NZ201745A - Apparatus for washing and classifying solid material in a pulsating liquid column - Google Patents

Description

017 4 5 Priority Date(s): 3-i.: %.

Complete Specification Filed: Class: faQ^/SK/PP.

Publication Date: ... .^1.!. ]?P.6j P.O. Journal, No: .

NEW ZEALAND PATENTS ACT, 1953 No.: Date: COMPLETE SPECIFICATION c<*\ * ■£ « ai ^ I °/ IMPROVED APPARATUS FOR WASHING AND GRANULOMETRIC SEPARATION OF SOLID MATERIALS IN A STATE OF SUSPENSION ±/We, ALUMINIUM PECHINEY, 28 rue de Bonnel, 6 9433 Lyon, France, a French company, hereby declare the invention for which Jc / we pray that a patent maybe granted to his/us, and the method by which it is to be performed, to be particularly described in and by the following statement:- - 1 - (followed by la) 201745 la IMPROVED APPARATUS FOR WASHING AND GRANULOMETRIC SEPARATION OF SOLID MATERIALS IN A STATE OF SUSPENSION The present invention concerns a novel apparatus for washing solid materials in a state of suspension in a liquid phase, to remove the solubilised materials therein, and/or for effecting precise granulometric separation of said materials, said apparatus being in the form of a pulsating liquid treatment column provided with perforated plates.

For a long time now the specialist literature has been describing many apparatuses, some of which were particularly suited to washing solids in aqueous suspension while others were more concerned with operations of classifying solid materials in suspension in a liquid phase.

However, apparatuses which have the double function of providing for washing, aqueous suspensions of solid materials, and classification thereof, have been proposed for the man skilled in the art.

It is with that aim that U.S. patent No. 3,119,721 described a pulsating liquid column for continuously washing an aqueous suspension of solid materials in which the liquid phase is an aqueous liquor containing, in a dissolved condition, materials which were solubilised, for example in an ore attack operation, which liquor is to be replaced in the washing operation by water which is introduced for that purpose.

.Tlir In accordance with the above-indicated U.S'. patent, the apparatus for carrying out such a washing operation, and which can permit washing of an ore suspended in a liquid phase comprises a vertical column, within which are disposed perforated plates or other obstacles which extend horizontally across the column and which are vertically spaced from each other, means for introducing solids or a suspension at one of the ends of said column, and further means for introducing a treatment liquid into said column in the vicinity of the other end, means for extracting a liquid phase or a suspension from the first end mentioned, and means for extracting solids in suspension in the treatment liquid at the other end, and finally, means for generating a pulsating effect in the content of the column so as to cause the solid materials and the liquid phase to pass through the perforations in the plates which extend horizontally across the column.

Thus, using such an apparatus, it is possible for example to wash an aqueous suspension resulting from the operation of attacking an ore, by intimate contact between solid materials which are displaced downwardly in the column and a treatment liquid which rises in the column, being introduced at the bottom of the column.

The provision of the perforated plates or other obstacles, which are disposed in the column at suitable intermediate distances, causes fractionating of the suspension between the plates forming the stages in the treatment, as well as forced displacement of the suspension, that phenomenon being promoted by a pulsating effect caused by introducing the treatment liquid at spaced intervals of time.. 201745 Thus, using that apparatus and in accordance with the mode of procedure associated therewith, it appears to be possible for the suspension of washed solids which no longer contains anything more than a small part of the materials in solution to be ddscontinuously extracted at the bottom of the column, by introducing the washing liquid discontinuously at the bottom of the column, and in a pulsating mode, a part of the washing liquid issuing with the extracted suspension while the other part rises upwardly in the column from which it is discharged by an overflow effect with the feed liquid and with almost all the materials in solution and the substances to be removed.

Now, in spite of all the precautions that the man skilled in the art might take, in particular by keeping strictly to reproducing the equipment described in the above-mentioned document and complying with the conditions of application of the process, it appeared to the applicants that the results obtained were disappointing. In fact, the efficiency of the washing action produced, when evaluated in accordance with the conventional rules relating to counter-flow washing, is found to be much lower than the theoretical value that the man skilled in the art could expect on the basis of calculation.

