This invention relates to processing of solids.

This invention has particular application to beneficiation of diatomaceous earth materials and particularly although not exclusively to a process adapted to remove such contaminants as clay and for illustrative purposes reference will be made to such application.

However, it is to be understood that this invention could be used in other applications where a solid contaminated with another material may be cleaned by washing the other material from the solid. In particular, the invention may be used in washing clay from another free flowing solid, such as sand or such like.

Diatomite deposits comprise an accumulation of fossil skeletons or frustules of hydrous silica from single-celled photosynthetic plants or algae which occur in both fresh and salt waters.

Processed diatomite minerals have wide commercial use in such products as filter aids, fillers or extenders, sorption agents, thermal insulation material and the like. The commercial value of diatomite is related to its requirements such as particle size, range of particle sizes and freedom from impurities.

Diatomite is used as a filter aid in the food and beverage, petroleum, chemical and mining industries. Filter aids must be insoluble in the liquid to be filtered and otherwise chemically inert. Diatomite may be used in a filtering process as a "pre-coat" or as a "body feed" depending on the nature of the material to be filtered. The shape and size of the filter aid particles is a relevant consideration for optimised performance. Particulate materials having a wide particle size distribution make efficient filter aids in terms of the clarity of filtrate, but their tendency to pack tightly

results in very low filtration rates. Coarse particles having a regular particle shape permit high filtration rates but will allow fine impurities to remain in the filtrate. Diatomaceous earths often comprise a mixture of substantially intact frustules together with a proportion of broken frustules.

Although many thousands of species of diatoms have been classified, diatoms of salt water origin have been generally preferred as a source of filter aid materials because the major contaminant in saltwater diatom deposits tends to be sand or grit which is easily removed. In contrast, the major contaminant in fresh water deposits tends to be clay which has been difficult and costly to remove. In addition it has been considered that a large variety of particle shapes of marine diatoms gives the most efficient filter medium.

Diatomaceous filter aids may be broadly categorized as falling into one of three grades - "natural", "calcined" and "flux-calcined". The production of diatomaceous filter aid from diatomaceous ore is termed beneficiation. Natural diatomite is usually prepared by crushing the raw ore in a hammer mill and then drying the crushed material to remove substantially all moisture. The crushed material is then classified in air cyclones to remove the sand contamination and to effect a rough particle size separation. This initial processing causes undesirable breakage of frustules during the milling and classification steps.

In order to achieve an upgraded filter aid of acceptable permeability, the natural diatomite processed as above may be calcined to high temperature and subsequently milled and pneumatically classified to remove coarse agglomerates and fines.

A superior grade of filter aid is prepared by calcination of natural product in the presence of a flux such as soda ash, followed by milling and pneumatic classification. Flux calcination has the effect of reducing

the surface area of the particles, agglomerating particles and rendering various impurities insoluble.

The main disadvantages of the three processes described above are high capital cost of processing plant and waste treatment and disposal systems, high energy costs, environmental, health and safety problems associated with dust laden air and the inability of these processes to remove clay as a contaminant from the ore. The calcined and flux- calcined products also have a quality disadvantage in that they contain significant proportions of cristobalite which is suspected of causing lung disease if inhaled.

It has been discovered that many deposits of diatoms of freshwater origin are comprised substantially of the species Melos±ra Granulata which, when removed of clay impurities, provides a filter aid with superior filtration qualities.

As traditional methods of dry beneficiation were found to be substantially ineffective in removal of clay from clay contaminated deposits of diatoms, attempts have been made to separate clay by wet beneficiation systems. Hitherto, wet beneficiation systems based on hydrocyclone or settling techniques have failed. Flocculation has also been unsuccessful.

The present invention aims to alleviate one or more of the above disadvantages and to provide a method and apparatus for beneficiation of a particulate solid which will be reliable and efficient in use.

