The Oldest Metals: Gold, Silver, and Lead

James Richard Fromm


Gold

Gold is probably the earliest known metal and has been found, as jewelry, in the most primitive of societies. This is because gold is found in nature in the form of more or less pure free metal and can be easily obtained by simple techniques such as washing or "panning" in stream beds. The major, and virtually only significant, impurities are the oxides of other metals. These can be removed by a process called cupellation. In cupellation, lead metal or its oxide litharge (PbO) are added to the gold, the mixture is placed in a crucible or ceramic pot, and the mixture is then heated in the open air. The impurities are dissolved in the molten PbO, and then partially blown off in the airblast, partially absorbed by the porous walls of the ceramic crucible, the cupel.

Modern methods of gold production use considerably lower grade ores than could be processed by ancient techniques. In these ores, the gold is present as finely divided metal particles in a large quantity of unwanted rock and clay (gangue) from which they must be removed. This can be done by several techniques, of which the most common is froth flotation. In froth flotation, which is also used to concentrate metal ores for copper, lead, zinc and other metals in modern practice, the finely crushed ore is mixed with oil and water in large tanks. The gold particles are better wetted by the oil while the gangue particles are better wetted by the water; air is blown into the tank to produce a froth of oil and the gold particles. The froth is lighter than water and is mechanically skimmed off for further processing.

Froth flotation is often used for the separation of metals and metal sulfides which are present as small particles from gangue. For gold ore processed in Yellowknife, N.W.T., Canada, froth flotation is used to separate the gangue from gold and from FeAsS2 (arsenopyrite). When arsenopyrite is roasted, the reaction which takes place is

2FeAsS2 + 5O2 Fe2O3(s) + 2SO2(g) + As2O3(g)

This reaction removes the arsenic and sulfur, leaving a more easily separated oxide slag. The sulfur dioxide and arsenic oxide, however, can be serious environmental pollutants. The arsenic oxide solidifies on cooling and is removed by particle precipitators and bag filters. It is then sealed away in unused mine tunnels. The sulfur dioxide is released to the atmosphere or used in the production of sulfuric acid.

In the case of gold, the process of froth flotation is often followed by cyanidation, a process developed in 1887. The crushed ore is treated with an aqueous 0.05 weight per cent NaCN solution (and enough CaO to neutralize any acid present in the rock, which would generate highly toxic HCN). The mixture is thoroughly aerated by blowing air in so that the gold is oxidized to Au(I). This occurs only in the presence of cyanide because the very stable soluble dicyanoaurate(I) ion Au(CN)2- is formed.

4Au(s) + O2 + 2H2O + 8CN- 4Au(CN)2-(aq) + 4OH-

The standard free energy change in this reaction is a large -871.18 kJ/mole reaction. Silver is removed by the same process as Ag(CN)2-. Treatment with metallic zinc reduces the complex ions back to the metal; the zinc dust is oxidized to Zn(II) at the same time:

2Au(CN)2-(aq) + Zn(s) Zn(CN)42-(aq) + 2Au(s)

The standard free energy change of this reaction is -124.68 kJ/mole reaction. The free cyanide is regenerated because in basic solution the very stable Zn(OH)42- complex ion is formed in preference to Zn(CN)42-:

Zn(CN)42-(aq) + 4OH- Zn(OH)42-(aq) + 4CN-

The standard free energy change in this reaction is -357.51 kJ/mole reaction.

An earlier process, used on gold and native silver ores, is amalgamation. Gold and silver dissolve in mercury to form amalgams, or liquid alloys. The gold and silver are recovered by distilling away the mercury. This process, known to the Romans and in common use in the Middle Ages, was probably responsible for Roman mercury production in Spain since mercury had no other use for them.

Silver and Lead

Silver and lead are generally found together both in nature and in history. Knowledge of them appears to originate in Asia, where they are found from about BC 3000, but in Europe they are found only from about BC 300. The usual ore is galena (PbS) which is normally found with Ag2S (argentite) impurities. Lead metal is not usually found free in nature, although some native silver is found as the metal. Another form in which silver is found in nature is as the insoluble chloride salt AgCl (cerargyrite or horn silver).

Production of Silver and Lead

The steps used to obtain the metals from their ores are two. The first or roasting step is simply heating in air; for lead, the reaction is:

2PbS + 3O2 2PbO + 2SO2(g)

The standard free energy of reaction is -389.4 kJ/mole lead sulfide and the reaction is spontaneous. In addition to producing litharge (PbO) and sulfur dioxide, the roasting of galena also gives rise to some PbSO4 and generally leaves some residual PbS unreacted. The second stage is the actual reduction of the oxide to the metal, known as smelting:

2PbO + C(s) CO2(g) + 2Pb.

The standard free energy of reaction is -18.58 kJ/mole of carbon dioxide produced and the reaction is not spontaneous under standard conditions. The reason the smelting process does work in practice despite the small free energy of reaction is partial coupling of the two steps plus the addition of heat from combustion of excess carbon.

The steps for obtaining silver from its ore are similar, and more favorable for the reduction (smelting) step; the roasting step is again spontaneous:

Ag2S + O2 SO2(g) + Ag2O.

The standard free energy of this reaction is -270.72 kJ/mole, so it is spontaneous as is the reduction step,

2Ag2O + C CO2(g) + 4Ag.

The standard free energy of the reduction reaction is -371.96 kJ/mole of carbon dioxide produced. In both cases the driving force for these reactions is the high favorable free energy of formation of both SO2 and CO2.

