It’s all about the method

— The following is an excerpt from the ninth edition of Mining Explained, published by The Northern Miner. The book sells for $30 plus $3 for shipping and is available by calling 1-800-668-2374. E-mail: northernminer2 @northernminer.com.

There are as many methods of extraction to choose from as there are sizes and shapes of orebodies. The shape and orientation of an orebody, the strength of the ore and surrounding rock, and the manner in which the valuable minerals are distributed are different for each ore zone. These and other factors will influence the selection of a mining method and the overall plan for developing the orebody.

Operating mines range in size from small underground operations, some of which have only a few production levels and may produce under 100 tonnes of ore per day, to the large open pits, some of which move tens of thousands of tonnes of ore and waste rock per day.

The primary opening into an underground mine can be either a shaft, a decline (also called a ramp) driven down into the earth, or an adit (a horizontal opening driven into the side of a hill or mountain). All have the same purpose — to provide access for people, materials and equipment and a way for ore to be brought to the surface.

Shafts are usually vertical (though they can be inclined) and equipped with hoists and headframes, that is, structures at the top that enclose the hoist.

Ramps, on the other hand, usually spiral downward at a gradient of about 15% to allow access into the mine by rubber-tired mobile equipment. In some cases, ramps are driven in a straight line to accommodate conveyor belts or have straight runs with switch back points. Ramps are generally less expensive to develop than shafts. But depending on the angle of the decline, the size of the opening and the ground conditions encountered, the total cost may be higher than the cost of developing a shaft to reach the same depth.

Horizontal or level mine workings are called crosscuts and drifts. Sometimes it is useful to open vertical workings between levels in an underground mine; these are called raises or winzes.

Surface diamond drilling will indicate whether a mineralized zone has sufficient potential to become an orebody. To outline the zone with greater accuracy, and confirm that the mineralization is continuous and the estimates of grade and tonnage are correct, surface work is followed by underground development and definition drilling from the workings. Only then can the developer make plans for production.

Diamond drill holes from surface can only tell part of the story. A mineralized zone may pinch or swell or be irregular, and a large amount of diamond drilling is needed to fill in the information between the first holes the developer drills. Sooner or later, drilling long holes from surface becomes prohibitively expensive, and the mine developer must decide whether to continue exploration from underground. If exploration goes ahead, a suitably sized exploration shaft or an access ramp is driven, allowing crews to get closer to the orebody.

The crews drive drifts and crosscuts from the shaft or ramp, and drilling areas are excavated at regular intervals to accommodate the underground drills. The mineralized body is then drilled from underground to determine the grade and tonnage.

Up to this point in the exploration program, the only samples taken from the orebody have been the drill cores. Once underground, however, it is possible to obtain much larger bulk samples of the ore by actual mining. The mineralized material removed during this operation provides a bulk sample for metallurgical testing, first at “bench scale” in a laboratory, and ultimately at “pilot scale” in an actual mill. The testing is used to determine the extraction method that will recover the most metal from the ore.