The Miller Process for Refining Gold

Gold Bars   Agnico-Eagle

Ever wonder how most gold is refined?  No for the most part they don’t use the Wolhwill Process rather they use the Miller Process.  Unlike the Wolhwill Process described earlier in this series the Miller Process doesn’t use electricity but rather uses a stream of chlorine gas that is passed through a crucible of molten gold that produces gold that is 99.95 Fine.  It works to remove all the impurities because gold is least affected by chlorine so all the other impurities are removed as chloride slag floating on the surface of the molten gold where it is skimmed off.

In this process the dore bar is melted in an induction furnace where the chlorine gas is introduced into the molten metal by a rotating lance.  The lance is controlled in a very precise manner in a way there is no break through of the chlorine gas to the surface of the molten metal.  This removes what are termed “By-metals” like silver, copper, nickel along with any other metals.  The process also removes amphoteric elements like sulfur, selenium, tellurium, arsenic and bismuth.  These elements are all removed from the molten metal as chloride slag that floats to the surface of the charge where they are skimmed off for further treatment.

The process takes from 2 to 4 hours to complete with the chloride fumes being removed and filtered by a two stage wet filter or a two stage dry scrubbing system that maximizes the recovery of the chloride as well as the cleaning of the exhaust air from the process.  In the process slag is poured off the top of the melt into a crucible and allowed to slowly cool so any gold that has been caught up with the chlorides will settle to the bottom where it can be recovered as a button of gold when the crucible has cooled completely; it is then added to the remaining molten gold.  In this process the particles of gold remaining in the crucible are carefully removed by hand as well as the separation molds when they have cooled.

Once the gold has cooled it is poured through a ceramic filter directly into the granulation equipment or anode molds if the Wolhwill process is used to further refine the gold.

The main advantages of this process are its quick turnaround time, size of the charge that can be from 20 kg to as large as you like.  The rotating lances are made from temperature resisting ceramics for long life and evenly introduce chlorine to the melt.  The use of modern induction furnaces keep the consumption low using far less kilowatts then coil type furnaces for the same size of melted product.

In this process the gold produced is 99.95 Fine that is good enough for most commercial purposes.  Most of the metals remaining in the gold are mostly silver and copper that if desired can be removed using the Wolhwill Process producing gold bullion that is 99.999 Fine.   

Refining Gold using Electrochemical Processes

An electrolytic cell used in the commercial refining of gold using the electro-chemical process.

This is a continuation of the previous article about the recovery of gold from concentrated stream heavies.  In this article we will examine the WolhwillProcess that was originally invented in 1874 and is still in use today. This process is used to produce gold that is 99.999% pure.  As an industrial-scale technique the Wohlwill process that was invented by Emil Wohlwill for refining gold using methods developed for electrochemistry.

The process makes use of a bar cast from impure gold that has been separated from concentrate like that in the first part of the process we explained in an earlier article.  This is done by pouring the gold and other metals into a crucible, melting then and pouring them into a mold producing what is called a “dore” bar composed of gold, silver and PGM.  If the amount of gold present in the dore bar is less then 95% it will be necessary to add enough pure gold to bring its content up to that level otherwise the other metals will be carried over in the finished product making the gold less then 99.999 fine.  The Dore bar acts as an anode in the process.

The cathode in this reaction is made from thin sheets of 24k gold that are spaced on each side of the Dore bar that acts as the anode.  Providing the anode is composed of more then 95% gold the remaining metals in the cathode will be deposited in the bottom of the electrolytical cell as anode sludge that can be recovered and the values of silver and PGM recovered in a later process.  

The gold along with the other metals are dissolved at the anode when current is delivered to the cell with pure gold coming through the chloauric from ion transfer where it is plated as pure gold at the cathode.  After the anode is completely dissolved the cathode is removed and melted down or processed for making the end product that is usually gold bullion pure gold like this is too soft for most other uses.

At 99.999 fine this gold is too soft for most purposes other then bullion so it must be alloyed with other metals most commonly silver although in some cases copper is also used.  For making jewelry the most common karats used are 14 and 18 k.  This is done by remelting the bullion and forming it into the shapes desired.

There are plenty of gold refiners that will refine the concentrate into pure gold, but they often keep the platinum values as part of the refining fee. 

How to make a Geopolymer Crucible for Refining Gold

You can easily make a crucible for refining gold by using an ancient technology that was allegedly used to build some of the stone blocks used in the pyramids of Egypt. This technology was rediscovered in the 20th century by a chemical engineer from France named Joseph Davidovits who named it geopolymer. This material can take the place of both conventional concrete and many ceramics, but for the purpose of gold prospecting we shall limit this discussion to making high-temperature crucibles.

Pouring gold from a crucible
Photo by Allen Drebert

The activator fluid is made from 32% sodium hydroxide dissolved in water. The solution also contains 32% sodium silicate. The sodium hydroxide solution is made first by dissolving 32 g of sodium hydroxide in 100 ml of water that is allowed to sit overnight. The activator solution also requires 250 ml of a 32% solution of sodium silicate that can be mixed the next morning.

The completed activator solution contains 100 ml of the sodium hydroxide solution, and 250 ml of the sodium silicate solution mixed together. The activator solution has to be mixed with a pozzolan material of which there are several you can use:

Coal fly ash

Volcanic ash

Ground glass

Ground brick

Coal fly ash and volcanic ash and do not have to be ground, but the other materials have to be ground as fine as possible. The nice thing about making crucibles from this formula is it doesn't take much to make a workable crucible.

The process works like this:

To three parts of pozzolan material and six parts of sifted sand add enough of the activator fluid so that it makes a paste that has “zero” slump when it is mixed. Because of the highly corrosive nature of this mixture you should wear rubber gloves whenever you are handling the material.

The easiest pozzolan material for most people to acquire is either ground glass or a ground-up brick. Glass can be extremely difficult to grind unless you heat it first to red heat with a torch, and plunge it immediately into cold water. This makes it much easier to grind, and glass is one of the most readily accessible pozzolan materials.

The mixture is kneaded into a clay like mass that is then applied to a mold. Coat the surface of the mold with a film of vegetable oil so the crucible will not stick when you try to remove it. The mold should be made of wood that has been coated with a layer of shellac before being used. In a pinch you can use an ordinary drinking glass for a mold providing you cover it with a film of vegetable oil first.

The mold should be covered with about 1/4 inch of the geopolymer material that is allowed to set overnight before removing it from the mold. After the crucible is removed from the mold that is ready for use.

If you have difficulty in making the geopolymer material into a clay like mass you can add one part of calcined clay. It doesn't take much to make a single crucible and we usually use a teaspoon to measure the amounts we need. 

Getting the fixings together for this can present some problems, but the most difficult are getting the sodium hydroxide that is usually sold under its common name “lye” in hardware stores as a drain cleaner. Do not use any other drain cleaning compounds for this project. The other chemical that is hard to find his sodium silicate that also has a common name “waterglass.” If this chemical is not available locally it can be ordered from Sheffield Pottery in 1 pound quantities. Sheffield ships all over the United States and Canada.

It was probably a variation of this formula that the Japanese used to stop the leak in the nuclear reactor that was damaged by the earthquake and tsunami that struck Japan on 11 March 2011.

You can read much more about the science of geo-polymers and Pozzicrete and by going here!