US3544282A - Simplified pretreatment for coarse flotation - Google Patents

Description

United States Patent Int. Cl. B02c 19/12; i303b 7/00; C01d 3/08 US. Cl. 23312 10 Claims ABSTRACT OF THE DISCLOSURE The coarse flotation of sylvinite wherein comminuted flotation-size sylvinite is first deslimed in brine; the resultant suspension liberated slimes is passed over a sieve bend unit to separate smaller particles of ore and also clay; the overside from the sieve bend unit is then passed onto a vibrating screen; and the oversive from the vibrating screen is subjected to coarse flotation.

BACKGROUND OF THE INVENTION This invention relates to an improved flotation process for the treatment of potash ores, and in particular to the coarse or granular flotation of potassium chloride from sylvinite ores.

Sylvinite ores are predominantly comprised of mixtures of sylvite (KCl) and halite (NaCl), and sometimes contain other potassium salts such as, for example, carnallite (KCl, MgCl 6H O). Generally speaking, flotation processes for the treatment of these ores comprise a series of preliminary steps intended to prepare the ore for flotation stage itself. The mine run ore is first comminuted to liberate sylvite. It is then sized to the appropriate particle size for flotation and then deslimed to remove at least the major part of the extremely fine particles (slimes) resulting from the diflerent kinds of impurities contained in the ore such as, for example, silicates, sulfates etc. In most cases this slime removal is achieved by treating the ore with an aqueous solution saturated with respect to the constituents KCl and NaCl (brine), this treatment being performed under strong agitation to liberate and disperse the insoluble impurities, particularly the clays.

After the solution of liberated slimes has been removed by any convenient means such as hydroseparator, cyclone, screen or the like, the ore is suspended in a suitable quantity of clear brine. The so-formed pulp is fed to the flotation cells either with or without a separate conditioning step. In the latter case, the flotation reagents are introduced in the first flotation cell(s).

It is well known that the preparation of the ore is especially important for performing a coarse or granular flotation, i.e., a flotation wherein separation product has a large particle size, for example, larger than 0.6-0.8 mm. (28-20 mesh Tyler). In view of the lower surface to weight ratio of such coarser particles, the detrimental effect of slimes and other impurities is manifested to a greater degree than in a standard flotation, i.e., wherein the separated product has generally a particle size lower than about 0.6-0.8 mm. (28-20 mesh).

Whereas, in a standard flotation up to 10 g./l. of clay slimes can be tolerated in the brine without a significant deleterious effect, in coarse flotation, the slime content of the brine must be lowered to less than g./l. in order to obtain satisfactory results. Furthermore, when treating an ore containing relatively large quantities of magnesium salts, standard flotation can be performed with 3,544,282 Patented Dec. 1, 1970 a brine containing up to 150 and even 200 g./l. of magnesium chloride (MgCI however, in coarse flotation, 30 to 50 g./l. of MgCl are excessive for good results.

According to known techniques, standard flotation and course flotation are often other performed in two separate circuits wherein the comminuted ore is first classifiied into two fractions of different particle size. Such a technique is particularly useful for obtaining a very large particle size product, having for example, up to 2.4 or 3.36 mm. (8-6 mesh).

Conversely, the entire bulk of ore can also be floated in one single circuit, but in this case a large particle size fraction must be separated either by sieving the floated product obtained from the rougher flotation cells, or by sieving the unfloated product from the cleaner flotation step(s). The method is preferred for obtaining a granular product having a particle size not more than 1.65 mm. (10 mesh).

Whatever the technique, coarse flotation in dual or single circuits, the slimes and impurities are a problem and must be removed as completely as possible. For this purpose, different techniques have already been proposed.

A method has been described, US. Pat. 2,931,502, in which the crushed ore is dry separated into two particle size fractions, it being preferred to use vibrating screens for this purpose. Both fractions are then treated separately to liberate and disperse the insoluble impurities, and sent to separate rake classifiers to remove the solution of slimes. Each fraction is then conditioned with appropriate reagents before entering its own flotation circuit.

According to a process described in US. Pat. 3,145,163, the mine run ore is dry screened to separate: (a) a fraction to be treated by coarse flotation, (b) a fraction of fine particles and (c) an oversize fraction which after regrinding is combined with the fine fraction before its introduction into the standard flotation circuit. After being suspended in brine, the coarse-size fraction is deslimed by passing it through a classifying apparatus before introducing it into the flotation stage itself. The standard-size flotation fraction is also sent to a separate series of classifiers to eliminate the slimes before entering the flotation cells.

Another method as described in US. Pat. 2,950,007 consists of first washing the entire mass of comminuted ore in brine, then sending it to a series of cyclones. to obtain: (a) a coarse particle size fraction, and (b) a fraction composed of the slimes and fine particles of ore. These two fractions are sent separately to classifiers for desliming, and then passed separately or in combination to the flotation step itself.

