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Science of the Comstock - Chemistry

Chemistry Topics:
Introduction
Ore Processing
Fire Assaying

Fire Assaying

Introduction

In order for people to define rock to be mined as ore, they have to know the amount of the metal of interest contained in the rock. If there is enough of that rock, and if there is enough metal that can be recovered by processing and that can be sold at a price greater than the expense to produce that metal, then the rock can be defined as ore to be mined. The assayer determines the amount of the metal in the rock.

The method used to measure the amount of metal (or other commodity) of interest is called assaying. The most common and reliable method for assaying rock for silver, gold or other precious metals (platinum group elements) is fire assaying. This method was used in the Virginia City mines and is still used today.

History

The fire assay method of determining the amount of precious metal in a rock was developed in ancient times (Haffty, J.; Riley, L.B.; Goss, W.D., A Manual of Fire Assaying and Determination of the Noble Metals in Geological Materials, United States Geological Survey Bulletin 1445, US Government Printing Office, Washington, D.C., 1977; p. 2 - 6). Archaelogical finds in Troy (about 2600 BC) and other places show that pure silver was made in the 25th century B.C. The process of collecting the precious metal from the rock using lead, and the removal of that lead leaving behind the precious metal (called cupellation) was invented in Asia Minor sometime after 2500 B.C. shortly after the discovery of the manufacture of lead from galena (lead sulfide).

The method was described by Georgius Agricola in 1556 in De Re Metallica (translated in 1912 by Herbert Clark Hoover and Lou Henry Hoover) (Hoover, Herbert C. and Lou H. Hoover, De Re Metallica, Dover Publications, Inc, N.Y., 1950, p. 219-265) as adding fluxes to separate the metal from the surrounding rock. These fluxes had to be specific to the type of ore, but included lead to concentrate the gold or silver. Agricola then described the melting of the ore and flux mixture in the assay furnace, then removal of the lead button to the cupel, a special crucible that abosrbed the lead as it melted in the furnace. The cupel with the lead button was placed in the muffle furnace until the fire had consumed all of the lead, then the bead composed of gold and silver, plus any other noble metals, was removed from the ashes of the cupel to be weighed and parted, or reacted with nitric acid to dissolve the silver and leave the gold and other metals not dissolved by nitric acid alone. Agricola described the technique in such detail that it can be readily compared to the method used today, which is essentially the same as that used reported by Agricola.

Fire Assaying Today

The technique of fire assaying still uses the same principles that were reported by Agricola.

The gold and silver must still be separated and removed from the other parts of the ore. A flux is added so the gold and silver can be collected by the lead and the other parts of the rock can be made into a readily separating glass slag. The standard flux to be added depends on the type of rock, but generally is defined by whether the rock is silicate or carbonate. If the rock is silicate, litharge (lead oxide, PbO), soda ash (sodium carbonate, Na2CO3) and borax (Na2B4O7) are added. The sodium carbonate and borax combine with the silicate in the rock to form a glass that floats on top of the molten lead, which contains the gold, silver, and other precious metals. If the rock is carbonate, silica (SiO2) in the form of sand must be added too in order to provide the silicate so the glass can form from the sodium carbonate and borax to create a good glass for the parts of the rock (the slag) other than the precious metals.
A reducing agent is added so the lead is changed (reduced) into lead metal. Again, the amount and type of reducing agent depends on the rock, but generally flour (source of carbon) is added. The reaction can be simplified to: 2PbO + C = CO2 + 2Pb.
The whole flux is thoroughly mixed with a known amount of carefully sampled powdered rock in a fire brick crucible. This crucible withstands the heat of the furnace and is thick enough that only some of the inner wall reacts with the fluxes to become a part of the glass slag, which floats on top of the molten lead.
The crucible is heated so the mixture of flux and rock sample melts (at a temperature lower than that of the rock alone as controlled by the flux composition).
As the lead is reduced, it drops through the liquid rock sample and collects (forms an amalgam with) the silver and gold that was contained in the rock. The lead with gold and silver collects at the bottom.
The liquid sample is poured into a mold so the glass slag and lead containing gold and silver (called the button) separate cleanly and solidify.
The slag is hammered away from the lead button.
The lead button that contains the gold and silver is then melted under oxidizing conditions in a bone ash cupel, a special crucible made of ashes of bone that is particularly absorbant to the lead oxide formed, so the lead (as lead oxide) is absorbed into the cupel or vaporized into the furnace and removed from the gold and silver.
The gold and silver bead that remains in the cupel at the end is cooled and weighed.
The amount of gold and silver individually can be determined by removing the silver by "parting" with nitric acid, in which the silver dissolves but the gold does not. After the nitric acid solution that contains the silver is carefully poured away from the gold bead, the remaining gold is fused (annealed) by heating, then cooled and reweighed. This yields the amount of gold in the sample, and by difference the amount of silver in the sample. (Other analytical techniques for the final determination of this concentrate of gold and silver and the platinum-group elements can be used, too.)
This small bar of gold was refined by the fire assay method.

Detailed descriptions of present day methods of fire assaying can be found in Hafferty, J., L. B. Riley, and W. D. Goss, A Manual on fire assaying and Determiniation of the Noble Metals in Geological Materials, U.S. Geological Survey Bulletin 1445, Washington DC, 1977; and Bugbee, E. E., A Textbook of Fire Assaying, John Wiley and Sons, Inc., New York, 1940.

Not all assayers use well established techniques, such as fire assaying, and some assayers have been known to be crooks (see section on Mining Scams).

