The Wohlwill process is an industrial-scale chemical procedure used to refine

gold Gold is a chemical element; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...

to the highest degree of purity (99.999%). The process was invented in 1874 by Emil Wohlwill. This

electrochemical Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference and identifiable chemical change. These reactions involve electrons moving via an electronically conducting phase (typi ...

process involves using a cast gold

ingot An ingot is a piece of relatively pure material, usually metal, that is Casting, cast into a shape suitable for further processing. In steelmaking, it is the first step among semi-finished casting products. Ingots usually require a second procedu ...

, often called a doré bar, of 95%+ gold to serve as an

anode An anode usually is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, which is usually an electrode of the device through which conventional current leaves the devic ...

. 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 A cathode is the electrode from which a conventional current leaves a polarized electrical device such as a lead-acid battery. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. Conventional curren ...

s for this reaction are small sheets of pure (24k) gold sheeting or stainless steel. Current is applied to the system, and

electricity Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...

travels through the

electrolyte An electrolyte is a substance that conducts electricity through the movement of ions, but not through the movement of electrons. This includes most soluble Salt (chemistry), salts, acids, and Base (chemistry), bases, dissolved in a polar solven ...

of chloroauric acid. Gold and other metals are dissolved at the anode, and pure gold (coming through the chloroauric acid by ion transfer) is preferably plated onto the gold cathode while other metals remain in the solution. 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 is the process of purification of a (1) substance or a (2) 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 w ...

method, the Miller process, which produces gold of 99.5% purity. For industrial gold production the Wohlwill process is necessary for highest purity gold applications. When lower purity gold is required, refiners often utilize the Miller process for its relative ease and quicker turnaround times and because it does not require a large inventory of gold, in the form of chloroauric acid.

References

{{reflist Metallurgical processes Gold

See also

References