— SUMMARY OF THE INVENTION — With the foregoing in view, this invention in one aspect resides broadly in a method for the beneficiation of a contaminated particulate solid comprising particulate solid and a contaminant, the method including the steps of:- forming a feed slurry of the contaminated particulate solid; subjecting the feed slurry to conditions of shear in

such manner as to disperse at least some of the contaminant in the feed slurry to form a dispersed slurry; passing the dispersed slurry through a settler-type separator; collecting from an underflow outlet of the settler-type separator a product slurry enriched in the particulate solid, and collecting from an overflow outlet of the settler-type separator a waste slurry enriched in the contaminant. Preferably, the contaminated particulate solid is diatomaceous earth contaminated with clay, and the method includes removing the clay from the diatomaceous earth.

In another aspect this invention resides broadly in a method for the beneficiation of a diatomaceous earth contaminated with clay, the method including:- forming a feed slurry of the contaminated diatomaceous earth; subjecting the feed slurry to conditions of shear in such manner as to disperse at least some of the clay in the feed slurry to form a dispersed slurry; passing the dispersed slurry through a settler-type separator; collecting from an underflow outlet of the settler-type separator a product slurry enriched in the diatomaceous earth, and collecting from an overflow outlet of the settler-type separator a waste slurry enriched in the clay.

Suitably, the slurry enriched in diatomaceous earth is a slurry having particles substantially having a particle size of between 1 and 70 μm and having a reduced clay content.

The aqueous slurry may be formed by any known method, such as wet comminution of diatomite, dispersion of diatomaceous earth into water, or such like. Preferably, however, the slurry is formed by wet attriting of diatomite or diatomaceous earth in a high shear mixing apparatus.

The slurry so formed may be further treated or fed to the high shear apparatus as formed in the slurry forming process. Preferably, the slurry is treated by having particles of a size greater than 200 μm removed before passage through the settler-type separator.

The slurry is suitably formed with a solids content of between 1% and 40%, preferably in the range of 3% and 15%, but most preferably in the range of 3% and 10%.

The slurry may be subjected to conditions of shear in the presence of a clay dispersing agent, such as a surfactant, emulsifier, detergent or such like. If required the pH, REDOX or density of the slurry may be adjusted to modify the dispersion behaviour of suspended particles.

Ultrasonic energy may be employed as a means of inducing a shear force to the slurry or alternatively ultrasonic energy may be employed in addition to a mechanically induced shear force. Alternatively, the shear force may be induced by mechanical action such as by stirring or mixing, by homogenization, or by pumping or such like. Preferably the slurry is subjected to conditions of shear immediately before passage into the settler-type separator.

It is believed that the shear enhances or encourages the tendency of clay to disperse in water as platelets and/or into platelets more finely divided than in the absence of the application of the shear force.

If required the particulate product from the settler- type separator may be mixed with an aqueous diluent under high shear conditions to form a clay reduced slurry and then passed through a further settler-type separator such that particles within the clay reduced slurry are subjected to high inertial forces to separate clay particles and oversize particles therefrom. The settler-type separator may be taken to mean any separator using settling as the means of separation, such as gravity separators, centrifuge type separators, magnetic separators or such like. Preferably,

and in particular for the separation of clay from diatomaceous earth contaminated with clay, the settler-type separator is a cyclone-type separator.

The method of the invention in another aspect includes subjecting the clay contaminated slurry to a plurality of sequential beneficiating steps, including:- forming an aqueous slurry of diatomaceous earth; subjecting the slurry to conditions of shear whereby at least a portion of included clay is dispersed in the slurry; passing the slurry through a settler type separator such that particles within the slurry are subjected to high inertial forces; and collecting from an outlet of the settler-type separator particulate diatomaceous earth product having a particle size of between 1 and 70 μm and having a reduced clay content.

In another aspect, this invention resides in a particulate solid product produced from a contaminated particulate solid by a wet beneficiation process as hereinbefore described. In another aspect, this invention resides in a diatomaceous earth produced from a contaminated diatomaceous earth by a wet beneficiation process as hereinbefore described. The product diatomaceous earth produced by the process of this invention has been found to have a negligible concentration of cristobalite. The product diatomaceous earth has a cristobalite content less than 2%, and preferably less than 0.5% by weight.