Fire Assaying Model

Introduction

Model for Collection of Precious Metals in Fire Assaying

Fire assaying is the method in which the amount of precious metal in a sample is determined. This is done in several steps:

Mixing the weighed sample with various chemicals to make the sample melt at a relatively low temperature.
Heating the sample and chemicals in a furnace to form a slag that is easily removed from the lead and a lead button that contains the precious metal.
Removing the slag from the lead button.
Heating the lead button in a bone ash cupel so the lead is removed from the precious metals, leaving the precious metals in a small bead (doré) at the bottom of the cupel.
Weighing the doré bead to determine the fraction of precious metal in the original sample.

The second step, heating the sample to gather the precious metal into the lead as it falls through the molten sample, may be roughly modeled by the same process used to clarify water.

Key Concepts

Silt and clay in water are removed by coagulation with a flocculant. When this activity is used to model the collection of precious metals by the lead, the silt and clay particles represent the gold and silver atoms, while the flocculant represents the lead as it falls through the molten sample.

In the activity, the calcium oxide (lime, CaO) reacts with the water to make the solution basic as shown by the following reaction: CaO + H2O = Ca++ + 2OH-. The ammonium aluminum sulfate (ammonium alum, shortened to be just aluminum sulfate because the ammonium ion does not take part in the reaction) then reacts with the hydroxide to form the gelatinous hydroxide: Al(NH4)(SO4)2 in water yields aluminum ion in solution: Al+3 + NH4+ + 2SO4-2 . The aluminum ion reacts with the hydroxide: Al+3 + 3OH- = Al(OH)3, a gelatinous precipitate.

Skills: Observing, Recording, Investigating, Modeling

Time: 40 minutes

Audience: Teachers and students, grades 5 - 8.

Objective: To experiment with chemical and physical ways to clarify water.

Safety: Wear chemical splash goggles.

Advance Preparation

Prepare the finely ground rock either by grinding some rock in a ball mill or with a mortar and pestle or using a mixture of rock and dirt found around the yard.

Prepare a solution of aluminum sulfate by mixing two teaspoons of powdered aluminum sulfate, potassium aluminum sulfate (potassium alum), or ammonium aluminum sulfate (ammonium alum) into one liter of water.

Lime is used directly. Avoid contact with skin and eyes.

Tips

The flocculant produced by the aluminum sulfate in basic solution (aluminum hydroxide, Al(OH)3 ) forms over a period of several minutes.

Materials

For each student:

Clear, 16-ounce empty water bottles with lids, 2 per group

Pulverized dirt sample or fine silt and clay from outside
Calcium oxide (CaO) (lime), about a match-head sized bit will be used for the experimental bottle
Plastic spoon to pick out the bit of calcium oxide
Aluminum sulfate solution prepared as above, 25 milliliters per group

The Procedure

Prepare "dirty water" by filling the two bottles 1/8 full of finely ground soil. The exact measurement need not be precise, but the two bottles should have the same amount of soil in them. Fill the bottles to within 3 centimeters of the top with tap water. Be sure the lid on tight. Shake the bottles to be sure all of the dirt is wet and mixed.
The first bottle is the control. Add no flocculant (aluminum sulfate and lime) to this bottle. The second bottle will have the aluminum sulfate plus lime added as flocculant.
To the experimental bottle, use a plastic spoon to add a small amount of lime to the solution to make the sample basic. Test that the solution is basic by using pH paper. (Litmus paper should turn blue; pH should be 10 or greater.) Now add 25 mL of the aluminum sulfate solution. Add 25 mL water to the control bottle to see if merely adding the water makes a difference.
At the same time (use two hands or two partners), slowly invert both bottles five times making sure the dirt is off the bottom. Set the samples down upright and make observations every 30 seconds. Concentrate on seeing how the suspended particles are collected by the falling aluminum hydroxide particles.

Suggested Follow-up Discussion

(Possible answers in italics, but other answers might also apply.)

This activity models the concept of the molten lead collecting the gold or other precious metal atoms in the molten rock during the fire assay method of analyzing rocks for gold content. What might be similar in the two processes? (The gelatinous aluminum hydroxide precipitate is distributed throughout the solution, then starts falling toward the bottom as the particles get larger. The lead is also distributed throughout the melted rock sample, and as bigger particles accumulate it starts to fall through the melted rock.) What might be different? (The water is less viscous than the melted rock; the lead is much more dense than the aluminum hydroxide precipitate.)

Think about possible reasons the clay particles might be caught by the gelatinous aluminum hydroxide precipitate. What might be the reasons gold might be caught by the lead? (The clay particles might be physically trapped by the gelatinous aluminum hydroxide precipitate. The clay particles might be attracted to the aluminum hydroxide precipitate because of slight electrical charges on different parts of the particles. The gold might be attracted to the lead because they are both metals; or it might be forced out of the melted rock because it is still a metal, not in a compound with the other elements in the melt.)

Extensions

Water treatment plants use several flocculants, usually including the aluminum sulfate. A field trip to a water treatment plant would expand understanding of the uses of flocculants.

The chemistry of attractions of one molecule to another involves the structure of each of the molecules. Partial charges on one part of the molecule may attract oppositely charged parts of another molecule such as is found in hydrogen bonding. Study the locations of partial charges on the aluminum hydroxide molecule and on clay particles. For the gold and lead atoms, partial charges are not a factor because these elements occur here as atoms not combined into molecules with other elements. Study the properties of gold and lead atoms to see if they might be attracted because they are similar in electronic structure.