With that consideration strongly in mind, the applicants, continuing their research, discovered and perfected an improved technology in respect of the above-mentioned column, which gives high-performance results which are truly close to those which could be expected on a theoretical basis. 201745 According to the invention, the improved apparatus for permitting operations of washing solid materials in suspension in a liquid phase to remove the solubilised materials therein and/or to effect operations of precise granulometric separation of solid materials in suspension in said phase comprises a vertical column in which horizontal perforated plates are disposed, means for the feed of suspension to be treated and treatment liquid and for extraction of the treated suspension, and a pulsating means, characterised in that, for a column comprising N actually installed perforated plates, each having a surface area S and a perforation coefficient p, being the ratio between the total area of the perforations and the surface area of said plate, the pulsating means supplying the sum V of the volumes which are transferred alternately upwardly and downwardly per unit of time, the liquid serving for the washing operation or for granulometric separation being of a relative density dn while the liquid issuing by overflow action is of a relative density do, and the solid materials in a state of suspension are of a relative density ds, g being acceleration due to gravity, the diameter 0 of the perforations and the mean distance 1 between said perforations are so selected that a coefficient associating the technical characteristics of said column, as defined by the relationship: 2 K1 = V PS N dn .do l.g do - dn ds is adjusted to a value which is at least equal to 10 and which is preferably between 20 and 300.

According to the invention, the improved apparatus comprises a vertical treatment column which is intended to permit very intimate contact between solid materials which are moving downwardly in the column, and a liquid treatment phase. 50J745 V ) ? ) The solid materials to be treated are generally introduced in the form of an aqueous and/or organic suspension in which substances may be dissolved, while solution and/or a liquid of organic origin, possibly in the form of a mixture or an emulsion.

The vertical treatment column may be cylindrical over its entire height. However, the region of the column which contains the plates is preferably of hyperbolic shape or is formed by a combination of cylinders and/or truncated cones forming the envelope of a theoretical hyperbola. The theoretical hyperbola is such that, with v being the kinematic viscosity of the liquid phase at the level of a plate having a surface area S, the ratio S/v is constant over the entire height of said region.

In a first arrangement, the vertical column comprises in its upper portion a cylindrical collecting region, in its middle portion, a cylindrical region of sufficient height to accommodate the N plates and of a diameter which is less than or equal to the diameter of the upper cylindrical region, the upper and middle regions being connected together by an inverted truncated cone, the large base of which is fixed with respect to the upper cylindrical region and finally, in its lower portion, a conical region, the base of which is connected to the middle cylindrical region.

In a second arrangement, the vertical column comprises, in its upper portion, a cylindrical collecting region, in its middle portion, an inverted rotationally symetrical hyperbolic region, the large base of which is connected to the foregoing cylindrical region and the small base of-" which, being disposed downwardly, is extended by a the liquid treatment phase may be water, an aqueous 6 - 201745 cylindrical region forming the lower part of the column which terminates with an inverted rotationally symmetrical cone.

In accordance with another arrangement, the vertical column is also provided in its upper portion with a cylindrical region, in its middle portion, with an inverted truncated cone, the large base of which is fixed with respect to the upper cylindrical region and is of the same diameter while the small base is disposed downwardly of said column and finally, in its lower portion, a cylindrical region, the diameter of which is identical to that of the small base of said truncated cone, which is extended by a rotationally symmetrical cone, the base of which is connected to the lower cylindrical region.

Finally, in accordance with the last arrangement, the vertical column is formed in its upper, middle and lower portions, by successive cylindrical regions of decreasing diameters, each region being joined to the previous region by level connecting rings or inverted truncated cones, the lower cylindrical region being extended downwardly by an inverted rotationally symmetrical cone.

