Valuable minerals from the ore are separated by the combination of water and mechanical action. The small pieces of ore bearing the valuable minerals pass through the screen and are concentrated further in sluices. The larger ones (those that do not pass through the screen) are moved to a waste stack using a conveyor. continues to import a big part of its total consumption of the precious metal from some of the countries mentioned above. Since rhenium and its compounds are used in very small amounts, very little is known about their toxicity. So far, only a few rhenium compounds have been tested for toxicity, and these include rhenium trichloride and potassium perrhenate. In 2007, worldwide demand for iridium reached 3,701 kilograms (119,000 troy ounces). Distribution of these were as follows: electrochemical uses (1,100 kilograms); electrical uses (780 kilograms); for catalysis (750 kilograms); and other applications (1,100 kilograms). Iridium is found at highest concentrations within the Earth's crust in three specific types of geologic structures: in impact craters, in igneous deposits, and in deposits reworked from either of the first two. Of these, 5 are made of platinum, 1 of palladium, 9 of silver, and 18 of gold. And of all these precious metal coins, the South African gold bullion coin Krugerrand is perhaps the most popular. In 1967, the South African Mint Company introduced the Krugerrand, with the intention of circulating it as currency. Platinum is traded on the London Stock Exchange (as ETF, under the ticker symbol LSE: PHPT) and on the New York Mercantile Exchange. One way of investing in platinum is through platinum ingots. Platinum ingots are first assayed and hallmarked before being sold on commodity markets. Another way of investing in platinum is through platinum coins. Their respective "largest" titles, however, carry further qualifications. The Welcome Stranger Gold Nugget: The exact distinction given of this gold nugget is: "the largest alluvial gold nugget ever found". It was discovered on February 5, 1869 at Moliagul, a small township in Victoria, Australia about 37 miles west of the city of Bendigo.