Gold Precipitation Methods

Gold precipitate is precipitated in the process of refining gold metal in the form of dust or fine particles are colored a brownish, and the precipitate obtained from the reaction of the chemical separation of metallic gold with aqua regia. In decision precipitates in aqua regia solution that has been contained gold metal content is required several stages. And eventually precipitated from these precipitates can be dissolved to be made in a metallic gold.

Some stages are used to obtain precipitates gold result from the process of refining gold ore with aqua regia solution can use the following steps.

• To neutralize the residual HNO, add urea [CO (NH2) 2] to the Aqua Regia which has been filtered. Take one quart of tap water and heat to boiling. Remove from heat and add one pound of urea to the water.  will use to adjust the pH of the acid. Slowly, add the water/urea to the acid. The acid will foam with the addition of the water/urea. Do not add the water/urea so quickly that the acid foams out of its container. When the acid stops reacting to the addition of water/urea, stop adding the water/urea. You've just raised the pH of the acid from 0.1 To 1.0, killing the nitric acid but not the hydrochloric.

it will get a reaction :

6 HNO3   5CO(NH2 ) 2 = 8N2   5CO2   13H2O   

• After the reaction is complete then Enter Sodium Bisulphite and observe the reaction.In theory, every single gram of gold requires 1.89 grams of Sodium Bisulphite. However, it should be added more, about 1.5 times longer.

Then the reaction is :

2HAuCl 4 + 2NaHSO3 = 2Au + 4HCl + Na2 SO4 + SO2

• Wait about 30 minutes, and immediately the acid will change to a muddy brown appearance as brown particles of gold form in the water. This brown "mud" is, despite its appearance, pure gold. And then dispose of the liquid solution slowly to the remaining precipitate only way destilled filtered and then rinsed with water. Sodium Bisulfide If not then there are several alternative reagents that can be used as a substitute such as Sodium Sodium Bisulphite Metabisulfide (SMB), oxalic acid, sulfur, and Sulphur Dioxide or Copperas (Ferrous Sulphate) 
• The acid should now be a clear amber color with a brown mud at the bottom. Pour off the acid into another container. Then pour it gently - the land of ammonia solution (30 ml Aqua ammonia has been dissolved into 100 ml mineral water) to precipitate until the pH reached 8.It will get a Gold Fulminating sediment. Do not pour off any of the mud. The mud is pure gold. Remember fulminant sediment is very dangerous, not to dry because it has the nature of the High Explosive. 
• Wash the precipitate to remove excess ammonia. When all the acid is poured off, add tap water to the mud. Stir and let the mud settle. Pour off the water into the container with the acid. If you have a filter, use it. Do not pour off any particles of brown. Repeat this rinsing 3-4 times or more Wash several times until the pH reaches close to 7. Rinse once with aqua ammonia. White vapors will appear. The aqua ammonia cleans impurities from the gold mud while, at the same time, it neutralizes any acid still clinging to the gold mud.
• Give the mud a last rinse, this time with distilled water.Using distilled water, wash the mud into a small Pyrex beaker Visionware pot. Give the mud a chance to settle and then pour off the water. Put the beaker on a hot plate to dry the mud. Do not preheat the hot plate or thermal shock may cause the beaker to break. and then in the process of melting metal to form gold.