HK1018231B - Method for separating mixture of finely divided minerals - Google Patents

Description

Method for separating a finely divided mineral mixture

USSN08/350913 relates to a method for separating a finely divided mineral mixture into its specific components. More particularly, the invention relates to a method of performing such separation by a novel selective coagulation technique in which a dispersed aqueous slurry containing a mixture of minerals is selectively flocculated with an anionic polymer by adding a fatty acid, such as oleic acid, and a polyvalent metal cation, such as calcium chloride. The fatty acid and polyvalent metal cation selectively coat an ingredient in the composition, which ingredient subsequently flocculates with the anionic polymer. Preferred for use in the present process are dispersants for anionic polymeric salts. The invention is particularly useful for separating colored impurities from kaolin clay.

The advantages of this process can be enhanced according to the invention by treating the dispersed slurry with soda ash.

When the particles in the mineral or powder mixture are large enough, e.g., greater than 325 mesh, the components of the mixture can be separated by physical means such as air or magnetic separation processes alone. When the particles in the composition are fine, more sophisticated techniques are required for effective separation. The conventional approach is to separate finely divided minerals, such as particles finer than 325 mesh (U.S. sieve), by forming the mixture into an aqueous slurry and adding chemicals thereto that cause the desired separation. Another widely used method is froth flotation. In froth flotation of phosphate or oxidized minerals from siliceous gangue, it is conventional practice to employ a fatty acid scavenger and a salt promoter. The mineral particles coated with the capture agent are separated from the gangue in the form of foam. Foaming agents are generally employed and aerated. When froth flotation is used on very finely divided (muddy) minerals such as certain kaolin clays, froth flotation of colored impurities in the clay using fatty acid scavengers becomes difficult and requires the use of clay dispersants to keep the clay particles dispersed during the froth flotation process. The flotation advantages of slurried minerals, particularly colored titaniferous impurities from fine particle size kaolin, are described in Green et al, U.S. patent 2990958. This process is commonly referred to in the art as super flotation. Super flotation has been carried out on a large scale for decades to upgrade kaolin clay. The process has been extended to other industrially valuable minerals such as cassiterite (tin oxide), phosphate slimes, fluorides and other sulfide-free minerals. Another commercial kaolin flotation process is known as TREP, which uses calcium chloride and oleic acid, see Bacon et al, U.S. patent 4472271. In the case of kaolin clay containing a large amount of mud. Conventional froth flotation techniques are not capable of removing colored materials.

The "selective flocculation" is another process widely used in industry for the separation of finely divided minerals and powders. For clays, certain processes employ anionic polymers to selectively flocculate the clay, leaving dispersed impurities and subsequent separation. Several industrial variations of selective flocculation employ weakly anionic polymers, such as hydrolyzed polyacrylamide, to selectively flocculate impurities in the clay, leaving behind dispersed purified clay. See, for example, U.S. patent 3837482 to Sherida, U.S. patents 3701417 and 3862027 to Mercade, U.S. patent 3371988 to Maynald et al, and U.S. patent 4604369 to Shi.

Earlier in the history of froth flotation, it has been proposed to add an anionic polymeric flocculant to a mineral slurry that has been treated with a fatty acid scavenger. Froth flotation was then carried out for separation, see U.S. Pat. No. 3138550 to Woolery.

In order to selectively adsorb flocculants on specific components of a mixture, various methods are proposed in the following documents: (Yu and Attia, "Flocculation in Biotechnology and separation Systems," (edited by Y.A. Attia), p.601, Elsevier, Amsterdam, 1987,; Behl, S. and Moudgil, B.M., mineral and metallurgical processes, 5, 92, 1992 and, Behl, S. and Moudgil, B.M., journal of colloid and interface science (U.S., 160, 1993). One such method involves selectively blocking active sites on inert or non-flocculating constituents to adsorb the polymeric flocculant. This may be achieved by adsorbing the low molecular weight fraction of the flocculant as a dispersant and/or a site blocking agent prior to exposing the particle surface to the flocculant.

Both froth flotation and selective flocculation processes have drawbacks, particularly when used on slurry minerals. When froth flotation is used to froth kaolin clays where the majority of the material is in the submicron size range, even super flotation does not achieve a technologically acceptable level of separation of the colored materials in the kaolinite based on achieving a suitable recovery of pure kaolin. Similar difficulties arise when GREP is employed to process ultrafine clays on an industrial scale. Selective flocculation processes using anionic polymers on the other hand typically result in flocs with very low settling rates unless large amounts of salt are used to promote floc settling. This requires more money to perform multiple washing steps since the presence of salt with the clay will adversely affect the rheology of the clay.

