Abstract: A method of detecting changes in a surface state and of monitoring said surface state, in particular for a body whose surface is at a given temperature and is in contact with a flowing liquid at a given temperature which may optionally be different from the temperature of said surface, wherein a reference metal body is placed in said liquid, said body having a surface at a temperature which is adjustable independently of the temperature of said liquid, and said body acting as an electrode which is optionally raised to an adjustable potential, and the surface state of said body is detected by measuring the intensity of light radiation reflected from at least one mirror-forming portion of the surface of the reference body.

Abstract: An apparatus for carrying out a liquid-liquid extraction process by co-current contact between an extraction liquid and a liquid from which impurities are to be extracted includes a mixing compartment in the form of a vertical column having an open lower end and equipped in its interior and substantially throughout its length with a rotary element having surfaces extending perpendicularly or obliquely to the axis of the element. The open lower end of the column extends into a settling compartment in the form of a leak-proof container provided with means for determining the level of the extraction liquid in the container. The extraction liquid is recycled from the container by being converted to a gas and then reintroduced into the column. Compressed air is controlled electronically to supply make-up extraction liquid and any necessary reagents into the settling container to ensure that the level of the extraction liquid is maintained constant.