In another aspect, this invention resides in an apparatus for the beneficiation of a contaminated particulate solid comprising a particulate solid and a contaminant, the apparatus including:- mixing apparatus wherein the contaminated particulate solid may be mixed with a liquid to form a feed slurry; shear means in fluid connection with the mixing

apparatus and operable to provide a dispersed slurry from the feed slurry; a settler-type separator assembly in fluid connection with the shear means; an overflow outlet for the settler-type separator, and an underflow outlet for the settler-type separator.

The settler-type separator assembly may include separation means for separating the contaminant from the particulate solid by density difference between the contaminant and the particulate solid.

Preferably, the apparatus is adapted for use for the beneficiation of contaminated diatomaceous earth suspended in water or an aqueous medium and particularly although not exclusively to a process adapted to remove such contaminants as clay.

Preferably the separation means for includes a cyclone- type separator or a plurality of individual cyclone-type separators each having respective overflow and underflow outlets and respective feed inlets in fluid connection in parallel. The cyclone-type separator may comprise for example a separator known by the trade mark MULTICLONE.

It is also preferred that the settler-type separator assembly includes a plurality of individual cyclone-type separators each having respective overflow and underflow outlets and respective feed inlets in fluid connection in parallel. Moreover, the beneficiation apparatus may include a plurality of said settler-type separator assemblies operably connected in parallel or a plurality of said settler-type separator assemblies operably connected in series.

The beneficiation apparatus may also be arranged so that each settler-type separator assembly includes a diluent junction connecting the feed inlet with a diluent inlet. Additionally, when the settler-type separators are connected in series, it is preferred that each diluent junction except

the last in the series operatively connects the overflow outlet from a downstream settler-type separator assembly with the underflow outlet of an upstream settler-type separator assembly, and the last said diluent junction in the series is operably connected to a diluent feedstock.

Alternatively, when the settler-type separators are connected in series, it is preferred that the feed inlet of each downstream settler-type separator assembly is operably connected to the underflow outlets of two upstream settler- type separator assemblies and the overflow outlet from each settler-type separator assembly is operably connected to an overflow outlet main.

~ BRIEF DESCRIPTION OF THE DRAWINGS -- In order that the invention may be more readily understood and put into practical effect, reference will now be made to the accompanying drawings which illustrate typical embodiments of the invention and wherein:-

FIG. 1 is a schematic illustration of a typical MULTICLONE hydrocyclone separator as may be used in accordance with the invention;

FIG. 2 is a process flowsheet for typical commercial beneficiation plant for diatomaceous earth materials according to the present invention, and FIG. 3 is a graph illustrating comparative performance as a filter aid between products according to the invention and commercially available products. Referring to Fig. 1, a cyclone separator assembly 10 has a bottom mounted inlet 11 which feeds a plurality of cyclone tubes 14. The fluid flowing into each cyclone tube 14 is split between an underflow 15 and an overflow 16 in each respective cyclone tube 14. The underflow 15 from each cyclone tube 14 is fed into a receiver chamber 17 having a receiver inlet 19 and a receiver outlet 18. The fluid received from each overflow 16 passes out through the outlet

12. The fluid flow in each case is in a direction indicated by each respective arrow 13.

In use, a pressurised slurry feed containing particulate matter is introduced into the inlet 11 and passes through each respective cyclone tube 14. By adjusting the fluid feed flow through the cyclone tubes 14, a particle size cut off range may be selectively adjusted to achieve a product having a particle size distribution within a desired range. It will be appreciated that the cyclone physical dimensions, such as diameter, may be selected to provide for a desired particle size diameter.

A typical hydrocyclone element employed as the cyclone tube 14 of FIG. 1 in the process according to a preferred embodiment of the invention comprises an injection moulded plastics body approximately 90 mm in length with an internal diameter of approximately 10 mm in the feed inlet region.

Hydrocyclone elements of the type described above have been found to be particularly effective in the classification of clay bearing diatomaceous earths. It is believed that the porous nature of the diatoms causes them to behave in a hydrocyclone as if they were solid particles of significantly smaller diameter. Accordingly, very fine-tuning of operational parameters of a hydrocyclone system is required to achieve a desired cut-off size for porous particles such as diatomaceous earths.