USSN08/350913 relates to a novel method for separating a mixture of finely divided solids, which is significantly different from known froth flotation and selective flocculation processes. This method overcomes many of the drawbacks of the prior art froth flotation and selective flocculation separation methods and, when used with kaolin clays, provides a means for producing a novel kaolin pigment product. The method employs selective flocculation of constituents in a previously dispersed aqueous slurry of mineral, preferably a slurry dispersed with sodium silicate and sodium polyacrylate. The slurry is dispersed such that the particles do not agglomerate with each other. The dispersion slurry is pretreated to effect selective flocculation by the addition of a fatty acid and a water soluble source of polyvalent metal cations. The amount of fatty acid and polyvalent metal cation is insufficient to flocculate the constituents in the dispersion slurry. When the anionic polymer is added to the pretreated dispersed slurry, a dense flocculated phase is formed immediately and settles rapidly as a dense viscous colloidal bottom layer; the upper layer is a dispersion slurry containing non-flocculated mineral particles. The flocculated phase also contains all of the fatty acids and multivalent cations added to the slurry. The lower density gelatinous layer can be easily separated from the rest of the slurry by decanting or other conventional unit operations. The slurry is not froth flotation after introduction of fatty acid and polymer as described in Woolery (above); nor is froth flotation used to separate the low flocculated phase from the upper dispersed phase.

After mixing with the mixture of sodium silicate and sodium polyacrylate, the raw material may be treated with soda ash, or may be mixed with a mixture of three components of soda ash, sodium silicate and sodium polyacrylate. In both cases, soda ash must be added prior to the treatment step. It can be seen that a product with good brightness can be obtained with soda ash in the process.

The addition of soda ash to the pre-treatment step changes the properties of the flocs (flocs formed upon addition of the flocculant). The size of the flocs has a significant impact on the process. Small, dense flocs are desirable because they contain small amounts of clay or dispersed slurry, thereby helping to improve recovery. Increasing the amount of soda ash will form a more dense floc. On the other hand, the size of the flocs is controlled by the addition amount of sodium polyacrylate. However, the addition of excess soda ash can coagulate the clay, thereby affecting the treatment step. In addition, soda ash is known to be a buffer, so its addition will help to control dispersion, especially in highly alkaline solutions (common in caustic/silicate solutions). Solutions with high pH will adversely affect the properties of the flocs used for selective flocculation, as they will cause the suspension to be excessively dispersed.

In a preferred embodiment, the invention is practiced with an impure kaolin clay containing dispersed particles of at least one colored titanium-containing impurity and kaolin clay and the impurities are so fine that they do not react satisfactorily to conventional froth flotation processes, such as superflotation or TREP. According to the invention, the dispersant used for purifying this kaolin is preferably sodium silicate supplemented with sodium polyacrylate and soda ash. Examples of such ultrafine kaolins are mined in east georgia in the united states; the clay has an average particle size below 0.5 microns and may be treated by selective flocculation with a weak anionic polymer, followed by the addition of large amounts of salt to encourage floc settling and multiple washing steps.

We believe that the present invention has significant breakthroughs in the handling of very finely divided mineral mixtures, which is a significant economic benefit over the currently used techniques. For example, a kaolin clay product having a very high brightness (90% GE brightness or higher) can be obtained without froth flotation. In some cases, high brightness kaolin products can be obtained without the need for conventional post-treatment processes to increase brightness, such as bleaching and magnetic separation. This is illustrated by the fact that our process allows for significant reduction in the amount of colored impurities, thereby producing a kaolin clay product having the desired brightness without the need for conventional post-treatment procedures. In some cases, the initial coarse removal step (which is necessary in most kaolin treatment processes) can be eliminated because coarse particles can be removed in the settled flocculated impurity layer. The process of the present invention does not incorporate insoluble salts that are introduced in prior selective flocculation processes. Because the step of washing for many times is saved, the cost of the kaolin treatment process is greatly reduced. In fact, polyvalent metal cations present in the kaolin clay raw material or introduced during the treatment process can be substantially quantitatively removed from the flocculated layer and thus do not adversely affect the rheological properties of the pure kaolin clay. The treated kaolin product has significantly better rheological properties.

