats, they will continue cutting for as long as the diamond protrudes from the tool.