By sequential treatment of the underflow with fresh water in each of a plurality of successive cyclone separation stages the amount of clay detected in the overflow diminishes until the resultant diatomaceous product is substantially free of clay. In diatomaceous ore bodies possessing an initial clay content of around 50%, between 4 and 10 MULTICLONE stages have been found to be effective in removal of substantially all clay and to produce a very high grade product being substantially free of clay contamination.

The use of clay dispersion agents such as surfactants, pH modifiers or other agents which may affect the electrical double layer surrounding the clay particles are useful in maintaining the stability of the colloidal suspension of clay particles or at least in reducing the tendency of the clay to flocculate. It is most important that in achieving effective separation of the clay particles from diatoms according to the invention that the clay is dispersed as much, as finely and/or as evenly as possible in the slurry prior to cyclonic separation. To this end ultrasonic energisation of the slurry, mechanically induced shear and/or dispersing agents may assist in improving dispersion of the clay.

Once the clay has been substantially removed from the diatomite, the product may then be further classified into differing particle size ranges by the use of cyclonic separators operated at progressively lower pressures drops, or progressively larger diameter cyclones, to give progressively larger particle cut-off diameters. This may be achieved by using larger diameter cyclone elements or by varying operating conditions which affect particle cut-off diameter.

A particularly advantageous feature of the process according to the invention is believed to be that the diatomaceous earth product and the waste products in the form of clay and oversize particles are all handled in the form of slurries thus avoiding the environmental and/or health and safety problems and/or hazards associated with dust laden gas.

Referring to FIG. 2, which shows a typical flow chart for a commercial diatomite beneficiation process according to the invention, a stockpile 20 of diatomaceous earth is gathered from a deposit and if required may be subjected to a preliminary milling operation to break up large lumps of hard material.

The raw clay bearing diatomite from the stockpile 20 is fed via a conveyor 21 into a plurality of wet attriting vessels 22 together with clear wash water 23 to form a slurry of about 5% to 40% solids. The slurry is subjected to high shear mixing by respective rotary paddle mixers 24 operating within a wide range of angular velocities, such as for example about 600 rpm.

The slurry then passes to a centrifugal pump 25 wherein the slurry is subjected to a further high shear mixing action before being pumped to a screen 26 to conduct a preliminary separation to remove coarse lumps of ore, pebbles and other coarse contaminants greater than for example 3 mm in diameter and a middle fraction of particles greater than 150 μm in diameter. A portion of the middle fraction may be returned via a return line 26a to the wet attrition vessels 22 for further processing while the coarse material and unwanted middle fraction are directed via a discharge line 27 to a settling pond (not shown).

The screened slurry is then directed via a feed line 28 to a further centrifugal pump 29 and thence via a manifold 30 to a plurality of pumps 31 which provide a preferred inlet pressure to the MULTICLONES in the range of from 100 kPa to 700 kPa. The pumps 31 then pump the slurry to the respective feed inlets of a first bank of MULTICLONES 32, 33, 34, 35, 36, 37 shown generally at A. Depending on the grade or quality of product required, classification may be carried out by either of two process sequences shown schematically above and below the broken line 20a.

The underflow (product) from the MULTICLONES 32 and 33 and MULTICLONES 34 and 35 are respectively directed via a pump 46 to the feed inlets of two further MULTICLONES 38 and 39 in the second bank shown generally at B. The combined underflows of the MULTICLONES 38 and 39 are then directed via a centrifugal pump 40 to a MULTICLONE 41 in bank C. The underflow of the MULTICLONE 41 is then directed to a further

MULTICLONE 42 via a centrifugal pump 43 and thence to a still further MULTICLONE 44 via a pump 45. The process outlined in FIG. 2 shows five banks of units in series, however, it will be appreciated that the preferred range of banks os from 3 to 10 banks of cyclones or MULTICLONES in series.

The underflows of the MULTICLONES 36 and 37 are subjected to sequential beneficiation steps through the banks of MULTICLONES 32, 33, 34, 35, 36, 37, 38, 39, 41, 42 and 44 shown generally at A B C D and E, each of the banks A B C D and E being connected in series with the underflow from a preceding bank being used as feed for the next successive bank.