The present process can remove titanium oxide (rutile and anatase and mixtures thereof) from kaolin, even when the titanium oxide and kaolin are in very fine particulate form. The process may also separate other non-sulphide minerals from other silicates. It can be used to separate certain iron-containing sulphides, such as pyrite. The method of the invention can be used to remove apatite (calcium phosphate) from silicate minerals in phosphate minerals, concentrates and pre-selected ores, even when the added material is sludge-like. The process can be used to separate muddy cassiterites (tin oxides), iron oxides, wollastonite, alkaline earth metal carbonates such as dolomite, limestone and magnesite from silicate gangue in ores, concentrates and pre-concentrates. Naturally occurring zeolites containing alkaline earth metal ions, such as chabazite, can be separated from the silicate gangue. Examples of minerals present in various silicate gangue are feldspar, montmorillonite clay, fine quartz, phosphorus clay and kaolin. The silicate species remain in the dispersed phase during these separations. The process of the invention can also be used for the beneficiation of ilmenite, nickel ore, anatase and bauxite. Generally, any agent-coated mineral that can be selectively froth floated by the combination of fatty acids and multivalent cation promoters can be separated as a colloidal flocculated lower layer using the process of the present invention.

The method is suitable for separating all or most of the material finer than 325 mesh (U.S. sieve). Grit is defined as +325 mesh (U.S. sieve) particles, i.e., particles remaining on the 325 mesh screen can be removed from the feed prior to or during the process. The invention is of most significant commercial value in the separation of ultrafine minerals, such as minerals in which at least 50% by weight of the particles are in the submicron size range. The use of this method for such fine mineral mixes represents the most significant cost reduction that can be achieved. This can be illustrated by the fact that costly pre-or post-treatment conventional steps can be dispensed with or they can be carried out more simply.

The present invention for treating ultrafine kaolin from the united states of east georgia will be described in more detail below. The colored impurities are mainly titanium oxide (rutile and anatase). Typical titanium oxide (TiO)₂) The analytical amount was 2.0 to 4.5% by weight based on the dry weight of the mined coarse-grained clay removed. However, brightness was already obtained with clay raw materials having an analytical content of titanium oxide of 0.6 to 6.0%To accept the improvement. Typically, a portion of the iron is located in the structural lattice of the kaolin crystals. The iron is present in small amounts, for example 1.0% by weight or less, based on the dry weight of the coarse-grained clay removed. These clays have a low reactivity towards oxidative or reductive bleaching and do not give a satisfactory response to the known flotation processes.

Typical particle sizes of georgia clay vary from 80% finer than 2 microns to over 95% finer than 2 microns e.s.d. (equivalent spherical diameter). At least 50% by weight of the e.s.d. is generally finer than 0.4 micron. These clays therefore fall within the general definition of a slimy mineral, as used in the froth flotation technique.

Eastern georgia clays are becoming increasingly important to the paper industry due to their outstanding high shear rheology and as co-pigments matched with carbonates. The removal of titanium dioxide impurities improves the brightness and dark color (less yellow) of the clay to produce a more matched carbonate co-pigment.

The preferred primary dispersant for use in the practice of the present invention is sodium silicate. We have found that a composition obtained by combining sodium hydroxide with a sodium silicate solution, such as sodium N  brand sodium silicate, having the same ratio of sodium oxide to silica as sodium silicate, does not lead to the same results as those obtained by removing a large amount of titanium dioxide from east georgia kaolin with sodium silicate.

The sodium silicate primary dispersant may be added as a dry material or as a solution in water. When added as a solution, the concentration of silicate is not critical. The procatalyst is added to the clay containing 5 to 70% solids, preferably 50% or more solids, based on the weight of the dry clay, using 3 to 9 pounds per ton, preferably 6 pounds or more per ton of sodium silicate (dry weight). Excess sodium silicate will coagulate the suspension, which will adversely affect the selective flocculation process. When the amount is insufficient, the slurry will not disperse, which will adversely affect the selective adsorption of the flocculant.

A water-soluble sodium or ammonium polyacrylate dispersant, such as C-211 sodium polyacrylate, may be added to the slurry previously dispersed with 0.1 to 0.8 pounds per ton of sodium silicate (based on the dry weight of the clay) to ensure that the clay is in a dispersed state throughout the operation. Typical molecular weights of polyacrylate dispersants may be between 2000 and 20000. The acrylate dispersant is necessary for high recovery of pure clay. A preferred viscosity of a suitably dispersed slurry for the purposes of the present invention is 600CPS or less at 20 revolutions per minute, as measured using a spindle number 2 in a Brookfield viscometer. The pH of the kaolin slurry before the sodium silicate is added is typically 5-7. After the sodium silicate is added, the pH is typically 7-11, and sodium or ammonium polyacrylate generally has no effect on the pH of the slurry.

