An alchemist at work trying to transmute base metals into gold
The precipitation of copper from an aqueous solution of copper sulphate by scrap iron is known as cementation, because the precipitated metal is usually cemented on the added metal. This reaction was known to alchemists and was one of the reasons that led them to believe that iron was transmuted into copper. They believed that it would be possible to transmute iron or any other metal into gold; however, what was needed was an elixir, or a catalyst in modern terms, to help accelerate the process.
Cementation is said to have been practiced in China in the tenth century. It was first applied in Europe in the thirteenth century in the Bohemian mining district Graslitz and in the Herrengrund mining area in Lower Hungary (modern Slovakia). In Graslitz, copper sulphide was smelted to matte, then roasted in heaps to form copper sulphate, leached with water, and finally metallic copper precipitated by iron. At Herrengrund, copper was recovered from mine water by cementation. Around 1497, another cementation plant was in operation at Schmöllnitz at the foot of the Tatra Mountains in Slovakia. Cementation was also applied in the Middle Ages for the recovery of copper from copper-containing pyrite by exposing the heaps of pyrite to air and rain. The sulphuric acid generated dissolved the copper minerals, forming a copper sulphate solution; copper was then precipitated using scrap iron. None of these operations, however, is mentioned by Agricola.
In modern terms, the process can be predicted from the electrode potentials: the metal with the more positive (oxidation) potential will pass into solution and displace a metal with a less positive potential, provided the solutions are dilute and the metal ion is uncomplexed. In the early 1960s, cementation of copper from dilute leach solutions was conducted on a large scale when dump and heap leaching operations became widespread as a cheap and rapid method of recovery.
The iron used for cementation was in the form of scrap, cans, or sponge. Cans were heated in a rotary kiln to remove the tin plating and paint from the surface, and the solder from the seams. They were then passed through a hammer mill, a toothed roll, or a shredding machine to make them more compact and less bulky. Stockpiling of iron usually presents a problem, as it occupies large areas and oxidizes with time. This oxidation not only means an iron loss, but also a decrease in the quality of cement copper produced from such material.
The pH level should be about 2 to prevent the hydrolysis of Fe2+ and Fe3+ ions. Appreciable amounts of iron dissolved in the acid present with the generation of hydrogen. Ferric iron also formed and its presence contributed to the dissolution of more scrap:
Fe + 2Fe3+ Æ 3Fe2+
In practice, two to four times the theoretical amount of iron was consumed. Due to the presence of trace amounts of arsenic and phosphorus as impurities in scrap iron, the conditions were favourable for the formation of the highly poisonous gases arsine, AsH3, and phosphine, PH3, by the following reactions:
Fe3As2 + 6H+ Æ 3Fe2+ + 2AsH3
Fe3P2 + 6H+ Æ 3Fe2+ + 2PH3
Methods and Equipment
Small-scale cementation reactions are usually conducted in rotating drums. Due to the rotation, the precipitated metal flows out with the exit solution and is collected by filtration (Figs. 2 and 3). Large-scale cementation reactions are conducted in launders, which are narrow tanks about 170 metres long, 1.3 to 3 metres wide, 0.3 to 1.3 metres deep, and laid at a slope of about two per cent. They are continuously charged with scrap iron cans, and the copper-bearing solution flows by gravity at the upper end of the launder (Figs. 4 and 5). Fire hoses or hydraulic slushers are used to agitate the cans, knock the cement copper from the cans, and slush the copper along the launders to the settling tanks where it is then collected. The flow rate of the solution through the launder has an important effect on the quality of cement copper produced. A high flow rate helps to produce a uniform movement of solution through the iron in the launder. This prevents the formation of low-acid stagnant areas that favour the deposition of hydrolytic products. Furthermore, because the cementation reaction is fast, slow flow rate of solution will result in excessive dissolution of iron by the acid.
Precipitation cones were invented by Kennecott engineers in the 1960s and were installed to replace the launders at their extensive heap leaching operation at Copperton Mill in Bingham Canyon, Utah. The inverted cone is a continuously operated unit. The vessel in Figure 6 consists of a tank of 4.2 metres in diameter and 7.3 metres in height, into which is mounted an inverted cone of 3 metres in diameter and 3 metres in height. The outer tank contains a 45º-sloped false-bottom floor from one side of the tank, to a bottom side discharge at the opposite side. The annular space between the inner cone and the tank is covered by a stainless steel screen mounted as a continuation of the cone and is anchored to the cone and tank. The cone supports a series of pressure nozzles arranged in such a manner as to create a vortex when the copper-bearing solution is pumped into the cone. The inner cone and the area of the tank above the stainless steel screen are filled with shredded detinned iron scrap. The injection of the solution through the mass of iron has the effect of not only rapidly precipitating copper, but also removing the metallic copper from the iron surface, thereby exposing clean, fresh iron. The pressure and velocity of the solutions in the lower conical section move the copper precipitated upward and out of the cone into a reduced velocity zone created by the larger diameter of the holding tank. The copper settles down through the stainless steel screen and accumulates on the sloped false bottom of the tank. Copper can then be discharged either intermittently or continuously. Cement copper produced in cones is purer than that from launders. Kennecott built 26 cones (Fig. 7).
The product is usually contaminated by clays, copper oxide, iron oxide, and metallic iron. A finer powder is obtained at high Cu2+ ion concentration. Furthermore, at a constant Cu2+ ion concentration, a fine powder is obtained at high H2SO4 concentration. The product is usually compacted and melted in a copper smelter. In some cases, where the leach solution contains slimes, it was desirable to avoid the filtration step and therefore a process known as leach-precipitate-float is used. In this process, the leach solution containing slimes makes contact with iron to precipitate copper, which is subsequently recovered by flotation. Drying cement copper is not an easy operation because of its tendency to form oxide. Typical cement copper contains 80 per cent to 90 per cent Cu.
In a few cases, the cement copper was purified and marketed as a powder. The cement copper was dissolved in H2SO4 in the presence of oxygen; thereby Fe2+ is converted to Fe3+, which can be precipitated and filtered off. The pure copper sulphate solution was then treated by hydrogen under pressure and high temperature to yield pure copper powder. Digestion with a CuSO4 solution, thus displacing the iron content by copper, was also suggested as a means of purification.
The End of an Era
Launders are no longer used and Kennecott demolished its precipitation cones in the 1980s when the operation was shut down. The process has now been displaced by solvent extraction – electrowinning which came into existence in the late 1960s. More than 20 per cent of all the copper produced worldwide is now by hydrometallurgy.
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