Through the above-described prior art processes work to a degree, they have serious drawbacks in that they are both complicated and expensive.

In the first two cases, the whole bulk of ore must pass onto vibrating screens to separate the coarse particle fraction. It is indeed well known that these sieves, when treating dry ore to separate fractions above and below 1-2mm. (14-8 mesh), have a low capacity per hour and necessitate much supervision and maintenance, primarily because of the plugging and wearing of the cloths. Moreover, each separated fraction for each flotation circuit is treated in a separate slime removal unit, thereby resulting in a relatively complex and expensive systems.

In the third case, the entire mass of ore is treated in cyclone separators. As cyclones can only operate with 3 fication which, apart from the expense, increases the complexity of the apparatus.

SUMMARY OF THE INVENTION A principal object of the present invention is to provide an improved flotation process.

Another object is to provide an improved process for coarse flotation.

A further object is to provide an improved process for treating sylvinite ores.

A still further object is to provide a novel combination if apparatuses which is less expensive and/or easier to maintain than prior art systems for beneficiating sylvinite by coarse flotation.

Upon further study of the specification and claims other objects and advantages of the present invention will become apparent.

To attain these objects, a system is provided cimprismg:

(a) Comminuting and sizing the ore so that most, if not all, the particles have a size within the limit allowable for flotation, a substantial proportion of particles being larger than 0.6 mm. (28 mesh);

(b) Suspending the comminuted ore in brine, and desliming same by conventional methods;

Passing the deslirned suspension containing liberated slimes to a sieve bend unit;

(d) Passing the oversize from the sieve bend unit onto a vibrating screen;

(e) Collecting the oversize from the vibrating screen as the deslirned coarse particle size fraction, and

(f) Subjecting said coarse particle size fraction to flotation.

One of the main advantages of this invention is that a large proportion of the ore to be treated is separated by one single run on an inexpensive apparatus. Owing to this first separation, only a fraction of the ore to be treated is sent to the vibrating screen, this fraction representing, for example, to /2 of the. total quantity of the ore. The sieve bend unit employed requires sub stantially no maintenance owing to the absence of moving parts, and has a very low energy consumption since it is fed by gravity.

A sieve bend unit is a screening unit constructed of curved spaced bars which are transverse to the flow of the treated material which is fed as a pulp tangentially to the curved screening surface.

Exemplary of a commercially-available sieve bend unit suitable for carrying out this step of the invention is the DSM screen manufactured by the Dorr Oliver Company.

DETAILED DISCUSSION OF THE INVENTION As the first step, mine run sylvinite ore is conventionally ground (Wet or dry) to achieve a sufficient degree of liberation of the crystals of sylvite and halite for effective separation by flotation. The degree of comminution varies as some sylvinite ores are ufliciently liberated when they have a particle size up to 2.4 (8 mesh), or even up to 3.4 mm. (6 mesh), whereas others must be comminuted to less than 2 mm. (10 mesh), and sometimes much lower.

On the other hand, to effect a coarse flotation, it is, of course, important that the ore be not too finely ground. It is a general rule that at least 20-25% of the particles in the ore must be larger than 0.6 or 0.8 mm. (2-8 or 20 mesh). Moreover, if the nature of the ore permits it, the proportion of particles larger than 0.6 or 0.8 mm. (28 or 20 mesh) is preferably about 40-50% or even up to 70% In the second step, the ground sylvinite is then deslirned under conventional conditions. For example, it is vigorously agitated in a diluted brine so that substantially all the dispersible insoluble impurities are suspended therein. This can be performed in any conventional apparatus 4 known for this treatment, such as rotating drums, agitated tanks and the like.

In the third step, the entire suspension (containing the liberated slimes) coming from the desliming stage passes directly to a sieve bend unit. The usual solids content in desliming operations, for example, 30-50%-of solids, is also suitable for the operation of the sieve bend unit. Consequently, it is unnecessary to dilute the suspension after desliming by addition of a supplementary quantity of brine. This is another advantage of the system.

In the fourth step, the oversize from the sieve bend unit which is roughly drained is passed to a vibrating screen. For this purpose, it can be mixed with clear brine in order to bring the solids content down to 25-35%, for example. A more concentrated suspension, for example, containing 40-50% of solids, can also be sent to the vibrating screen, a portion of the clear brine being kept as washing medium for the solids retained on the vibrating screen. The fact that it is possible to wash the oversize on the vibrating screen without increasing the quantity of circulating brine is most beneficial since it permits the removal of the adhering slime-containing brine. The oversize from the vibrating screen, constituting the coarse fraction, can be sent directly to flotation cells without any further treatment. Moreover, it is sufficiently drained, and its solid content can reach -95%. This high solid content is also advantageous for the addition of flotation reagents in the conditioning steps before the flotation cells, for it is known that the reagent is better distributed on a pulp having a high solids content.