After the addition of the primary dispersant and the acrylate (co-dispersant), the dispersed kaolin slurry is a thin fluid that has the appearance of an emulsion. Substantially no stratification or flocculation occurs when resting. As previously mentioned, the slurry is dispersed so that its particles do not agglomerate. The degree of dispersion may not be the same as that of the slurry dispersed to the minimum viscosity (i.e., rheologically dispersed slurry).

In accordance with the present invention, the treated slurry is treated with 1 to 10 pounds soda per ton. All weights are on a dry weight basis. The same amount of soda ash can be used in the primary dispersion stage as sodium silicate or sodium polyacrylate dispersant or as a mixture of both. The soda ash can be added in the form of dry materials or in the form of solution. By using soda as a "buffer", the original clay can be effectively mixed in fresh water or silicate-rich reclaimed mineral water, see example 4.

The fatty acids used in the process to pretreat the impure clay (or other added material) for selective flocculation may be of the type commonly used in oxide mineral froth flotation, for example C12-18 fatty acids. Oleic acid is preferred. Mixtures of fatty acids and resin acids may be employed, for example mixtures of tallow fatty acid and sulfonated fatty acid. The amount of fatty acid may vary with the level of impurities in the kaolin (or the relative amounts of non-silicate minerals in the other minerals that may be covered with sulfuric acid and multivalent cations) and is typically in the range of 1 to 10 pounds, most commonly 3 to 5 pounds per ton (based on dry clay). When too much fatty acid is used, a film (or separate phases) is seen on the surface of the slurry, which film contains fine colored lumps, thus preventing them from settling after flocculation, and when the amount of fatty acid is insufficient, the separation of the process is not very good. The addition of a foaming agent is not beneficial.

The salt comprising the polyvalent metal cation may be added to the slurry at the same time as or prior to the addition of the fatty acid. When treating a mineral ore, a preselected mineral ore, or a beneficiated mineral ore that contains solids that provide multivalent cations in the slurry, it may not be necessary to add any other source of multivalent cations. Suitable salts containing multivalent cations are soluble in water at the pH of the slurry to which the salt is added. Particularly preferred are salts containing divalent metal cations, particularly calcium, magnesium and barium. Other polyvalent metal cations that may be used include aluminum, iron, tin, titanium, manganese, and rare earth elements. When treating clays, preferably colorless cations such as calcium and magnesium should be used. The anion in the preferred salt is chloride, although nitrates, sulfates, acetates or formates may also be employed. The salt may be added as a dry material or as an aqueous solution, typically in an amount of about 0 to 4 lbs/ton, most preferably 2 lbs/ton of dry clay. When an excess of salt is used, non-selective flocculation of the unsuitable slurry occurs, which interferes with the ability of the polymer to selectively flocculate the titanium oxide. In addition, excess salt (relative to fatty acids) requires one or more rinsing steps, which adds significant cost to the process. When no salt is added, the flocs formed are very small and can adversely affect the separation process.

The flocculating agent used in the process is highly anionic, being a homopolymer or copolymer of vinyl acrylic acid, carboxylic acid anhydride and carboxylate monomers with a suitable nonionic monomer. Examples of nonionic monomers are acrylic carboxylic acid amides and carboxyalkyl esters. Acrylic acid (or salts thereof) and acrylamide and copolymers are preferred for kaolin treatment. Since the polymer is highly anionic, it consists mainly of acidic propylene groups.

The flocculants successfully used in this process are highly anionic high molecular copolymers of sodium acrylate and acrylamide, which have 50% by weight of acrylate groups and a molecular weight of over 5 million. Preferred polymers have 95% or more acrylate in the copolymer and a molecular weight of 10 to 30 million, preferably 25 million. The polymers used in the accompanying examples were obtained from Sharpe Specialty Chemical Co. and include Sharpflor^TM9990, 9993, 9950, 9954 and 8581. The production process of these polymers is application specific. Theoretically, they can be prepared by copolymerization of acrylamide and acrylic acid (anionic monomer) or by partial hydrolysis of polyacrylamide.

The fatty acids and salts are typically added to a previously dispersed slurry containing 10% to 50% solids. Minimal dilution occurs when these agents are added, so the solids content of the slurry is essentially unchanged. The pH of the slurry is typically 6.5-10 after addition of the fatty acid and salt.