A treatment region is created within the vertical column, between the above-mentioned upper and lower cylindrical portions. The treatment region is provided with horizontally disposed perforated plates which are equally spaced from each other. The perforated plates may also form groups, each of which comprises one or more equally spaced plates, the distance between each group generally being greater than the distance between the plates 201745 One of the ends of the column is provided with means for introducing the suspension to be washed and/or elutriated and means for discharging the liquid phase charged with solubilised substances and/or elutriated materials, while the other end of the column is provided with means for extracting the washed and/or classified solid materials and means for introducing the liquid washing and/or elutriation phase.

Finally, disposed in the lower cylindrical region of the column is a pulsating means for generating within the space in the column, a rising and falling movement of the suspension through the perforated plates disposed in the treatment region.

By expanding their research and carrying out experiments to perfect the apparatus according to the invention, the applicants were led to find and establish that the various technical parameters of a column for washing and precise granulometric separation of materials in a state of suspension, and the many parameters associated with the various agents being treated and producing the treatment action, were associated in relationships defining two coefficients and K2• Thus, for a vertical column comprising N actually installed perforated plates, each of a surface area S, with a perforation coefficient p, and provided with a pulsating means for supplying the sum V of the volumes transferred alternately upwardly and downwardly per unit of time, with the washing liquid being of a relative density dn and the liquid issuing in an overflow mode at the upper portion of the vertical column being of a relative density do which is sufficiently different from dn, and with the solid materials in a state of suspension being of a relative density ds, the diameter 0 of the m 301745 perforations and the mean spacing 1 between said perforations are so selected that a coefficient K defined by the above-mentioned relationship: K1 = V pS N dn .do l.g do - dn ds (1) is s,et to a value which is at least equal to 10 and which is preferably between 20 and 300.

When the relative density dn of the washing liquid and the relative density do of the overflow liquid have a relative difference of less than 2%, a coefficient K^ defined by the following relationship: zr _ V_ • 0 2 pS v (2) in which v is the kinematic viscosity of the liquor at the level of a plate having a surface area of S, a perforation diameter 0 and a perforation coefficient p, the coefficient K2 must be associated with the above-mentioned coefficient , said coefficient being at least 100 and preferably being between 300 and 5000.

In regard to those two relationships, the applicants found by experiment that the perforation coefficient p, being the ratio between the total area of the perforations in a plate and the surface area of the plate must be between the limits of 0.001 and 0.25 and preferably between 0.005 and 0.1.

All the parameters characterising the values to be given to and must be in a coherent system of units.

Thus in the SI metric system of units, lengths (0 and 1) are expressed in metres, areas (S) are expressed in square metres, volumes (V) are expressed in cubic metres, times are expressed in seconds, the acceleration due to gravity (g) is expressed in metre per second squared,- and j; . kinematic viscosity (v) is expressed in metres squa] second. r20E£B»86; s&BJ X In addition, all those parameters are well known to the man skilled in the art and can "be easily defined "by him in each specific situation.

Thus, the settling area S, which is also referred to as the surface area of the plate, is defined in accordance with conventional, well-known rules in regard to settling, to provide for a desired production tonnage.

Likewise, the volume V which is alternately transferred upwardly and downwardly in the vertical column per unit of time is set at a value which is at least equal to the volume required to produce transfer of the solids in a state of suspension from one stage to another, on the basis of a given hourly production rate. The volume V is displaced discontinuously and in a pulsating mode at the bottom of the column by virtue of the provision of means for producing the pulsation effect, such that the washing liquid is partly discharged with the washed and extracted suspension, while the other part rises upwardly in the column from which it issues by an overflow action with the feed liquid and almost all the materials in solution.

The applicants found that the level of efficiency of the treatment increases in proportion as the instantaneous flow rate of suspension produced by the pulsations in each direction approaches a continuous flow during each fraction of the cycle, that is to say, during each forced displacement of the suspension upwardly and downwardly.

The number of plates n theoretically required is obviously determined by the man skilled in the art in dependence, on the degree of washing or classification desired, using the rules of the art. • The number N of plates which are actually installed is always less than twice the number n of plates which are theoretically preaired in the vertical column.

