High quality requirements particularly as regards concentration and the maximum quantities of undesirable ions such as Mo, V and P are stipulated by the processors for the end product, so-called "yellow cake" produced by the uranium ore mines. Uranium is generally precipitated as diuranates from the aqueous solutions produced by the ion exchange and solvent extraction processes under precise conditions.

The diuranate can be precipitated from alkaline solutions by two processes:
1. with sodium hydroxide at pH > 12 and 80°C:



2. with ammonia or magnesium oxide after prior acidification of the solutions.

In the first process vanadium is coprecipitated and the sodium diuranate contains a nominal stoichiometric excess of sodium. The vanadium can be removed by roasting the "yellow cake" in the presence of sodium carbonate at 850°C followed by washing. The sodium carbonate solution from the washing is converted into sodium hydrogen carbonate and the sodium hydroxide solution into sodium carbonate by passing hot carbon dioxide into the solutions, which are returned to the leaching process. The consumption of solid sodium hydroxide is 10 to 20 kg/t ore.

In the second process the alkaline solution is mixed with acid and the carbon dioxide liberated driven of by boiling. The resulting acidic solution is then neutralized with ammonia or magnesium oxide, whereupon uranium precipitates together with molybdenum and vanadium. The process is therefore only used if uranium ores have low concentrations of molybdenum and vanadium.

Uranium can also be obtained as uranium(Ⅳ) oxide from the alkaline solutions by reduction with hydrogen at 140 to 150°C under pressures of 6 to 10 bar in the presence of a nickel catalyst:



This very expensive process is no longer carried out industrially.

In the case of acidic solutions, uranium is generally precipitated with ammonia at pH 5 as ammonium diuranate, but magnesium and calcium hydroxides are also used as the precipitating agent:

Should it be necessary, excess sulfate and iron can be removed by preliminary precipitation with milk of lime or magnesium oxide to gypsum and iron hydroxide. If the phosphate concentration is too high, it can be recipitated
as iron phosphate by adding iron ions. A better crystalline ammonium diuranate is obtained by simultaneously blowing in air or steam during the precipitation. In the case of salt-rich acidic uranium solutions, precipitation can be carried out with hydrogen peroxide, this type of precipitation being very selective.

The thereby formed sulfuric acid is neutralized by adding magnesium hydroxide. Independently of the production method, the precipitated uranium concentrate is washed to remove adhering salt solution and then dried. The precipitates produced with ammonia are subsequently calcined in a multiple hearth kiln at 750°C, ammonia, sulfite and chloride being driven off and U₃O₈ being formed:

With sodium diuranate or uranoxy-hydrate a drying temperature of 120 to 175°C is sufficient.

Independently of their color (calcined uranium oxide is dark-green to black), the resulting uranium concentrate is known as "yellow cake". The name refers to the yellow color of the uranium precipitate.