The pumps 31, 40 and 43 are medium to high head pumps (100 to 1000 kPa) which not only feed their respective MULTICLONE but also subject the slurry to high shear mixing just prior to entry into the feed port of the respective MULTICLONE. It is believed that the high shear mixing action assists in dispersion of any clay particles in the slurry to enable efficient separation. The clay containing overflow from banks A B C and D is directed via a discharge line 47 to a settling pond. If required, clear water decanted from the settling pond may be used as a source of make-up fluid for the slurry in the wet attriting tanks 22 and/or it may be used for flushing the underflow from a MULTICLONE.

The overflow from bank E, being substantially free of clay, is returned via an underflow return line 48 to the attrition tanks 22 as a source of make-up fluid.

The underflow from bank E is then directed via a settling tank feed line 49 to a settling tank 50 and the clear supernatant liquid is decanted and directed via a decant line 51 to a "clear water" dam to be used as a source of clear flushing water in the MULTICLONES.

A thickened diatomite slurry containing about 30 - 50% solids is removed from a settling tank 50 and is directed via

a thick slurry line 52 to a rotary dewatering screen 53. Dewatered diatomite is then dried and conveyed on a conveyor 54 in a drying kiln 55 before being transported by a conveyor 56 to a storage hopper 57 for subsequent packaging in a bag house 58.

Dust particles created during the drying process are pneumatically conveyed to a wet dust collector 59 and collected in a slurry. Decanted clear water may be received from a decant receive line 60 from the settling tank 50 to make up the slurry. The dust slurry is directed via a dust slurry line 61 to an line 48 and thence to the attrition tanks 22.

Referring to FIG. 3, an alternative arrangement of the MULTICLONES 36, 62, 63, 64 and 65 shown generally at A B C D and E respectively, each of the banks A B C D and E are connected in series with the overflow from a following bank being returned to the preceding bank, so that there is substantially countercurrent flow of the overflow liquid.

Microscopic examination of diatomite produced according to the invention has shown that the product comprises mainly whole diatom frustules with a relatively uniform particle size distribution.

Comparative particle size analysis tests were conducted on a Malvern laser diffraction particle size analyzer. The products compared were commercially available "natural",

"calcined" and "flux calcined" products as well as diatomite produced by the invention and raw diatomite ore before processing. The results set forth in Table 1 show that product according to the invention is far more consistent in particle distribution than for that marketed under the trade mark "CELITE".

Table 1

Diatomite Mean Size Span

Product (D50) (D90-D10) D50

Celite 577 17.7 3.25

(Natural)

Celite 512

(Calcined) 19.9 2.77

Celite 545

(flux Calcined) 43.3 2.00

Product according to invention 15.6 1.62

The expression "span" is an indication of particle size range. Smaller "span" values indicate a narrow range of particle size distribution.

Evaluation of filter and products depends very much on the filtration application - both on the equipment used and the nature of the fluid and particles being separated.

Filter aid performance is by nature a compromise between high clarity of filtrate and filtration rate.

An arbitrary test was established in which an aqueous suspension of montmorillonite clay was filtered through a laboratory test batch filter. Each batch of feed comprised a suspension of 1% by weight of clay in 250 ml of water with 5 grams of filter aid added, the results of which is shown graphically in FIG. 4.

The results demonstrate that a basic product produced by the process according to the invention, a "natural"

diatomite, may perform competitively with a high grade calcined diatomite. The processed fresh diatomaceous earth 70 is compared with flux calcined marine diatomaceous earth 71, processed diatomaceous earth 72, processed calcined marine diatomaceous earth 73, natural marine diatomaceous earth 74 and raw fresh diatomaceous earth 75, where processed refers to the process of this invention. When the basic product produced by the process of this invention is upgraded by further size classification in a MULTICLONE hydrocyclone the product may show superior performance over the high grade flux calcined diatomite.

By selective classification of beneficiated diatomite or selective combination of various classification fractions in varying quantities, it is believed that products produced by the present invention may be selectively tailored to suit particular filtration applications.

The process according to the present invention enables highly cost efficient production of beneficiated diatomite products with substantially reduced capital costs but at the same time produces a product of superior quality.

It will of course be realised that the above has been given only by way of illustrative example of the invention and that all such modifications and variations thereto as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of the invention as claimed in the following claims.