In a process for separating liquid mixtures by solvent extraction in countercurrent, a rising or falling temperature gradient is provided in the extraction zone or in a washing zone, so that the concentration of dissolved components <PICT:0477567/IV/1> <PICT:0477567/IV/2> <PICT:0477567/IV/3> <PICT:0477567/IV/4> liquid mixture (other parts being at low, normal or raised temperature), or the temperature may be lowest at an intermediate point and rise towards both ends. The temperature gradient may consist of a series of steps, the apparatus comprising mixer (emulsifier), cooler and separator (settling tank) for each temperature stage. The concentration of dissolved components is preferably kept at about 10 to 20 per cent by volume. The process may be used for treating cracking-distillates and residues of mineral oil, tars, shale oil to remove wax and asphalt, to separate fractions richer and poorer in hydrogen, to improve any property which varies with the solubility, or to remove aromatic, or aromatic and naphthenic components. The process may be used with the processes described in Specifications 355,294 and 417,617, [both in Group III]; when used for treating petroleum (Duo-Sol process) in countercurrent with two selective solvents which are mutually only slightly miscible, the two solvents may be (1) aliphatic hydrocarbons or mixtures thereof, preferably paraffins other than methane and ethane, and (2) polar solvents (sulphur dioxide, methyl alcohol, nitro-benzene, nitromethane, naphthonitrile, benzonitrile, methyl rhodanide, aniline, cresol, benzyl alcohol, benzaldehyde, acetone, furfurol, thiophene aldehyde, and polar derivatives in general of furane, pyridine and thiophenes are specified). In one method, the solvent extract is washed with the pure extract or with a liquid rich in extract (reflux liquid); the process may be used for splitting the following mixtures by means of the selective solvent indicated, the reflux liquid being the first-named in the solvent in the zone is kept constant. The temperature gradient may extend only through that part of the extraction system between the respective entrances of solvent and component: (1) glycol ethers and glycol, by means of ether; (2) isopropyl acetate, isopropyl alcohol and water, by means of benzene; (3) ethyl alcohol and ethyl acetate, by means of calcium chloride solution; (4) glycol, glycerine and water, by means of chloroform; (5) amyl alcohol and methyl alcohol, by means of heptane; (6) sulpho-acids and the hydrocarbon oils from which they are produced, by means of amyl alcohol; (7) chlorhydrin and glycol by means of secondary alcohols containing 13 to 18 carbon atoms. Diluents for the selective solvent such as pentane, ether, benzene, naphthalene, chloroform may be added. Pressure may be normal or above or below atmospheric. Control of temperature to keep the concentration constant may be effected by hand or automatically, test-cocks being provided at various points or the raffinate being metered; in any given extraction process, the concentration can be kept constant either by maintaining a particular temperature in the settler and adjusting the flow of raffinate so as to maintain the level of phase separation in the settler constant, or by maintaining a particular outflow from the settler and adjusting the temperature so as to keep the constant level. As shown in Fig. 1, a mixture A + B is treated with solvent O which is brought to the desired initial temperature by a heat exchanger 8, and a temperature gradient is maintained by a coil 2, a liquid or vapour jacket 3, and a jacket 6 heated electrically. As shown in Fig. 2, a temperature gradient is maintained in each of two columns 10, 11; a mixture A + B is fed to the bottom of the column 10 and is treated with two mutually immiscible solvents O, P in countercurrent; the solvent O is fed in drops 27 to the top of the column 10 and takes up more of A than of B, the drops collect as a separate phase O + A + (B) below the level S; this mixture is led to the top of the tower 11 and washed to remove B with a mixture P + A fed to the bottom of the tower at 21; the liquid leaving the bottom of the tower 11 is a mixture O + A which is separated, e.g. by distillation at 22; the drops rising to the top of the tower 11 collect as a separate phase P + A + B above the level T, which is fed to the bottom of the tower 10; the component A is extracted in the tower 10 from the mixtures A + B and P + A + B, and a mixture P + B leaves the top of tower 10; test-cocks 17 may be provided on both towers; the tower 11 is provided with heating or cooling means 15, 16 by which the concentration in one or both phases is kept constant. As shown in Fig. 3, plant with stepped temperature gradient for extracting mineral oil with furfurol comprises settlers 1a--1d, mixers (emulsifier) 2a--2d, non-turbulence pumps 6a--6d, heat-exchangers 3a--3d; the extraction zone is not shown. An