Once the dissolution of silver and other metals by acid nitric, the bath is cooled and the residue is separated from the solution. The parting residue is impure gold and looks like a powder and must be filtered and washed with hot water in order to eliminate any trace of silver nitrate or other water soluble compound. In this way, gold has no nitric acid or any nitrate. The final point is detected by adding sodium chloride in to the filtrate solution. When there is not any milky turbidity, the washing process is complete. The washings are added to the first silver solution. The residue is pressed, dried, melted and poured into ingots. The gold thus obtained is usually 99.8% to 99.9% pure and the minor element is silver. It has been noted that the presence of silver is influenced by the addition of excessive dosages of nitric acid and the poor washing of the residue.
It has been noted that if the residue contain small quantities of impurities, it is not necessary to add much flux and small quantities of sodium carbonate and potassium nitrate are sufficient. In this case the slag has small quantities of impurities and the losses of gold are minimal. Nevertheless, if the presence of impurities is high, it is necessary to add borax and potassium nitrate and remove the slag with the impurities. Basically, the addition of potassium nitrate has an oxidizing effect on impurities, which are transported to the slag phase as oxides. When sufficient potassium nitrate is added, the final point is determined by the viscosity of the slag and when this is pasty, the addition of flux is correct. But, when the slag is extremely loose, it is necessary to add more flux in order to regulate the viscosity. Although this is the last part of the process, it is important to observe the slag. Experienced operators recognize the final point and the problems associated by the poor efficiency of the acid process.