Continuously Cast Good Delivery Silver Bars

For more than 100 years, Allgemeine Gold- und Silberscheideanstalt has been refining silver from various secondary materials in the well-known jewellery city of Pforzheim in southern Germany. In the past, almost all of the recycled silver was used for the manufacture of Allgemeine’s silver products, with the occasional surplus being sold in the form of grain.

With the substantial increase in silver prices in recent years, however, three tendencies became clear. Firstly, there was a steady increase in the amount of silver scrap available for refining, which led to Allgemeine increasing its refining capacity and production. At the same time, the higher price led to a fall in the consumption of some silver products, with a consequent fall in the demand for silver grain. Furthermore, with silver being increasingly seen as an affordable investment product, the rising market demand for London Good Delivery 1,000 troy ounce bars presented a challenge regarding the optimum method of producing these bars.

In Allgemeine’s case, a solution had to be found for the efficient melting and casting of the silver crystals originating from the electrolytic refining process. Normally, Good Delivery bars are cast in open moulds, usually made of cast iron. With large-scale production, performing this operation manually is very labour-intensive. In addition, because of silver’s tendency to absorb oxygen when molten, the final stages of solidification in open moulds often results in this oxygen being released with the formation of surface needles, which then have to be removed by grinding or wire brushing. Allgemeine developed the idea of using continuous casting to produce Good Delivery bars and thus obviate both the above- mentioned problems.

In 2008, when Allgemeine was granted accreditation by the LBMA for its gold and silver bars, expensive manual casting was still used. Yet, as a large-scale manufacturer of silver products, Allgemeine had experience of an extremely efficient continuous-casting facility with machinery for cutting processes and an experienced and well-trained team of experts for the manufacturing of semi-finished products. The decision was then made to use this kind of equipment for the manufacture of Good Delivery silver bars. The main stages in the process are described below.

1. Electrolytically recycled silver crystals are pre-melted in a 400 kW induction furnace with a capacity of 500 kg. After homogenisation of the melted material, a sample is taken and analysed spectrographically to determine the contents of 25 possible impurities. Simultaneously, the oxygen content is measured. Within a few minutes, the exact composition and quality of the melt in the furnace is known.
2. Transfer of the pre-melted material into inductive heated holding furnace and continuous casting in which the water-cooled ‘mould’ has the same cross-section as a Good Delivery. The furnace (see Figure 1) has a capacity of 1,100 kg and is stabilised at a temperature of 1,150°C. The casting is performed under steady conditions, the melted material is constantly fed in at a rate of 500 kg/hr. The cast section is cooled by a water spray (see Figure 2). With a two- or three-shift system, this continuous-casting technology yields an output of 8 to 10 tonnes per day.
3. Sawing into bar lengths is carried out by means of a programmable metal saw.
4. In order to produce the undercut on the end surfaces and to ensure the rounding of the resulting edges (to ensure the safe handling in the vault), the final stage involves the treatment of the bars in a CNC-controlled milling-machine. As this machinery is used for this process only, the resulting scrap can be returned to the melting step.
5. Marking of the bars on the end surfaces is carried out using a programmable dot matrix engraving system, immediately after determining their weight. The finished bar (see Figure 3) benefits from having extremely flat surfaces, rounded edges and clear markings. The only minor deviation is on the bottom surface, which shows track-like indentations resulting from the apparatus used to withdraw the continuous bar from the casting furnace (see Figure 4). Following examination of sample bars by a panel of LBMA inspectors in London, the bars were accredited as Good Delivery by the LBMA on 15 June 2012.

Overview

When compared to manual casting, this multi- level production mode seems to be rather complicated. The milling part is an additional step in the process and is what determines the overall capacity. Nevertheless, for Allgemeine, the continuous-casting technology is an efficient alternative for the manufacturing of bars. It fits in well with the skills and experience of its employees and the need to produce substantial amounts of silver bars for the market.

The system can be used both for the production of high-purity 999.9 silver or, by a calculated addition of copper, 999 fineness material can also be produced.