The solids in the slurry after addition of the fatty acid and salt are typically 20 to 45, with about 40% being preferred. After the fatty acid and salt are added but before the polymer is added, the slurry may be diluted with water, preferably with a lower mineral content.

The polymer is added as a solution having a concentration (by weight) of less than 0.5%. At higher concentrations, the flocculated material can clump due to mixing limitations. At very low concentrations, the amount of water added can be very large causing operational problems. In preparing the polymer solution, water having a lower calcium and magnesium content must be employed. The agitation must be sufficient to avoid degradation of the polymer while it is soluble in water.

Floc production was seen immediately after the solution of the polymer was added to the well-dispersed slurry pretreated with fatty acid and metal. It is not necessary to agitate the contents of the vessel to form floes. However, even severe agitation does not affect floc formation. After a few minutes of standing under quiet or semi-quiet conditions, the flocs settle as a sticky gelatinous bottom layer containing mostly all of the titanium-containing minerals in the starting clay. For DongGeya kaolin, the iron content of the clay remains essentially unchanged. But for kaolin containing free iron minerals, the iron is concentrated in the flocs. Unless the clay has had its coarse particles removed prior to treatment, the coarse particles will remain in the flocculated layer when the kaolin material is treated. The lower settled layer will generally be dark brown and significantly darker in color than the upper dispersed layer containing pure clay. The water in the slurry is mostly present in the upper clay-rich layer.

After the polymer is added, the fluid dispersion of the treated kaolin product is decanted in a drum tank, column, or the like, with the lower fluid containing the gelled material comprising large particles greater than 5 microns, impurities including colored materials, and some other minerals. Mechanical devices such as a doctor blade box or low shear centrifuge can also be used to separate the gelled floes from the dispersed product.

The latter part of the decantation process can have multiple opportunities to optimize the overall process yield and reduce the amount of residual impurities remaining in the dispersed phase. This will have an impact on the quality of the product being processed and the overall manufacturing cost of such products.

Due to the high viscosity imparted to the kaolin slurry by the addition of the flocculant, very small (even gel) flocs will remain suspended in the dispersed treated kaolin product. These flocs have a structure containing the fatty acids as impurities and do not settle after the initial addition of the polymer. These small flocs can be dispersed by adding a suitable dispersing agent such as C-211 (sodium polyacrylate).

Another method of treating this small amount of floe is to leave the small floe on a screen when operating in a batch mode. When the process is carried out in a continuous manner, clogging of the screen becomes a serious problem if the screen surface is not cleaned by a reagent which can cause floc deposition, which may be high pressure water or a reagent.

Further improvements in the purity, physical properties and brightness of the treated and kaolin products can be obtained by means of HGMS (high order ladder magnetic separator) having a magnetic field strength of greater than 2 tesla, preferably less than 5 tesla. Impurities additionally located in the mineral pore structure can be removed by a "scouring and grinding" process step prior to HGMS. The unit operation does not cause a significant change in the particle distribution of the slurry. This process allows release of entrapped impurities that were not removed by the process of the present invention initially performed.

Further brightness enhancement can be achieved using conventional reductive bleaching. Dithionite chemicals may be employed or formed in situ and are disclosed in US5145814 to Willis et al. Oxidative bleaching is advantageous when treating clays contaminated with organic impurities.

The method of the present invention can be used to reduce the level of colored impurities in kaolin clay materials that have been partially purified by, for example, froth flotation.

In laboratory tests, a simple helical agitator can be used in all process stages. Either batch or continuous operation may be used. In continuous operation, a squirrel cage mixer may be used to mix the dispersed slurry after the fatty acid and salt are added.

The following examples are intended to illustrate the most preferred mode of operation of the invention and are not intended to limit the invention thereto. In examples 1-5, the kaolin starting material used was from east georgia, usa. The typical particle size distribution is 80% by weight finer than 2 microns, with an average particle size of 0.3 to 0.4 microns.

All amounts are on a dry weight basis unless otherwise indicated. All screen sizes were obtained with U.S. sieves.