Any kind of vibrating screen can be used, provided the screen surface is a vibrating mass which agitates and separates the material during transit. The vibrations of such units are generally induced by electromagnetic means or by mechanical eccentricity.

The finer solids which pass through the vibrating screen can be treated separately, but they are generally combined with the fine fraction coming from the sieve bend unit. The combined fractions are then sent to one or more classifiers to remove the slimes. Any usual type of classifier can be used such as cyclones, bowl or rake classifiers and the like. Since these classifiers have to treat only part of the ore, their output and/ or number may be considerably reduced in comparison with conventional processes. The fine deslirned fraction can then be sent to the standard flotation cells with or without preliminary conditionmg. I

It is evident that the operation described above for a plant having two distinct flotation circuits can be used also in a plant having a single flotation circuit. In the latter case, the oversize from the vibrating screen and the deslirned fine fraction coming from the classifiers are recombined. It is also possible to perform the conditioning of one or both of these two fractions before combining them.

The process of the invention is particularly advantageous if the treated ore contains soluble magnesium salts in such quantity that they otherwise would interfere with the flotation and, in particular, with the coarse flotation.

The magnesium salts present in the brine pose a problem to which there is no easy economical solution. When the ore containing soluble magnesium salts is added to the brine to form a pulp, these magnesium salts (which are not present in a normal sylvinite brine) dissolve and modify the composition of the brine of the circuit. This change in composition may be such (as is the case, for example, with carnallite which increases the magnesium chloride content of the brine) that the flotation is very adversely affected. Furthermore, such difliculties are gen erally becoming more and more troublesome as the magnesium chloride content of the brine and the particle size of the treated product increase.

In known processes relating mostly to the treatment of carnallite ores, it is necessary to drawoif quantities of brine from the flotation circuits to lower the content of magnesiumchloride in these brines, such quantities being balanced by addition of clean brine free of (or containing less) magnesium salt. It has also been suggested to de-' compose the carnallite in a separate circuit and filter the obtained pulp in order to separate the brine rich in magnesium chloride, the filter cake being treated in a usual flotation circuit. In all these cases, it is ditficult and expensive to maintain the magnesium chloride content of the flotation brine at a sufliciently low value (less than 50 g. MgCl per liter of brine) so as not to impede the coarse flotation, i.e. to avoid reduction of the flotation velocity and to avoid an increase in proportion of large particles of sylvite in the tailings By using the process of the invention it is possible to treat only the coarse-particle-size fraction and to rid it of substantially all the magnesium chloride present. According to an embodiment of'the present invention, the ore suspension'corning from the desliming stage (where the decomposition of the carnallite takes place) is sent to a sieve bend unit where the larger particles are separated and partly drained. The oversize from the sieve bend unit is then passed to the vibrating screen where the retained solids canbe washed with a suitable quantity'of clean brine coming, for example, from the coarse flotation circuit. The oversize from the vibrating screen, very well drained and washed, contains substantially no more magnesium chloride due to the easy washing on the vibrating screen. In this particular case, two elements interfering with the coarse flotation (insoluble slimes and magnesium salt) are simultaneously removed.

The fine fraction of ore passing through the sieve bend unit and the vibrating screen can be treated as previously described in the case of sylvinite ore, i.e. it is substantially cleaned from insoluble impurities and sent to a standard flotation circuit separate from the coarse flotation circuit. Though the brine has a high magnesium chloride content, it can be tolerated if the quantity of magnesium salt present in the brine and brought by the ore does not exceed the limit tolerable for standard flotation.

If the quantity of magnesium salt in the brine is excessive, the operation is performed in two stages. In the first stage, the carnallite contained in the ore is decomposed with as small a quantity of brine as possible. By using an inexpensive technique such as hydraulic separation, for example, a portion of the brine very rich'in magnesium chloride is separated and discarded in order to lower the quantity of magnesium chloride present in the ore. In the second stage, the moist solid from the first stage is treated in the same manner as described in the preceding paragraph.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the specification and claims in any way whatsoever.

EXAMPLE 1 In an industrial unit treating 100 t./h. of a sylvinite ore containing 17.5% of K 0, the coarse fraction was separated by the combined use of a sieve bend unit and a vibrating screen, the efliciency of the process being controlled with respect to the particle size of the product and to the removal of the insoluble impurities.

The ore was comminuted so that 97.5% of the particles were smaller than 2.4 mm. (passing through 8 mesh Tyler sieve) and 43.6% larger than 0.8 mm. (oversize of 20 mesh Tyler sieve). The ore was then suspended in a brine saturated with respect to potassium chloride and sodium chloride, the average solid content being maintained at 45%. This suspension was passed through a series of four desliming cells, then on to a sieve bend unit having the bars spaced at a distance of 1.6 mm.