-/O - 201745 In addition, the diameter 0 of the perforations in each plate is generally greater than six times the diameter of the coarsest particles present in the suspension to be treated, and the spacing between two plates must be at least equal to the mean 5 spacing 1 between the perforations.

As regards the coefficients and they have been defined experimentally and the limits between which they may occur are those in respect of which tests have been carried out, both with suspensions and with solutions which are free of 10 solid phase.

The invention will be better appreciated by reference to the description of the apparatus shown in vertical section in Figures 1 to li.

Referring to Figure 1, the vertical treatment column 15 which is for washing and for selective granulometric separation of solid materials in a state of suspension comprises a middle cylindrical treatment region 1 above^which is disposed an upper cylindrical region 2 provided with an overflow arrangement 3, the middle and upper regions 1 and'2"being connected together 20 by the inverted frustoconical surface L|., and then a lower cylindrical region 5 which is extended by a rotationally symmetrical conical surface 8.

Disposed at the upper end of the column, that is to say, in the region 2, is a feed conduit 10 for introducing the 25 suspension to be treated into the column. The treatment liquid passes into the lower cylindrical region 5 by way of the duct 7 under the action for example of a pump (not shown).

Disposed in the middle treatment region 1 are the N plates 16 provided with perforations 17 which are spaced from each 30 other by a mean distance 1. The middle treatment region 1 which ■ is of substantial vertical extent is intended to permit very intimate contact between the liquid phase to be treated and that producing the treatment action, and the solid phase of the II 20174 -x- substances to be elutriated. The suspension of solid materials to be treated, in a liquid phase, being introduced by way of the duct 10, a mechanical pulsation means 6 disposed in the lower cylindrical region 5 of the column produces the upward-5 and-downward movement of the substance contained in the column. A fraction of the treatment liquid which is introduced by way of the duct 7 is displaced upwardly in the.column, coming into intimate contact with the solid materials being treated, by virtue of counter-flow circulation with respect 10 to those materials. That fraction of the treatment liquid is then discharged by way of the overflow arrangement 3 in the cylindrical collecting, region 2 of the column. The solid materials which, during the treatment, move dowmvardly in the column, are discharged with the other fraction of the treatment 15 liquid, by way of the duct 9, which is referred to as the underflow of "the column.

In the other figures of. drawings, the middle treatment region is a rotationally symmetrical hyperbolic surface 12 in the embodiment shown in Figure 2, with the smaller 20 section directed downwardly, while in Figure 3 the middle treatment region is formed by an inverted frustoconical surface 13 and in Figure i). the middle treatment region is formed by successive cylindrical regions 111 whose diameters decrease downwardly of th.e column, each cylindrical region 25 being connected to the next by an inverted frustoconical surface 15.

As regards separation by elutriation of the solid materials in a state of suspension in the liquid phase, this is effected . by transfer of the materials from one plate to the other, 30 under the action of the -volume of the suspension which is displaced by the pulsation effect, and then by re-suspending 2°17 45 the materials between each plate, which forms a treatment stage, the finer solid fraction migrating upwardly in the column while the coarser-grain solid fraction is displaced downwardly in the column. Thus, each treatment stage defined by the spacing between two plates forms a hydroseparator'which is fed by the association of the flows of suspension of the solid materials, coming from the stage immediately above, when the pulsation effect is propagated downwardly, and from the stage immediately below when the pulsation effect is upward. Hence, each stage T produces a suspension of solid granularraaterials when the pulsation effect is downwardly in the column, and a suspension of fine solid materials when the pulsation effect is upwardly in the column. The solid materials arriving at the underflow arrangement 9 of the column are therefore passed successively into a series of hydroseparators in which they were put into suspension again and re-settled so that in consequence their content of fine solid materials becomes less and less.

Hence, the level of efficiency in regard to separation of the solid materials into two fractions, one being a granular fraction and the other being a fine fraction, by means of the N + 1 hydrosepa~ation stages, is very much higher than that achieved with a conventional hydroseparator-.