Example 1

According to the teachings of USSN08/350913, a DongGeorgia kaolin clay stock was mixed at 60% solids with 7 lb/ton anhydrous sodium silicate and 0.6 lb/ton C-211 (sodium polyacrylate) using a Cowles mixer. The slurry was mixed for 15 minutes to completely reduce the coarse clay. The slurry was sieved through a 325 mesh sieve to remove coarse particulate matter. The resulting slurry was diluted to 40% solids. The pH of the slurry was 10.4. To this slurry, 5 lb/ton oleic acid and 2 lb/ton calcium chloride solution (38.5%) were added simultaneously while stirring. The obtained slurry was completely stirred at room temperature for 15 minutes. While stirring moderately, 0.25 lb/ton Sharpfloc was added to the slurry^TM9950 polymer. Sharpflor^TM9950 is a copolymer of polyacrylamide and polyacrylate having a negative charge of 95% and a molecular weight of over 10 million. The desired polymer is diluted to such a concentration that when added to the slurry, the amount of solids obtained is 20%. Colored flocs immediately appear. When the agitation is stopped, the flocs begin to settle quickly. The floc was allowed to settle for 30 minutes. The flocculating phase (brown gelled phase) represents about 30% of the slurry volume. The dispersed slurry was decanted, separated from the flocculation layer and passed through a 325 mesh screen to remove any small floes that remained with the pure kaolin slurry. The solids content of the decanted slurry was 10%. The slurry was then flocculated with alum and sulfuric acid and filtered in a Buchner funnel. The filter cake was dried in a microwave oven.

The results obtained show that TiO in DongGezhia kaolin₂The content is reduced from 4% to about 0.6%. The GE brightness increased from 80.0% to 90.4% with a clay recovery of 73%.

Example 2

According to the teachings of USSN08/350913, a DongGeorgia kaolin clay stock was mixed at 60% solids with 5 lb/ton anhydrous sodium silicate and 0.5 lb/ton C-211 (sodium polyacrylate) using a Cowles mixer. The slurry was mixed for 15 minutesSo that the coarse clay becomes completely small. The slurry was sieved through a 325 mesh sieve to remove coarse particulate matter. The resulting slurry was diluted to 40% solids. To this slurry was added 2 lb/ton calcium chloride solution (38.5%) followed by 3 lb/ton oleic acid. The obtained slurry was completely stirred at room temperature for 15 minutes. While stirring moderately, 0.33 lb/ton Sharpfloc was added to the slurry^TM9950 polymer. Sharpflor^TM9950 is a copolymer of polyacrylamide and polyacrylate having a negative charge of 95% and a molecular weight of over 10 million. The desired polymer is diluted to such a concentration that when added to the slurry, the amount of solids obtained is 20%. Colored flocs immediately appear. When the agitation is stopped, the flocs begin to settle quickly. The floc was allowed to settle for 30 minutes. The flocculating phase (brown gelled phase) represents about 30% of the slurry volume. The dispersed slurry was decanted, separated from the flocculation layer and passed through a 325 mesh screen to remove any small floes that remained with the pure kaolin slurry. The solids content of the decanted slurry was about 10%. The slurry was then flocculated with alum and sulfuric acid and filtered in a Buchner funnel. The filter cake was dried in a microwave oven.

The product had a GE brightness of 89.1% with a recovery of 49.1%. Example 3

According to the present invention, a Cowles mixer was used to mix the Donggoo kaolin clay raw material at 60% solids with 5 lb/ton anhydrous sodium silicate and 0.5 lb/ton C-211 (sodium polyacrylate). The slurry was mixed for 15 minutes to completely reduce the coarse clay. The slurry was sieved through a 325 mesh sieve to remove coarse particulate matter. The resulting slurry was diluted to 40% solids. The slurry was further treated with 5 lb/ton soda ash. The rest of the process is the same as in example 2. Upon addition of the flocculant, colored flocs immediately appear. When the agitation is stopped, the flocs begin to settle quickly. The floc was allowed to settle for 30 minutes. The flocculating phase (brown gelled phase) represents about 30% of the slurry volume. The dispersed slurry was decanted, separated from the flocculation layer and passed through a 325 mesh screen to remove any small floes that remained with the pure kaolin slurry. The solids content of the decanted slurry was about 10%. The slurry was then flocculated with alum and sulfuric acid and filtered in a Buchner funnel. The filter cake was dried in a microwave oven.

Comparing the results of example 2 with the results of the present invention (with soda ash addition), the results show an improvement in brightness from 89.1% to 91.6% at comparable recovery of 51.7%. This increase in brightness has significant economic benefits.

