Kaanji in Ayurveda terms implies to a fermented product prepared from cereals , specifically rice. Kaanji is used in purification process of many Ayurveda metals, minerals and gemstones.

General method of preparation.

Rice grains are cooked in water. The cooked grains are then transferred to an hollow earthen vessel. Water 3 times the quantity of cooked rice is then is poured in the earthen vessel over the cooked rice grains. The earthen vessel is sealed with a muslin cloth and kept undisturbed for 7 days. After that, the water (which turns sour due to fermentation) is filtered and stored for use. This water is called Kaanji.

References

http://nlam.in/gloss_disp.php?9bf31c7ff062936a96d3c8bd1f8f2ff3