The solvents are removed by evaporating in a vacuum and ammonium diuranate, (NH4)2U2O7, is precipitated by adding ammonia to neutralize the solution. The diuranate is then heated to yield a purified, solid U3O8, known as yellow cake. (Kennesaw State University )

On site, close to the mines, the uranium is concentrated and transformed into a yellow powder, commonly called yellow cake, which contains 75% of uranium. It is in this form that the uranium is marketed. (planete-energies.com)

The first stage of this process is to dissolve the yellow cake in nitric acid; solvent extraction using tributyl phosphate in a hydrocarbon solution extracts the uranium from the various impurities and then the extract is washed with dilute nitric acid to produce uranyl nitrate in a very pure form. (landmarkminerals.com)