Example 4

According to the present invention, a raw material of DongGeorgia kaolin clay is mixed at 60% solids with 5 lb/ton anhydrous sodium silicate and 0.5 lb/ton C-211 (sodium polyacrylate) in circulating white water using a Cowles mixer. The recycled white water is mining waste water that includes very fine clay and certain primary dispersants, such as sodium silicate (N  brand). It is commonly referred to commercially as "chalk" water. The slurry was mixed for 15 minutes to completely reduce the coarse clay. The slurry was sieved through a 325 mesh sieve to remove coarse particulate matter. The resulting slurry was diluted to 40% solids. The slurry was further treated with various amounts of soda ash (0, 2.0, 3.0, 3.5, 3.7, 4.0, 5.0 lbs/ton). To this slurry was added 1.3 lb/ton calcium chloride solution (38.5%) followed by 5 lb/ton oleic acid. The rest of the process is the same as in example 2. Upon addition of the flocculant, colored flocs immediately appear. For slurries treated with 2.0 lb/ton or more of soda ash, the flocs begin to settle quickly when agitation is stopped. The floc was allowed to settle for 30 minutes. The dispersed slurry was decanted and separated from the flocculated layer. The dispersed slurry was flocculated with alum and sulfuric acid and filtered in a Buchner funnel. The filter cake was dried in a microwave oven.

In the test without treatment with soda ash, it can be seen that the very fine flocs do not settle within 30 minutes. Thus, the recovery of the product is very poor. However, larger flocs can be seen and better separation can be achieved when the amount of soda ash is increased, thereby resulting in higher brightness and recovery. The results in table 1 show that the separation efficiency increases when the amount of soda ash in the system increases. Table 1: soda dispersant pair selective flocculationEffect of separation efficiency of Process soda wt% GEB wt% recovery of product, # per ton of Dry Clay% TiO₂Weight 090.013.10.632.087.560.01.063.088.261.70.813.588.761.10.643.788.957.00.684.089.955.70.455.090.246.70.45

Example 5

Example 5 shows the effect of treating a pre-dispersed slurry with soda ash as compared to adding soda ash to a sodium silicate/sodium polyacrylate mixture at the point of primary dispersion. The latter is preferred because it is easy to perform in production.

Example 5A

According to the present invention, a raw material of DongGeorgia kaolin clay is mixed at 60% solids with 5 lb/ton anhydrous sodium silicate and 0.5 lb/ton C-211 (sodium polyacrylate) in circulating white water using a Cowles mixer. The slurry was mixed for 15 minutes to completely reduce the coarse clay. The slurry was sieved through a 325 mesh sieve to remove coarse particulate matter. The resulting slurry was diluted to 40% solids. The slurry was treated with 5.0 lbs/ton soda ash. The rest of the process is the same as in example 2. Upon addition of the flocculant, colored flocs immediately appear. For slurries treated with 2.0 lb/ton or more of soda ash, the flocs begin to settle quickly when agitation is stopped. The floc was allowed to settle for 30 minutes. The dispersed slurry was decanted and separated from the flocculated layer. The dispersed slurry was flocculated with alum and sulfuric acid and filtered in a Buchner funnel. The filter cake was dried in a microwave oven.

The brightness of the product obtained was 91.5% with a recovery of 52.7%.

Example 5B

Example 5A was repeated with soda ash added as a third ingredient (together with sodium silicate and sodium polyacrylate). The other procedures of this experimental procedure were the same as those of example 5A. The product obtained had a brightness of 91.6% and a recovery of 38.8% these results show that with these three components, the dispersant mixture did not affect brightness. But smaller flocs can be seen and do have a significant impact on recovery.

Example 6

Example 6 shows a particularly preferred method of soda ash pretreatment to optimize the treatment of a slurry that has been aged too long to perform the desired purification.

The east georgia kaolin clay raw material was mixed at 60% solids with 5 lb/ton anhydrous sodium silicate and 0.5 lb/ton C-211 (sodium polyacrylate) to ph9.3 in a plant mixer. The dispersed slurry is freed of coarse particles by settling in a scraper box. The resulting slurry was diluted to 40% solids. This slurry was aged for more than one month. During aging, the pH of the slurry drops below 8.0. More dispersant was added to maintain the pH at 9.2.

The slurry was treated under the same conditions as in example 2. Upon addition of the polymer, the entire slurry flocculates and does not settle. The entire sample became a viscous mass.

But when the aged portion of the slurry was treated with 5 lb/ton soda ash before treatment with calcium chloride and oleic acid as in example 2, the slurry was reactive to flocculant addition. Upon addition of the flocculant, colored flocs immediately appear. For slurries treated with 2.0 lb/ton or more of soda ash, the flocs begin to settle quickly when agitation is stopped. The floc was allowed to settle for 30 minutes. The dispersed slurry was decanted and separated from the flocculated layer. The dispersed slurry was flocculated with alum and sulfuric acid and filtered in a Buchner funnel. The filter cake was dried in a microwave oven. The product had a brightness of 91.0% with a recovery of 50%.