Hence, the apparatus according to the invention is found to be highly efficient not only for the purposes of washing a suspension of a solid material in a liquid phase but also for producing highly accurate granulometric separation of solid materials into two classes, one being a fine fraction which is discharged with the overflow arrangement and the other being a granular fraction which is extracted at the "bottom of the column.

EXMPLE_1 This Example illustrates an effort to wash and classify trihydrate of aluminium in suspension in a sodic liquor. 13 - 201745 having a relative density of 1.28, at a temperature of 5L°Ct v/ith a mean granuloma try. of 60-65 p, by means of a pilot column designed in accordance v/ith the state of the art.

The column was of cylindrical-conical type, of an 5 overall height of ij. metres, comprising, in a downward- direction: - a first cylindrical collecting and overflow region of a diameter 0-^ = 2.25m >connected by means of a truncated cone to - a second cylindrical region which was 0.5 metre in. height and 0£ = 1.67 m diameter, containing 10 plates disposed at a spacing of lj.0 millimetres and apertured with holes 9 millimetres in diameter, disposed in accordance with a square mesh with a side of [j.O millimetres, corresponding to a perforation percentage p = 3.5/°> said region being connected by means of a truncated cone to - a third cylindrical region, 1 metre in height and with a diameter 0^ = 1.05 m, provided with 20 plates disposed at a spacing of !i0 millimetres and apertured v/ith holes 11 millimetres in diameter, in accordance with a square mesh with a side of IlO millimetres, corresponding to a perforation percentage of 6.26?£.

The third region was extended by a cylindrical portion of the same diameter and with a height of 0.5 m, provided with a pneumatic pulsating means and an intake duct for the washing water. The third region terminated with a cone opening at the underflow orifice.

The column was fed by the first cylindrical region, using 7.1 m-^/h of an aqueous suspension containing 5 tonnes of trihydrate of aluminium.

At the same time, in its lower cylindrical region, tie column received 5.5 m^/h of washing water, of which 2.8 m^/h was transferred towards the overflow. The total pulsation flow in the two directions represented 13-k- m /h. !& 3f -t: 20J.745 *3 3.7 mJ/h of a suspension containing 2.k tonnes of solid natter was extracted from the coluran, by -way of the underflow arrangement.

The respective relative densities of the liquid and 5 solid phases were as follows: column overflow: do = 1.2 washing water: dn = 1 solid: ds = 2„lj.

The ratio in respect of the concentration of dissolved 10 salts in the liquid feed and underflow phases was only 7.5> this being a result which should have been achieved with a theoretical number of plates n of 3» whereas the column contained 30 installed plates.

At the same time as the washing operation, the grains 15 of aluminium hydrate were classified.

The efficiency of the classification operation was defined by the method using the 'partition curve', which is known as the 'TROMP CURVE', as disclosed by TROMP K.F. 'Neue Wege f&r die Beurteilung der Aufbereitung von Steinkohlen' 20 Gluckauf 73 (137) 125/131 - l£L/l56, which gave a d^Q, a particle diameter having a probability of 0.5 of being discharged at the underflow arrangement, equal to 67 microns, and a partition curve imperfection ('imperfection index1) T . "7S - d2S . 0.25 5o The above-mentioned design and operating data on the column resulted in the coefficient K-^ being of a value of 0.65.

EXAMPLE_2 This Example illustrates the improved apparatus according 30 to the invention, which was used to effect washing and classification of trihydrate of aluminium in suspension in a , A ' ' t A 201745 sodic liquor with a relative density of 1.09, at a temperature of 52°C, with a mean granulometry of 60-65 P, "by means of a pilot column designed in accordance with the teaching of the invention.

