GB2198373A - Cyclones - Google Patents

Abstract

A cyclone comprises a separating portion having an inner cylindrical wall 12 and an outlet portion 20 consisting of a first frusto-conical portion extending inwardly of the cylindrical wall 12 and joined there to by a smooth curve 21 and a record frusto-conical portion extending into a tubular outlet 14 and joined to the first frusto-conical portion by a smooth curve 22. The inner surfaces of the outlet portion 20 are made of a material, e.g. ceramic plastics or lathed or ground metal, devoid of surface defects which would interfere with the flow of high density particles such as gold with a size below 44 microns. The cyclone may be used in the separation of solid/gas, solid/liquid or liquid/liquid mixtures. <IMAGE>

Description

CYCLONES This invention relates to a method of separating fine, dense minerals from lighter gangue material and to an improved cyclone for carrying out the method.

Compound water cyclones are known and conventionally have a cylindrical body having a tangential inlet and .^* compound cone section attached to the base of the cylinder r for the outlet for high density materials.

This compound cone has a long section at an angle of about 1200 to the cylinder wall, a central shorter section at about 750 and an external centrally located outlet angled at o to 200.

Most patents concerned with improving the efficiency of cyclones have concentrated on altering the geometry of the cyclones. One patent U.S. 2,827,334 proposes elimination of discontinuity in curvature to reduce wear.

Difficulty has been encountered with recovering alluvial gold and gold present as fines below 44 microns in size especially from 0.1 to 30 microns. In many cases assay of ore having such fine material has been difficult to achieve. This invention is in part predicated on the realization that minerals or native metals present in small crystalline particles as in alluvial deposits is often present in larger quantities than conventional analysis reveals. Further it has been discovered that much ultra-fine material is not recovered from the high density rejects.

To this end the present invention provides a method of concentrating high density minerals from mined ore by treating a slurry in a cyclone having a cylindrical body, and a bottom outlet comprising a compound cone having a first section extending inwardly of the cylindrical wall of the cyclone body and a second section extending from said first section into the outlet tube wherein the cyclone has one or more of the following characteristics a) a smooth curve forming the junction of the first and second sections of the compound cone; b) a smooth curve forming the junction of the first section and the wall of the cylindrical body; and c) the surface of the first and second sections being formed of a material devoid of surface defects which would interfere with the smooth flow of high density fine particles.

The material selected for the internal wall of the cyclone may be any suitably hard, smooth surface which can be formed in curved shapes. Ceramics, plastics and lathed or ground metals are all suitable materials.

In the prior art the importance of these qualities for recovering ultra fine material was not recognized.

A preferred embodiment of this invention will now be described with reference to the drawings in which figure 1 is a conventional compound water cyclone and figure 2 is a sectional view of the compound cone of the conventional cyclone and figure 3 is a sectional view of the cone section of this invention.

In the conventional cyclone as shown in figures 1 and 2, the ore containing slurry is fed into the cylindrical body 10 via the tangential inlet 11. The slurry follows a spiral path around the internal wall 12 of the body 10.

Less dense gangue flows out through the overflow discharge 13 and the higher density mineral is discharged through the underflow discharge 14. The base of the cylindrical body is a compound cone 15 terminating in the underflow discharge 14. The cone 15 comprises two sections 16 and 17. Section 16 extends at an angle of 1200 from wall 12 and section extends at an angle of 75" from section 16 towards outlet 14.

In figure 3 the references 12, 13 and 14 are the same as in figures 1 and 2 except that the lower end of overflow tube 13 may vary in height relative to outlet 14.

The base 20 of the cylindrical body 10 in this invention comprises a curved section 21 at the junction of wall 12 with the base 20 and a second curved section 22 at the junction of base 20 and the beginning of the underflow discharge 14.

In the conventional cyclone as shown in figures 1 and 2 the base section 15 is usually formed of metal, e.g.steel which is lathed to form the two angular surfaces 16 and 17. In contrast, in the present invention the base 20 is formed from suitable metal or moulded from a ceramic or synthetic plastic material to provide an ultra-smooth surface with few surface defects greater than 10 microns. A suitable synthetic plastic material is polypropylene with the necessary wear resistance capabilities. Alternatively lathed steel which is smooth and free of surface defects and which is subsequently case hardened, is also suitable.

