Liquation

Liquation is a metallurgical method for separating metals from an ore or alloy. The material must be heated until one of the metals starts to melt and drain away from the other and can be collected. This method was largely used to remove lead containing silver from copper, but it can also be used to remove antimony minerals from ore, and refine tin. Liquation is a metallurgical method for separating metals from an ore or alloy. The material must be heated until one of the metals starts to melt and drain away from the other and can be collected. This method was largely used to remove lead containing silver from copper, but it can also be used to remove antimony minerals from ore, and refine tin. The first known use of Liquation on a large scale was in Germany in the mid-15th century. Metal workers had long known that Central European copper ore was rich in silver, so it was only a matter of time until a method was discovered that could separate the two metals. The 16th century process of separating copper and silver using liquation, described by Georg Agricola in his 1556 treatise De re metallica, remained almost unchanged until the 19th century when it was replaced by cheaper and more efficient processes such as sulphatization and eventually electrolytic methods. Liquation requires that the silver-rich copper first be melted with approximately three times its weight in lead, as silver has a greater affinity with lead, most of the silver would end up within this rather than the copper. If the copper is assayed and found to contain too little silver for liquation to be financially viable (around 0.31% is the minimum required ) it is melted and allowed to settle so that much of the silver sinks towards the bottom. The ‘tops’ are then drawn off and used to produce copper while the silver-rich ‘bottoms’ are used in the liquation process. The copper-lead alloy created can be tapped off and cast into large plano-convex ingots known as ‘liquation cakes’. As the metals cool and solidify the copper and the silver-containing lead separate as they are immiscible with each other. The ratio of lead to copper in these cakes is an important factor for the process to work efficiently. Agricola recommended 3 parts copper to 8-12 parts lead. The copper must be assayed to accurately determine how much silver it contains, for copper rich in silver the top end of this ratio was used to make sure the maximum amount of silver possible would end up within the lead. However, there also needs to be enough copper to allow the cakes to keep their shape once most of the lead has drained away, too much copper and it would trap some of the lead within and the process would be very inefficient. The size of these cakes has remained consistent from when Agricola wrote of them in 1556 to the 19th century when the process became obsolete. They were usually between 2 ½ to 3 ½ inches (6.4 to 8.9 cm) thick, about 2 feet (0.61m) in diameter and weighed from 225 to 375 lbs (102 kg to 170 kg). This consistency is not without reason as the size of the cakes is very important to the smooth running of the liquation process. If the cakes are too small, the product would not be worth the time and costs spent on the process, if they are too large then the copper would begin to melt before the maximum amount of lead has drained away. The cakes are heated in a liquation furnace, usually four or five at once, to a temperature above the melting point of lead (327°C), but below that of copper (1084 °C), so that the silver-rich lead melts and flows away. As the melting point of lead is so low a high temperature furnace is not required and it can be fuelled with wood. It is important that this takes place in a reducing atmosphere, i.e.one with little oxygen, to avoid the lead oxidising, the cakes are therefore well covered by charcoal and little air is allowed into the furnace. It is impossible to stop some of the lead oxidising however and this drops down and forms spiky projections known as ‘liquation thorns’ in the channel underneath the hearth. The older and relatively simple method of cupellation can then be used to separate the silver from the lead. If the lead is assayed and found not to contain enough silver to make the cupellation process worthwhile it is reused in liquation cakes until it has sufficient silver. The ‘exhausted liquation cakes’ which still contain some lead and silver are ‘dried’ in a special furnace which is heated to a higher temperature under oxidising conditions. This is essentially just another stage of liquation and most of the remaining lead is expelled and oxidised to form liquation thorns, though some remains as lead metal. The copper can then be refined to remove other impurities and produce copper metal.