The column was of a cylindrical-conical type, with an overall height of I}, metres, comprising, in a downward direction: - a first cylindrical collecting and overflow region • with a diameter 0^ = m, which was connected by means of a truncated cone to - a second cylindrical region, 0.5 metre in height and with a.diameter 0£ of 1.07 m, containing 10 plates disposed at a spacing of 0 millimetres and apertured with holes with a diameter of 6 millimetres, disposed in accordance with a square mesh with a side of lj.0 millimetres, corresponding to a perforation percentage p of 1.8%, that region being connected by means of a truncated cone to - a third cylindrical region with a height of 1 metre and a diameter 0^ of 0.97 m, provided with 17 plates disposed at spacings of I4.0 millimetres, and apertured with holes 9 millimetres in diameter, in accordance with a square mesh with a side of lj.0 millimetres, corresponding to a perforation percentage of h°/o.

The third region was extended by a cylindrical portion of the same diameter and with a height of 0.5 m, which was provided with a pneumatic pulsating means and ah intake duct for the washing water. The third region terminated in a cone portion communicating with the underflow orifice.

The column was fed by way of the first cylindrical region O v/ith 11.3 m /hour of an aqueous suspension containing 6 tonnes of trihydrate of aluminium.

At the same time, the column received in its lower cylindrical region, 3-1 m^/hour of washing water, of which 1.6 tit was transferred towards the overflow. The total pulsation flow in the two directions represented li^.9 m^/h. — 4 ■%.- 201745 -j*r- 3 2.3 m /h of a suspension containing 1.9 tonne of solids was extracted from the column, by way of the underflow arrangement.

The respective relative densities of the liquid and solid phases were as follows: column overflow: do = 1.09 washing water: dn = 1 solid: ds = 2.4.

The ratio in respect of the concentration of dissolved salts in the liquid feed and underflow phases was only 103, that being a result which should have been achieved with a theoretical number of plates n of 15.5, whereas 27 plates were installed.

At the same time as the washing operation, the grains of aluminium hydrate were classified.

The degree of efficiency of the classification operation was defined by the method using the 'partition curve' method which gave a d^Q, diameter of particles having a probability of 0.5 of discharge at the underflow, equal to 73 microns, and a partition curve imperfection ('imperfection index'): ^75 ~ ^25 1 = = °-l5 50 The above-indicated column design and operating data gave a coefficient of 10.

EXAMPLE_3 This Example illustrates the improved apparatus according to the invention, which was used to wash and classify trihydrate of aluminium in suspension in a sodic liquor with a relative density of 1.09, at a temperature of 52°C, with a mean S V"»» — p (20fFiEBt8a& 'W J7. 2 017 45 granulometry of 60-65 p, using a pilot column designed in accordance with the teachings of the invention.

The column was of a cylindrical-conical type, with an overall height of I4. metres, comprising, in a downward direction: a first cylindrical collecting and overflow region with a diameter 0^ = 1.60m, connected by means of a truncated cone to - a second cylindrical region with a height of 0.5 metre and a diameter = 1.07 m, containing 10 plates disposed at spacings of forty millimetres, and apertured with holes 5 millimetres in diameter, disposed in accordance with a square mesh with a side of I).0 millimetres, corresponding to a perforation percentage p = 1.2%, said region being connected by means of a truncated cone to - a third cylindrical region with a height of 1 metre and a diameter.0 of 0.97 m, provided with 17 plates disposed at spacings of I4.O millimetres and apertured. with holes 6.5 millimetres.in diameter, in a square mesh with a side of 1+0 millimetres, fcorresponding to a perforation percentage of 2.1%. . "" ,7V .... .. " The third region was extended by. a cylindrical portion of the same diameter and with a height of 0.5 m, provided with a pneumatic pulsating means and an intake duct, for the washing water. The third region terminated with a cone communicating with the underflow orifice. „ ...

The column was fed by means of the first cylindrical region, with 11 m^/h of an aqueous suspension "containing 5.8 tonnes of trihydrate of aluminium. .

At the same time, in its lower cylindrical region, the column received 2.7 m^/h of washing water, of which 1.1 m^/h was transferred towards the overflow. The total pulsation flow O in the two directions represented llj.,1 m /h. 201745 2.L ra^/h of a suspension containing 2 tonnes of solid matter was extracted from the column by way of the underflow.