Such a material enables the formation of the curved portions 21 and 22 which eliminates any turbulence and ensures a smooth flow of particles.

It is thought that the high density materials are sustained in a relatively fast moving layer over the surface of section 20 and the absence of sharp angular changes eliminates turbulence and maintains very small high density particles in this layer. The measured residence time of the high density materials in the cyclone is generally much lower than that of lighter gangue which is thought to be held in turbulent fluid adjacent overflow 13. By locating the end of the overflow 13 the degree of removal of lighter gangue can be varied. The optimum distance for 8 or 10 cm.

diameter cyclones is about 2.5 cm. above the contact point of overflow 13 and section 20. However clearances between 1 cm. and 5 cm. are effective.

As is conventional, the inlet tube 11 is generally 1.5 times the internal diameter of the cyclone above the junction of section 20 with wall 12. This may vary from 1.5 to 4 or 5 times and refers to the distance from the junction of section 20 and wall 12 to the top of the inlet pipe.

By using the present invention, improved recovery through underflow 14 is achieved for very fine minerals of particle sizes below 30 microns and down to 1 micron.

Because of the very smooth surface the fast moving layer of denser mineral is maintained and no upward deflections from surface imperfections occur. Further the absence of turbulence at sections 21 and 22 also ensures that fine particles are not drawn away from this layer.

It has been found with this invention that recovery of gangue through overflow 13 is usually proportional to feed concentration while recovery of denser materials through underflow 14 is proportional to the concentration of the denser materials in the feed.

By utilizing lower pulp density, e.g. as low as 1 or 20 gms. per litre, improvement in the fine material recovery is also experienced. This is thought to be due to the ease of maintaining a thinner dense material layer.

Conventionally pulp density is of the order of 100 to 250 gms. per litre.

The present invention also has application in other fields of separation. Separating sand from sewerage is particularly difficult with conventional cyclones because pulp density of sewerage is low and pressures are low.

However, by using gravity a fall of less than 2 metres in height enables sufficient pressure for the cyclone of this invention to separate the majority of sand particles.

In one application with a clearance between outlet 13 and section 20 of 35 to 40 mm for the upper outlet one pass achieves 97% of feed into concentrcte with pulp densities of 0.25 to 1.5% so that two passes renlove virtually all the sand.

The present invention is not limited to solid/liquid separation but because of its unique performance characteristics finds application in separating solid/air mixture or liquid mixtures.

Thus in the separation of particles in the treatment of dust or flue gases high separation rates of fine particles can be achieved.

Liquid mixtures and emulsions are separable using the cyclone of this invention where a difference in density exists. A greater diameter opening for the outlet orifice 14 enables the denser liquid (e.g. water in oil/water mixtures) to be collected in the underflow and enables the less dense liquid (e.g. oil) to be collected in the overflow.

In all applications of the cyclone of this invention there is no restriction as to the major carrier fluid. For reasons of cost and convenience water is the most useful.

Examples of this invention using a cyclone of 75mm internal diameter for recovering alluvial gold will now be described.

Example 1 The following is an example of the recovery obtained in a single pass using the cyclone of this invention.

Fountain Head (Northern Territory) alluvial ore was used which was assayed at 0.65 gms. of fine alluvial gold.

Using a conventional cyclone only negligible gold is recovered.

The results were : First pass Tailings (overflow) 0.31 gms. per ton Concentrate Size + 150 microns rate 43 gms. per ton - 150 to + 75 microns rate 30.7 gms. per ton - 75 microns 204.5 gms. per ton The calculated recovery was 0.37 gms. per ton or 54%.

Second pass Tailings - trace only Concentrate Size + 150 microns nil - 150 to + 75 microns 2.3 gms. per ton - 75 microns 75.6 gms. per ton The calculated recovery was 0.2 gms. per ton.

Example 2 This example compares the cyclone of this invention with a standard compound water cyclone.