extract (furfurol + aromatics + some raffinate) at 94 DEG C. is fed by the line 18 to mixer 2d, mixed with extract passing through 4d from the settler 16c at 78 DEG C., the mixture is pumped through cooler 3d and the lines 7d, 16d to the settler 1d where two phases are formed at 86 DEG C. One phase from the settler 1d is treated in similar fashion with extract from the next lower settler; and so on in stages, a purified extract being fed to the lowest settler from line 4a in admixture with the extract from settler next above: there may be seven stages, the temperatures in the settlers being 86, 78, 71, 63, 55, 48, and 40 DEG C. The extract solution from the lowest settler is used as cooling agent in five of the coolers such as 3c, 3d, whilst 3a and 3b require a colder agent. The cooling may be regulated by means of cocks 13, 14 and byepasses 10, 11. The extract solution, preheated by use as cooling medium is distilled to separate the furfurol from the aromatic extract; the distillation may be effected at reduced pressure in "multiple-effect" apparatus, an extract containing furfurol being drawn off from the main fractionating column and led to the line 4a, furfurol and furfurol + water being drawn off at other points. The raffinate discharge tubes 4b--4d are provided with meters 21b--21d. Temperature in the settlers is regulated by this metered outflow of raffinate and by the controlled coolers 3a--3d. The mixer shown in Figs. 3a, plan and elevation, comprises a cylindrical casing with inlet 37a for the continuous phase outlet 37 for the mixture, and a hollow axial pipe 35 forming the inlet for the phase to be dispersed. The pipe 35 is connected with a number of perforated pipes 36, Figs. 3a, forming a rotating cage, and fixed rods 38 are provided in the casing to hinder rotation of the body of liquid. The liquid from the pipes 36 therefore forms drops, the size depending on the speed of rotation of the cage. The settlers or phase separators (not shown in detail) comprises nests of tubes, opposite the open ends of which is mounted a baffle plate, whereby liquid flowing through the tube impinges on the baffle and is forced to return outside the tubes where separation into phases takes place. Discharge tubes at the upper and lower regions of the separator are provided. British Specifications 380,954, 445,317, and 451,881, [all in Group III], and French Specification 790,206 also are referred to.ALSO:<PICT:0477567/III/1> p <PICT:0477567/III/2> <PICT:0477567/III/3> <PICT:0477567/III/4> In a process for separating liquid mixture by solvent extraction in countercurrent, a rising or falling temperature gradient is provided in the extraction zone or in a washing zone so that the concentration of dissolved components in the solvent in the zone is kept constant. The temperature gradient may extend only through that part of the extraction system between the respective entrances of solvent and liquid mixture (other parts being at low, normal or raised temperature), or the temperature may be lowest at an intermediate point and rise towards both ends. The temperature gradient may consist of a series of steps, the apparatus comprising mixer, (emulsifier), cooler, and separator (settling tank) for each temperature stage. The concentration of dissolved components is preferably kept at about 10 to 20 per cent by volume. The process may be used for treating cracking-distillates and residues of mineral oil, tars, or shale oil to remove wax and asphalt, to separate fractions richer and poorer in hydrogen, to improve any property which varies with the solubility, or to remove aromatic, or aromatic and naphthenic components. The process may be used with the processes described in Specifications 355,294 and 417,617; when used for treating petroleum (Duo-Sol process) in countercurrent with two selective solvents which are mutually only slightly miscible, the two solvents may be (1) aliphatic hydrocarbons or mixtures thereof, preferably paraffins other than methane and ethane, and (2) polar solvents (sulphur dioxide, methyl alcohol, nitrobenzene, nitromethane, naphtho-nitrile, benzonitrile, methyl rhodanide, aniline, cresol, benzyl alcohol, benzaldehyde, acetone, furfurol, thiophene aldehyde, and polar derivatives in general of furane pyridine and thiophene are specified). In one method the solvent extract is washed with the pure extract or with a liquid rich in extract (reflux liquid), the process may be used for splitting the following mixtures by means of the selective solvent indicated, the reflux liquid being the first-named component: (1) glycol ethers and glycol, by means of ether; (2) isopropyl acetate, isopropyl alcohol and water, by means of benzene; (3) ethyl alcohol and ethyl acetate, by means of calcium chloride solution; (4) glycol, glycerine and water, by means of chloroform; (5) amyl alcohol and methyl alcohol, by means of heptane; (6) sulpho-acids and the hydrocarb