What Sort of Ore Sorter?

To miners, getting the lead out is more than a metaphor for haste. Separating what you want from what you don't is a crucial problem.

Mineral industries use many processes to separate valuable products from waste rock. Hand sorting, one of the oldest techniques, is still the best in some applications. The human eye and hand team up well to distinguish differences in certain readily apparent physical properties, such as color or brightness. In Alaska, hand sorting is still used for recovering coarse gold from placer concentrates, metals from waste processing facilities, and coarse tin from coarse gravel on the Seward Peninsula.

Slightly more sophisticated is the sluice box, which takes advantage of the difference in density (mass per unit volume) between particles of gold and of rock. For a given volume, gold weighs about seven times as much as quartz.

Another separation process, froth flotation, takes advantage of how easily a mineral particles' surface can be wetted by water. Some minerals resist wetting, so they are more likely to be surrounded by air. This property, which can be enhanced by certain chemicals, can be used to induce particles of such minerals to float and thus be separated. Froth flotation is the principle process that will be used to recover the lead and zinc minerals from Cominco's Red Dog Mine.

All separation processes take advantage of differences in basic properties between various minerals. Detecting some of those properties requires technology that considerably extends sensory capabilities beyond those of human beings.

Automated ore sorting developed to replace hand sorting. With automation, a computer replaces the human brain and a sensing device takes over for eyes---perhaps a video camera, and x-ray detector, or an electrical current meter. Most often, automated sorting takes four steps. First, the crushed rock or gravel is screened into size ranges. Sorting devices work most efficiently when the size of the largest particles is no greater than two or three times the size of the smallest particle

After screening, the material is spread evenly on a conveyor or inclined chute and passed through a zone where properties of the rock, either natural or induced, are sensed. These properties most often involve low-level radioactivity or some property of light, such as reflectance or fluorescence, but other characteristics are sometimes employed. For example: diamonds fluoresce when bombarded by x-rays, common salt reflects much light, and uranium ores give off gamma rays.

If a rock particle shows a high enough level of the sensed property, it is physically separated from the moving stream of rock. These find-and-shunt processes all happen very quickly, as one might imagine, and automated sorting would be impossible without computers. Typical sorting systems process material at rates of 500 to 1000 particles each minute.

Automated sorting is highly ore-specific, with different treatments needed for different minerals, and not all ores can be sorted with today's technology. South Africa now has the largest application for sorting metallic ores, because gold there is associated with uranium minerals. However, even unglamorous commodities like talc, limestone, gypsum, and salt are sometimes processed with sorting technology.

Because Alaska has large and diverse mineral resources, the state also has great potential for developing and using automated sorting. Even in one of our oldest endeavors, placer gold mining, the technology may have a place. At some mines, a sorter might have advantages for monitoring the oversize rock that doesn't go through the sluice box, spotting any significant-size gold nuggets or gold-in-quartz cobbles. And, while many of us might get very excited about picking small gold nuggets out of placer concentrate, probably most miners would rather turn this tedious chore over to a machine--at least, to a trustworthy one.