. The tests on the oversize from this sieve bend unit showed that the average solids content was 72% and that 75% of the particles had a size larger than 0.8 mm. (20 mesh). This oversize (average 38 t./h.) was taken up in a clear brine to bring the solids content down to 30%, and then sent to a vibrating screen.

The oversize from the vibrating screen (average 30 t./h.) had an average solids content of and 89.5% of the particles were retained on a sieve of 0.8 mm. (20 mesh Tyler).

Tests have shown that on entering the sieve bend unit, the suspension contained 5.88 g. insoluble clay impurities per g. dry ore. On leaving the vibrating screen, there remained only 0.145 g. clay per 100 g. dry ore, which corresponds to a clay removal of 97.5%.

EXAMPLE 2 In the unit described in Example 1, there was treated a carnallitic sylvinite ore containing 17.6% K 0 and 6% carnallite (that is 2.1% of MgCl The ore was comminuted and sized so that 97% of the particles were smaller than 2.4 mm. (8 mesh Tyler) and 45% were larger than 0.6 mm. (28 mesh). The ore was then suspended in brine and deslimed, the decomposition of the carnallite into KCl and MgCl taking place during the desliming stage. The suspension was then passed to a sieve bend unit and the oversize of this unit was sent to the vibrating screen. A continuous washing was performed on the vibrating screen with 14 m. /h. of fresh water, thereby displacing the major part of the impregnating brine rich in magnesium chloride.

Tests have shown that on leaving the desliming step,

the brine had an average MgCl content of g./l., whereasin the oversize from the vibrating screen, washed and drained, and which constituted the coarse fraction, the brine contained only 26 g./l. MgCl It is thus shown that with the process of the present nvent on, it is possible to have a coarse flotation circuit n which the magnesium chloride content of the brine is sufficiently low so as not to interfere with the flotation. It 1s, of course, necessary to avoid a brine transfer from the coarse flotation circuit to the standard flotation circuit, it, therefore, being necessary to balance the additions and losses of water in the coarse particle circuit (this being part of usual technique in flotation).

The preceding examples can be repeated with similar success by substituting the generically and specifically de- S CIIbCd reactants and operating conditions of this inventron for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this mventlon, and Without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Consequently, such changes and modifications are properly, equitably, and intended to be, within the full range of equivalence of the following claims.

What is claimed is:

1. A process for separating sylvite, which process compnses the steps of:

(a) comminuting and sizing a sylvite-containing ore so that substantially all the particles therein are of flotation-size, a substantial proportion thereof being larger than 0.6 mm.;

(b) agitating resultant comminuted ore in brine to liberate slimes;

(c) passing the brine suspension containing liberated slimes to a sieve bend unit;

(d) passing resultant oversize from said sieve bend unit onto a vibrating screen to obtain a particle size separation;

(e) collecting resultant oversize from the vibrating screen; and

(f) subjecting said oversize from the vibrating screen to flotation.

. 2 A process as defined .by claim 'l wherein said sylvitecontaining ore is sylvinite. 3. A process as defined by claiml wherein said over; size from said sieve'bend unit represents 6 to /2 of the comminuted ore.

4. A process as defined by claim 1 wherein said 'over-' size from said vibrating screen is subjected to'flot'ation in a coarse particle size flotation circuit.

5. A process as defined by claim 1 wherein saidrsylvite-j containing ore. contains carnallite, and wherein said carnallite is decomposed during said desliming.

6. A process as defined by claim 1 wherein said sylvite containing ore contains carnallite, and comprising the fur ther steps preceding the desliming step, of decomposing the carnallite with a small quantity of brine; separating resultant brine rich in magnesium chloride from the resultant wet solid; and passing the resultant wet solid to the desliming step (b).

7. A process asdefined by claim 1 wherein said sylvite: containing ore is sylvinite, said substantial proportion comprises at least 20% by weight of the ore, said oversize from the sieve bend unit constitutes to V2 of the com-..

particle size flotation. g 9 References Cited UNITED STATES PATENTS l 2,916,142 12/1959 Fontein 209-274 2,931,502 4/ 1960 Schoeld 209-164 2,950,007 8/1960 Smith 209-166 2,968,525 1/1961 Clark 23-89 X 3,008,655 11/1961 Adams 241-20 3,145,163 8/ 1964 Dewey -209-12 3,341,135 9/1967 Wilson 241-20 5 3,380,666 4/1968 Barnhill 241-20 3,446,443 5/1969 Clark ....a 209-12 ROBERT c. RIORDON, Primary Examiner D. G. KELLY, Assistant Examiner US. 01. X.R.