Abstract: Red mud bauxite residue produced in the Bayer Process is treated in a plurality of centrifuges and repulp tanks to successively wash and dewater the red mud. A final centrate liquor is obtained in a first upstream centrifuge stage which is rich in alumina and caustic values for respective recovery and reuse. The concentrated red mud cake produced in the final downstream washing/centrifuge step is low in caustic and alumina concentration permitting environmentally friendly disposal. The concentrated red mud cake is reslurried and mixed with bauxite sand for disposal in a dry stacking impoundment area.

Abstract: A photo diode formed by a substrate of Cd.sub.x Hg.sub.1-x Te covered with a layer of CdTe forming a p-n junction. The substrate may be p-type in which case the layer is n-type or vice versa. An array of photo diodes may be formed by covering the substrate with semi-insulating CdTe and forming islets of In on the CdTe. Heating causes In to diffuse into the CdTe doping it n-type. This results in regions of n-type CdTe surrounded by semi-insulating CdTe each region forming, with the substrate, a photo diode. The heating also causes diffusion between the Cd.sub.x Hg.sub.1-x Te and CdTe to give a graded heterostructure. Electrical connections are made to the substrate, and each n-type region. The n-type CdTe region may alternatively be formed by molecular beam epitaxial growth techniques using a beam of In dopant.