Claims (23)

1. A method for selectively separating finely divided mineral particles from a mixture of kaolin mineral particles, comprising:

a) forming said mixture into a dispersed aqueous slurry using sodium silicate and sodium polyacrylate as dispersing agents;

b) adding to said dispersed aqueous base slurry a fatty acid and a source of polyvalent cations, said slurry not being flocculated unless a mineral in the slurry provides a polyvalent cation;

c) introducing a high molecular weight organic anionic polymer without adding a blowing agent to said slurry, thereby forming flocs which can settle as a dense lower layer; and

d) separating said settled layer from other materials in the slurry,

the improvement comprising adding soda ash as a dispersant after step (a) and before step (c).

2. The process of claim 1 wherein said soda ash is present in an amount of 1 to 10 pounds per ton of dry kaolin clay.

3. The method of claim 2 wherein the flocculated mineral comprises colored titanium oxide.

4. The method of claim 2 wherein at least 50% by weight of the mineral particles in said slurry are in the submicron size range.

5. The process of claim 2 wherein in step (a) said dispersing agent is sodium silicate.

6. The process of claim 5 wherein sodium polyacrylate is also added in step (a).

7. The process of claim 1 wherein a sodium polyacrylate dispersant is added to the settled layer from step (d) prior to the addition of additional polymer.

8. The process of claim 1 wherein sodium polyacrylate is added to the dispersion slurry from step (d).

9. The method of claim 1, wherein said fatty acid is oleic acid.

10. The method of claim 1 wherein said polyvalent metal salt is calcium chloride.

11. The method of claim 1 wherein said slurry is diluted after step (b) and before step (c).

12. The method of claim 1, wherein said polymer is a highly anionic polyacrylamide or copolymer of acrylamide.

13. The method of claim 12, wherein said polymer has a molecular weight of greater than 5 million.

14. The method of claim 12, wherein said metal oxide mineral comprises titanium oxide, said dispersant comprises sodium silicate and sodium polyacrylate, said fatty acid is oleic acid, said polyvalent metal salt is calcium chloride, and said anionic polymer is polyacrylamide.

15. The method of claim 14, wherein sodium silicate is present in an amount of about 5 to 10 lbs/ton, said sodium acrylate is present in an amount of about 0.5 to 1.0 lbs/ton, said oleic acid is present in an amount of about 2 to 8 lbs/ton, said calcium chloride is present in an amount of about 1 to 5 lbs/ton, and said anionic polymer is present in an amount of about 0.1 to 1 lbs/ton.

16. The method of claim 14 wherein said slurry in step (a) is about 60% solids and is diluted to about 40% solids prior to step (b) and further diluted to about 20% solids prior to step (c).

17. The process of claim 2 wherein said alkaline carbonate is selected from the group consisting of calcium carbonate, magnesium carbonate and magnesium/calcium carbonate and said silicate mineral comprises clay.

18. The method of claim 2, wherein said phosphate mineral is apatite and said silicate mineral comprises clay.

19. A method according to claim 1, wherein the mixture of kaolin mineral particles to be separated is a finely mineralized, eastern georgia kaolin clay raw material containing particles of colored titanium oxide impurities, the method comprising:

a) forming said raw clay into a dispersed aqueous slurry by adding sodium silicate and sodium polyacrylate;

b) adding soda ash in an amount of 1-7 pounds per ton of said kaolin clay material;

c) adding oleic acid and calcium chloride to said dispersed aqueous base slurry, said slurry not flocculating;

d) introducing anionic polyacrylamide having a high charge density without adding a foaming agent to said slurry, thereby forming flocs which settle as a dense lower layer; and

e) the settled layer is separated from the other materials in the slurry used as the purified kaolin dispersion.

20. The process of claim 19 wherein a silicate dispersant is added to the settled layer from step (e) prior to adding additional polymer.

21. The process of claim 19 wherein a sodium polyacrylate dispersant is added to the purified kaolin dispersion from step (e).

22. The process as recited in claim 19, wherein in step (a), said slurry is aged and thereafter in step (b) soda ash is added, the addition of soda ash increasing the recovery of purified kaolin.

23. The process as recited in claim 19, wherein the slurry formed in step (a) is obtained by mixing a raw kaolin clay in circulating water containing sodium silicate, and the addition of soda ash in step (b) increases the recovery of the purified kaolin.