Seeking help regarding the use of thiourea method for gold extraction – why does the addition of alkali result in precipitation?

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First, sulfuric acid is added, along with an appropriate amount of ferric sulfate; then thiourea is used to dissolve the gold. After that, liquid alkali is added to adjust the pH to 11.5–12. The resulting precipitate is filtered and dried at low temperature. Sodium carbonate, borax, nitrate, and glass powder are added to this precipitate; after mixing, the mixture is placed in a crucible. Salt is placed on top of the crucible, and the mixture is heated using a blowtorch or furnace. Sir, under the conditions of acidic oxidation using thiourea to dissolve the gold, and after adding liquid alkali, the precipitate that forms contains iron hydroxide. How is the gold then separated out? And what chemical reactions occur when those additional substances are added? What is the function of each of them? Please help me out, expert