The respective relative densities of the liquid and solid phases were as follows: column overflow: • do = 1.09 v/ashing water: dn = 1 solid matter: ds = 2,1}.

The ratio in respect of the concentration of dissolved salts in the liquid feed and underflow phases was only 220, that being a result which should have been achieved with a theoretical number of plates n of 23, whereas 27 plates were installed.

At the same time as the v/ashing operation, the grains of aluminium hydrate were classified.

The level of efficiency of the classification operation was defined by the method using the 'partition curve', which gave a d^. diameter of particles having a probability of 0.5 of discharge at the underflow, equal to 75 microns, and apartition curve imperfection ('imperfection index'): cL7? — dp? 1 = 2d„ = 0-1* 5o The above-indicated column design.and operating data resulted in a coefficient of 30.

'V* ' /v / |K201BEBS»: -19- -201745 - }*-

Claims (10)

WHAT WE CLAIM IS

1. An apparatus for permitting operations of washing solid materials in suspension in a liquid phase to remove the solubilised materials therein and/or to effect operations of precise granulometric separation of solid materials in suspension in said phase comprising a vertical column in which horizontal perforated plates are disposed, means for the feed of suspension to be treated and treatment liquid and for extraction of the treated suspension, and a pulsating means, characterised in that, for a column comprising H actually installed perforated plates, each having a surface area S and a perforation coefficient p, being the ratio between the total area of the perforations and the surface area of said plate, the pulsating means supplying the sum V of the volumes which are transferred alternately upwardly and downwardly per unit of time, the liquid serving for the washing operation or for granulometric separation being of a relative density dn while the liquid issuing by overflow action is of a relative density do, and the solid materials in a state ox suspension are of a relative density ds, g being acceleration due to gravity, the diameter 0 of the perforations and the mean distance 1 between said perforations are so selected that a coefficient associating the technical characteristics of said column, as defined by the relationship: - W N dn do K-^ = }_pS] .. l.g do - dn . ds is adjusted to a value which is at least equal to 10 and which is preferably between 20 and 300.

2. An apparatus according to claim 1 characterised in that, when the relative density dn of the washing liquid ana the relative density do of the liquid issuing in the overflow mode have a relative difference of less than 2%,. * :N -2o- 201745 there is associated with the coefficient K-^, a coefficient which is defined by the relationship: K - 2- £ 2 ~ pS * v wherein V is the kinematic viscosity of the liquor -at the level of a plate, which is set to a value of at least 100 and preferably between 300 and 5000.

3. An apparatus according to claim 1 or claim 2 characterised in that the perforation coefficient p, being the ratio between the total area of the perforations of a plate and the area of the plate, is between the limits of 0.001 and 0.25 and preferably between 0.005 and 0.1. k.

An apparatus according to claims 1 and 2 characterised in that the N perforated pistes which are actually installed in the column are installed in a region of cylindrical shape in said column.

5. An apparatus according to claims 1 and 2 characterised in that the N perforated plates which are actually installed in the column are installed in a region of hyperbolic shape in said column.,

6. An apparatus according to claims 1 and 2 characterised in that the N perforated plates which are actually installed in the column are installed in a region formed by the combination of a plurality of coaxial cylindrical-conical shapes of said column.

7. An apparatus according to claim 6 characterised in that the region formed by the combination of a plurality of coaxial cylindrical-conical shapes, of said column is the outside 2 3) ■ ... ! -2.1- 20174$ envelops of a hyperbolic shape.

8. An apparatus according to claim 5 or claim 7 characterised in that, with \> being the kinematic viscosity of the liquid phase at the level of a plate having a surface area S, the ratio S/v is constant over the entire height of the region of hyperbolic shape.

9. An apparatus according to claim 1, substantially as hereinbefore described with particular reference to any one of the accompanying Figures 1 to 4.

10. An apparatus according to claim 1, substantially as hereinbefore described with particular reference to either of the foregoing Examples 2 and 3. ***** Dated this day of February 1986 A J PARK & SON Agents for the Applicant