A refractory ore generally contains sulfide minerals, organic carbon, or both. Sulfide minerals often trap or occlude gold particles, making it difficult for the leach solution to complex with the gold.

In the event that there are high levels of copper or silver present, leaching of the precipitate using nitric or sulfuric acids may be required. Gold parting is primarily the removing of silver from gold. Various techniques have been practised; salt cementation from ancient times, parting using distilled mineral acids from medieval times, and in modern times using chlorination using the Miller process and electrolysis using the Wohlwill process.

Most metals occur in nature in their oxidized form (ores) and thus must be reduced to their metallic forms. The ore is dissolved following some preprocessing in an aqueous electrolyte or in a molten salt and the resulting solution is electrolyzed. The metal is deposited on the cathode (either in solid or in liquid form), while the anodic reaction is usually oxygen evolution. Several metals are naturally present as metal sulfides; these include copper, lead, molybdenum, cadmium, nickel, silver, cobalt, and zinc. In addition, gold and platinum group metals are associated with sulfidic base metal ores. Most metal sulfides or their salts, are electrically conductive and this allows electrochemical redox reactions to efficiently occur in the molten state or in aqueous solutions.

Electrowinning, also called electroextraction, is the electrodeposition of metals from their ores that have been put in solution via a process commonly referred to as leaching. Electrorefining uses a similar process to remove impurities from a metal.

Electrowinning is the oldest industrial electrolytic process. The English chemist Humphry Davy obtained sodium metal in elemental form for the first time in 1807 by the electrolysis of molten sodium hydroxide.

Electrorefining of copper was first demonstrated experimentally by Maximilian, Duke of Leuchtenberg in 1847.

The H CycleBecause of its low molecular weight hydrogen can leave Earth's atmosphere.

It has been suggested that this occurred on a grand scale in the past and that this is why today the Earth is mostly irreversibly oxidised.

Water is produced when hydrogen is burned.

Molecular clouds of H2 are associated with star formation. Hydrogen plays a

vital role in powering stars through proton-proton reaction and CNO cycle nuclear fusion.

Throughout the universe, hydrogen is mostly found in the atomic and plasma states whose properties are quite different from molecular hydrogen. As a plasma, hydrogen's electron and proton are not bound together, resulting in very high electrical conductivity and high emissivity (producing the light from the Sun and other stars). The charged particles are highly influenced by magnetic and electric fields.

Water is formed by the combination of oxygen and hydrogen. During photo synthesis hydrogen is produced by the dissociation of water which forms glucose after combing with carbon dioxide.

The N cycle

Many small nodules are present in the roots of leguminous plants which contain bacteria named Rhizobium leguminosarum. These bacteria convert atmospheric nitrogen into nitrates. Plants absorb nitrate salts through their roots .

Apart from this, a fraction of nitrogen in atmosphere changes in to nitric acid during lightning which forms nitrates with rain water in soil.

Thae C cycle

Most of the carbon within organisms comes from the carbon dioxide (CO2) in the air.The atmosphere is 0.03 mol % in CO2. However, the greatest physical reservoir of carbon is not atmospheric carbon dioxide but instead is located in the Earth's crust and is not easily accessible to biological organisms. Figures 17.27 and 17.28 show the reservoirs of carbon and the

transactions that take place in the carbon cycle.

1. Carbon fixation. Photosynthetic carbon fixation is responsible for the vast majority of the carbon fixed in nature. It involves the use of light energy to "fix" atmospheric carbon dioxide into sugars (and subsequently into other biological molecules):

CO2 + H2O + energy -----> (CH2O)n + O2

Photosynthetic carbon fixation may be done in both oxic and anoxic environments. In the reaction above, use of water as an electron donor results in the formation of oxygen. This is the chemistry done by oxygenic organisms such as algae, cyanobacteria, and green plants. Alternate electron donors are used by photosynthetic carbon fixers, such as the phototrophic green or purple sulfur bacteria, living in anoxic environments.

Carbon occurs in all known organic life and is the basis of organic chemistry. When united with hydrogen, it forms various hydrocarbons which are important to industry as refrigerants, lubricants, solvents, as chemical feedstock for the manufacture of plastics and petrochemicals and as fossil fuels.

When combined with oxygen and hydrogen, carbon can form many groups of important biological compounds including sugars, lignans, chitins, alcohols, fats, and aromatic esters, carotenoids and terpenes. With nitrogen it forms alkaloids, and with the addition of sulfur also it forms antibiotics, amino acids, and rubber products. With the addition of phosphorus to these other elements, it forms DNA and RNA, the chemical-code carriers of life, and adenosine triphosphate (ATP), the most important energy-transfer molecule in all living cells.

A metallurgical assay is a compositional analysis of an ore, metal, or alloy. Silver is assayed by titration, gold by cupellation and platinum by inductively coupled plasma optical emission spectrometry (ICP OES).