Cast gold bars are usually produced directly from melted gold. However, the way in which melted gold is used to manufacture these bars can vary. The three main types of gold bars being manufactured are:
(1) Large cast bars with approximate weights.
(2) Small cast bars (1000 g or less) with specific weights.
(3) Long flat cast bars, notably for the manufacture of minted bars.
Large Cast Bars:
Large cast gold bars are usually manufactured to an approximate weight, so the refined gold at the required purity is directly poured into the casts
Induction furnaces are usually used to melt gold. Gold is melted in bulks in large holding crucibles. Gold crucibles are then tilted, and a relatively small amount is poured into pre-heated transfer crucible. At this stage, using vacuum tubes, small samples of gold are tested. Melted gold in the transfer crucible is then shifted to pre-heated, dressed bar moulds (generally cast-iron) of the required size. During pouring process, the mould sits on a balance. When the balance records the required weight of gold, the pouring is stopped. A few seconds after pouring process, the bar is dropped into a tank of water for cooling. Alternatively, the mould can be allowed to cool gradually. This cooling procedure is called “quenching”. The bar is then cleaned to remove stains of any kind and then weighed.
Small Cast Bars:
Small cast bars, having specific weight, are produced using refined gold, which has been tested for the required impurity. Refined gold is usually available in two basic forms: granules and small cut pieces. The choice of basic form depends on the method of manufacturing.
First of all, the required form of gold is balanced and the required amount is weighed. Usually, gold is weighed slightly more than the weight of the bar, in order to cater for manufacture losses.
Traditional method: In traditional small cast bar manufacture method, melted gold is poured into moulds; this method is still used by many manufacturers. Gold is first placed in a crucible of appropriate volume and is then melted in a furnace at a controlled temperature of 1200C. Care should be taken to avoid contamination of gold. The melted gold is then poured into a pre-heated bar mould, which has been dressed with carbon smoke or graphite. After that, the mould is cooled and the bar is cleaned and weighed.
Modern method: Using modern methods, a lot of small cast bars can be manufactured simultaneously. The gold is poured into bar moulds of the required size. A number of moulds are simultaneously fed into induction furnace for melting at controlled temperature of 1200C. Melted gold is then pushed into a “Cooling Tunnel”. When it comes out, the bars are removed from casts, cleaned and then carefully weighed.
Long Flat Cast Bars:
Long flat cast bars can be manufactured either from cast iron moulds in a pack or from a continuous casting machine.
Traditional Method: A number of cast iron blocks of required width, thickness and length are clamped together vertically, so that the flat base of one mould forms the side of the other mould. These moulds are filled with melted gold and then cooled. When the gold has solidified, the moulds are unclamped and the bars are removed.
Modern Method: Continuous casting machine
The modern methods of manufacturing use casting machines to produce bar stock of required width and thickness on a continuous basis.
Post time: 12-17-2016