The test equipment and cyclone set up were similar in each case. The cyclone(s) were mounted above feed and tailings tanks each of 300 litre capacity, connected to a centrifugal pump fitted with a variable speed drive, and outlet pressure gauge. The cyclone spigot aperture and vortex finder depth could be varied, but was set at 25 mm (one inch) above the spigot opening, for each cyclone.

Test Conditions Feed Size 100% - 250 pm Feed pulp density 2% solids w/v Feed rate 100 1/minute Test Procedure Samples of Fountainehead Tailings which were provided were screened to remove +250 pm material and then made up to slurry of 2% solids. Slurry was pumped through the cyclone(s) at the required flowrate of 100 1/minute, and the total overflow and underflow products collected over a test run of 1-2 minutes duration. The overflow product was allowed to settle, was decanted, filtered, weighed, and sampled for assay. The underflow concentrate was filtered, dried, screened on a 106 micron screen, and the +106 micron and -106 micron products weighed and submitted for assay.

Gold assays were carried out using Code MET 5/3 Fire Assay.

Results The metallurgical balances for each cyclone run are shown in Tables 1 and 2. A comparison shows that in the two tests carried out, the back calculated head grades for the samples supplied for testing were as follows: Invention Standard Cyclone Back-calculated head grade 0.45 g/t Au 0.26 g/t Au h' 'withstanding this, the results of the cyclone of this inve:zxon were better than those obtained from the standard cyclone, as shown below.

Invention Standard Cyclone Final tailing grade 0.08 g/t Au 0.19 g/t Au Final concentrate grade 24.8 g/t Au 5.2 g/t Au Upgrade ratio (conc:feed) 55:1 20:1 Recovery to u/flow (total) 84.028 29.558 Recovery to -106 pm u/flow product 58.188 24.30% NOTE: Gold grades and recoveries represent fire-assayable gold.

DISCUSSION It is noted that the tests were of a demonstration type and were conducted without variation or adjustment of operating parameters. The validity of comparison between the metallurgical results achieved may have been affected by variations in properties of the feed sample; this however does not affect the observation that the cyclone of this invention was able to recover fire-assayable gold at least as fine as 106 microns topsize by gravity concentration.

Thus, the present invention provides a unique means of improving recovery of mineral present in small particle sizes from ores and in many cases reveals a higher presence of mineral than conventional analysis reveals.

Claims (8)

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A method of concentrating high density particles from slurries comprising passing the slurry into a compound water cyclone having a cylindrical body, an inlet in a wall of the cylindrical body and a bottom outlet at the base of the cylindrical body, said bottom outlet comprising a compound cone having a first section extending inwardly of the cylindrical wall and a second section extending from said first section into an outlet tube wherein the bottom outlet has at least one of the following characteristics: a) a smooth curve forming the junction of the first and second sections of the compound cone; b) a smooth curve forming the junction of the first section and the wall of the cylindrical body; and c) the surface of the first and second sections being formed of a material devoid of surface defects which would interfere with the smooth flow of high density fine particles.

2. A method as claimed in claim 1 wherein the slurry is of native gold bearing ore and at least 50% of the gold particles of less than 100 microns in size are recovered.

3. A compound water cyclone having a cylindrical body, an inlet in a wall of the cylindrical body and a bottom outlet at the base of the cylindrical body, said bottom outlet comprising a compound cone having a first section extending inwardly of the cylindrical wall and a second section extending from said first section into an outlet tube wherein the bottom outlet has at least one of the following characteristics: a) a smooth curve forming the junction of the first and second sections of the compound cone; b) a smooth curve forming the junction of the first section and the wall of the cylindrical body; and c) the surface of the first and second sections being formed of a material devoid of surface defects which would interfere with the smooth flow of high density fine particles.

4. A cyclone as claimed in claim 3 wherein the first section is ringer than the second section and is disposed at an angle of bout 1200 to the cylinder wall and said second section is angled about 750 to the cylinder wall.

5. A cylone as claimed in claim 3 wherein the bottom section is made from a material selected from: a) ceramics b) high density or hard plastics c) lathed or polished metals

6. A method according to claim 1 substantially as hereinbefore described.

7. A method according to claim 1 substantially as hereinbefore described in the Examples.

8. A compound water cyclone substantially as hereinbefore described with respect to and as shown in Figure 3 of the accompanying drawings.