Wohlwill process
Encyclopedia
The Wohlwill process is an industrial-scale chemical procedure used to refine gold
to the highest degree of purity (99.999%). The process was invented in 1874 by Emil Wohlwill
. This electrochemical
process involves using a cast dore ingot
, often called a Doré bar
, of 95%+ gold to serve as an anode
. Lower percentages of gold in the anode will interfere with the reaction, especially when the contaminating metal is silver or one of the platinum group
elements. The cathode
(s) for this reaction are small sheets of pure (24k) gold sheeting. Current is applied to the system, and electricity
travels through the electrolyte
of chloroauric acid
. Gold and other metals are dissolved at the anode, and pure gold (coming through the chlorauric acid by ion transfer) is plated onto the gold cathode. When the anode is dissolved, the cathode is removed and melted or otherwise processed in the manner required for sale or use. The resulting gold is 99.999% pure, and of higher purity than gold produced by the other common refining
method, the Miller process
, which produces gold of 99.95% purity.
The Wohlwill process is necessary for highest purity gold applications, and when lower purity gold is required refiners often utilize the Miller process due to its relative ease, quicker turnaround times, and because it does not require a large inventory of gold, in the form of chloroauric acid, on site at all times.
A variant of the Wohlwill process is the fizzer cell. In a fizzer cell, the decaying anode is separated from cathode by a ceramic cell or nanofilter through which electric current can pass, but dissolved ions cannot. To recover gold from a fizzer cell, one removes the gold chloride laden solution and selectively precipitates the gold using a selective reducing agent such as ferrous sulfate, hydrazine
(which is highly dangerous and explosive
), or other lixiviant
s. A fizzer cell which utilizes a carbon rod instantly introduces contaminants into the solution. Carbon rods through which electrical current passes, begin to break down immediately upon application of current. This perceptibly worsens contamination of cell over time.
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
to the highest degree of purity (99.999%). The process was invented in 1874 by Emil Wohlwill
Emil Wohlwill
Hans Emil Wohlwill was a German engineer of electrical chemistry. He invented the Wohlwill process in 1874.- Literary works :...
. This electrochemical
Electrochemistry
Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...
process involves using a cast dore ingot
Ingot
An ingot is a material, usually metal, that is cast into a shape suitable for further processing. Non-metallic and semiconductor materials prepared in bulk form may also be referred to as ingots, particularly when cast by mold based methods.-Uses:...
, often called a Doré bar
Dore bar
A doré bar is a semi-pure alloy of gold and silver, usually created at the site of a mine. It is then transported to a refinery for further purification.The proportions of silver and gold can vary widely...
, of 95%+ gold to serve as an anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
. Lower percentages of gold in the anode will interfere with the reaction, especially when the contaminating metal is silver or one of the platinum group
Platinum group
The platinum group metals is a term used sometimes to collectively refer to six metallic elements clustered together in the periodic table.These elements are all transition metals, lying in the d-block .The six...
elements. The cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
(s) for this reaction are small sheets of pure (24k) gold sheeting. Current is applied to the system, and electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
travels through the electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
of chloroauric acid
Chloroauric acid
Chloroauric acid is a inorganic compound with the formula HAuCl4. This pale yellow compound is a common precursor to gold in a variety of purposes. The term chloroauric acid is also sometimes used to describe other gold chlorides....
. Gold and other metals are dissolved at the anode, and pure gold (coming through the chlorauric acid by ion transfer) is plated onto the gold cathode. When the anode is dissolved, the cathode is removed and melted or otherwise processed in the manner required for sale or use. The resulting gold is 99.999% pure, and of higher purity than gold produced by the other common refining
Refining
Refining is the process of purification of a substance or a form. The term is usually used of a natural resource that is almost in a usable form, but which is more useful in its pure form. For instance, most types of natural petroleum will burn straight from the ground, but it will burn poorly...
method, the Miller process
Miller process
The Miller process is an industrial-scale chemical procedure used to refine gold to a high degree of purity . It was invented by Francis Bowyer Miller. This chemical process involves blowing a stream of pure chlorine gas over and through a crucible filled with molten, but impure, gold...
, which produces gold of 99.95% purity.
The Wohlwill process is necessary for highest purity gold applications, and when lower purity gold is required refiners often utilize the Miller process due to its relative ease, quicker turnaround times, and because it does not require a large inventory of gold, in the form of chloroauric acid, on site at all times.
A variant of the Wohlwill process is the fizzer cell. In a fizzer cell, the decaying anode is separated from cathode by a ceramic cell or nanofilter through which electric current can pass, but dissolved ions cannot. To recover gold from a fizzer cell, one removes the gold chloride laden solution and selectively precipitates the gold using a selective reducing agent such as ferrous sulfate, hydrazine
Hydrazine
Hydrazine is an inorganic compound with the formula N2H4. It is a colourless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. Approximately 260,000 tons are manufactured annually...
(which is highly dangerous and explosive
Explosion
An explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. An explosion creates a shock wave. If the shock wave is a supersonic detonation, then the source of the blast is called a "high explosive"...
), or other lixiviant
Lixiviant
Lixiviant is a liquid medium used in hydrometallurgy to selectively extract the desired metal from the ore or mineral. It assists in rapid and complete leaching. The metal can be recovered from it in a concentrated form after leaching. Lixiviant in a solution may be acidic or basic in nature. Most...
s. A fizzer cell which utilizes a carbon rod instantly introduces contaminants into the solution. Carbon rods through which electrical current passes, begin to break down immediately upon application of current. This perceptibly worsens contamination of cell over time.