Photographic Uses of Silver

Silver is essential in black-and-white photography. Photographic film and paper are simply coated plastic film and coated treated paper. The coating consists of microscopic crystals of AgBr held in a binder such as gelatin; the size and uniformity of distribution of the crystals determines the speed and sensitivity of the film. The exposure of film to light causes photoreduction of a few silver ions in the silver bromide crystals to metallic silver. The number of silver ions reduced is directly proportional to the intensity of the light so that an image can be formed on the film.

When the film is removed from the camera it has no visible image. It must now be treated with an aqueous solution of a mild reducing agent (developing). As the film is developed, each photoreduced silver atom catalyzes the chemical reduction of all or most of the silver ions in the same microscopic crystal, amplifying the effect of the original light many millions of times. The result of this is the formation of a visible image in silver metal.

Once the image has been developed, the excess AgBr is removed by treatment with aqueous sodium thiosulfate solution (hypo) in a fixing process:

AgBr(s) + 2S2O32- Ag(S2O3)23- + Br-

The complex ion forms with the silver ion Ag+ but not with silver in metallic form, so the developed image is not removed. This leaves a negative fixed image (negative) on the film, which can now be exposed to additional light without destruction of the image. Normal positive prints are obtained by repeating the process using either paper or film.

Toxicity of Lead

Lead is a slow cumulative poison to humans and other animals, and it can affect people exposed to small doses over a long period of time as well as those who receive larger doses over a short period of time. Fatal effects are observed if its concentration is high enough, but the usual effect is nonfatal. Effects of low-level lead toxicity in children are believed to include lowered intelligence, other neurobehavioral derangements, and impaired hearing. Lead generally builds up in the bones of affected individuals and also shows up in their blood.

Blood levels are usually used as indicators of an individual's health in cases of suspected lead poisoning. An individual having a blood lead level of 80 micrograms/100 mL (0.8 ppm) would be diagnosed as definitely sick. A blood level of 0.6 ppm would mean the individual was probably sick, while 0.4 - 0.6 ppm would indicate a high risk. A level of 0.2 ppm is the modern mean level for adults (U.S.A.). Some effects in children have been noted at levels near 0.2 ppm.

Lead level in bones, on the other hand, are of more archaeological interest. A level of 60 ppm is sometimes found today in the U.S.A., while values of about 40 ppm are found for the 1865 era, U.S.A., and lead bone levels of about 4 ppm are found in North American Indian skeletons of around 1500. More typical modern bone values would be 20 - 40 ppm. The level of lead in soft tissue is usually less, 0.1 - 28 ppm, and more variable.

Lead is toxic in the form of the Pb(II) cation, whose principal toxic effect is inhibition of hemoglobin synthesis. The reaction inhibited is the formation of the heme nucleus itself.

Normal lead levels in intake vary considerably. In foods the lead levels are 0.1 - 0.3 ppm, and in beverages they are 0.02 - 0.03 ppm. Modern public water supplies normally have lead levels well below 0.1 ppm (the lead comes from lead solder in the pipes). The current U. S. Environmental Protection Agency standard of 50 ppb may be lowered. Electric drinking fountains have been found to supply water with higher lead content due to their lead fittings and solder.

Lead levels in air vary from 0.17 micrograms per cubic meter in unpopulated rural areas through 4 micrograms per cubic meter in urban, densely populated areas. Values near freeways may reach 10 - 25 micrograms per cubic meter, and in a long highway tunnel, extreme values of 50 micrograms per cubic meter are possible. The air values originate from use of tetraethyllead, Pb(C2H5)4, as an antiknock additive in gasoline. As environmental regulations have reduced or eliminated the use of lead in gasoline, these values have steadily dropped.

Abnormal sources of lead intake for humans are primarily two. First, drinkers of large amounts of moonshine whisky drink in lead from the lead solders of stills. Second, children, especially toddlers in poor urban areas (of 2255 lead poisoning treatments, New York City, 1970, all but 90 were children 6 years of age and under) from eating the lead-based paints used prior to about 1950. These used compounds such as PbSO4, PbCrO4, and PbCO3 as pigments. Modern paints contain little or no lead.

Lead balance in the body is important. Most of the lead taken into the human body is excreted; a rough daily maximum balance is, for maximum normal intake and probable upper normal limits of lead content: solid foods, 1200 micrograms per day; beverages, 30 micrograms per day; public water, 200 micrograms per day; air intake, 225 micrograms per day, for a total maximum normal lead intake of 1655 micrograms per day. The maximum normal daily output of lead from the human body is about 2000 micrograms per day.

The details of the normal or average input and output are as follows. The air inhaled contains, on the average, 15 - 19 micrograms per day, of which 35 - 55% is exhaled, leaving a net air intake of 7 - 8 micrograms per day; of this perhaps 7 - 13% is absorbed by the lungs. Food and water contain about 250 - 350 micrograms of lead per day; of this, perhaps 5 - 10% is absorbed and eventually reaches the blood. Smoking contributes about 500 micrograms of lead per cigarette; the amount of this which is absorbed is unknown.

The corresponding average daily output would be, in the urine, 10 - 40 micrograms per day; in the feces 100 - 400 micrograms per day; and in the sweat, 10 - 40 micrograms per day, for a total of about 120 - 480 micrograms per day. Clearly, the average input and output are approximately in balance. It is only when a human body takes in a high level of lead that the lead accumulates in the body.


Copyright 1997 James R. Fromm