Field of the Invention

This invention relates to surface-modified powders of natural and artificial minerals based on Ca and/or Mg such as MgO, talc (3MgO SiO2-H2O), Mg(OH)2, CaCO3, CaMg(CO3)2, MgCO3, Ca(OH)2, CaSO and mixtures thereof. The invention also relates to methods for their production and their use. The invention makes it possible to alter the crystallographic nature of the particles as well as their surface charges and to form powders of improved performance and compatibility with a variety of materials, such as plastics and cellulose, and lowers the cost of their production. The present invention is particularly important in the production of powders for the paper industry, for the fertilizer industry, for the cosmetic industry, for the paint industry, for the ceramic industry, for the plastic and rubber industries, etc.

Background of the Invention

Surface Properties of Materials

The surfaces properties of materials are dependent, mainly, on their bulk properties and the environment in which they are formed. Reviews of this issue are given in many books and articles (e.g., in "The Chemical Physics of Surfaces"; S. R. Morrison; Plenum Press; New York (1990). Reference is made to Chapter 8 in the above book "The Solid/Liquid Interface" page 297, which includes explanations regarding the relationships between the surface electric charges, the bulk properties and the media properties).

Application of Surface-modified Powders

The literature is replete with patents, articles, reviews and books concerning the production and beneficiation of minerals (e.g. "Mineral Processing"; E. J. Pryor; Elsevier Publishing; Third Ed.; 1965) and their uses, especially after surface modifications, in the plastics compounding, in the paper industry, in the ceramic industry, in the paint industry, etc. (e.g. "CaCO3 Fillers - Market Trends and Developments"; J. Revert' e i Vidal; Industrial Minerals; November 1994, "Plastic Compounding - Where Mineral Meets Polymer"; M. O'Driscoll; Industrial Minerals; December 1994, "Surface Modification of Mineral Fillers"; R. Goodman; Industrial Minerals; February 1995, "Magnesium Hydroxide Flame Retardant (NHFR) for Plastics and Rubber"; O. Kalisky et al; Chimica Oggi/Chemistry Today; June 1995 and references therein). The above reviews illustrate the importance of the physical properties, and especially the surface characteristics, of fine powders that are used as fillers in a large variety of applications.

Size Reduction

Powders are intensively used in numerous applications, e.g. fillers and flame retardants in the paper and in the plastics industries, as raw materials for ceramics and cements, as constituents in cosmetics, etc.. In order to make effective use of powders, their particle size distribution should be controlled, usually reduced, and their surface properties should be compatible with those of the substrates with which these powders are to be used.

Grinding or milling of materials are common technologies for size reduction. However, they require the expenditure of high energies, especially in the sub-micron range, at which the high surface area of the particles increases the rate of their coalescence. The high cost of such operations is increased by their low productivity and by the requirement for equipment made of special materials that withstand the high attrition

and minimize the contamination ofthe final fine powders. Generally, two processes are used in the art - dry and wet grinding/milling. In order to increase the production rates of both types of processes and to afford better qualities of grinding milling, aids, such as dispersants like sodium hexametaphosphate, etc., are usually employed.

Another approach for obtaining fine powders involves their controlled recrystallization or precipitation by reacting suitable reactants.

In the cases described above it is of much importance that any slurry of fine powders be stable, namely, that the fine particles will not undergo coalescence and the solids will not separate or precipitate even after long periods of time. This can be achieved, among other methods, by imparting large electric charges to the surfaces of the fine particles.

A recent Israeli patent application IL 113283, filed April 6, 1995 by the same applicant herein, the description of which is incorporated herein by reference, discloses the use of certain carboxylic acids to improve the production of fine powders.

CaCO ₂ Powders

A typical and most important example is powdered CaCO3- Knowing the surface properties of calcite and understanding of how to modify them in order to improve its separation from other minerals and its properties as a major filler in the plastics and the paper industries, are of prime importance.

The known art allows to obtain, quite easily, negatively charged chalk powders by grinding/milling calcite or precipitating it by reacting e.g. Ca(OH)2 and CO2 in water to form PCC (precipitated calcium carbonate). In both cases the products obtained are negatively charged with small electric charges that do not prevent the coalescence ofthe fine particles and their precipitation out of the produced slurries. Moreover, the calcite

powders obtained do not attract carboxylic acid salts and cellulose, which are negatively charged.

It should be noted that in some instances the surface ofthe calcite are coated with minor impurities that turn the overall surface charge to weakly positive. However, this does not improve much the relevant properties of the calcite powders.

Operation at pH values below 7 gives rise to weakly positively charged calcite. However, operating at this pH range renders the calcite chemically unstable and yet the positive charge is not large enough to prevent the coalescence of the fine particles or to attract carboxylic acid salts and cellulose sufficiently. Operation at pH values above 7, which is chemically preferred, gives rise to negatively charged calcite particles with a small charge - (cf. "Fine Particle Processing" - Proc. International Symp. on Fine Particles Processing; Las Vegas; Feb. 24-28, 1980; S. Mori et al - "An Improved Method of Determining the Zeta Potential of Minerals Particles"; pp. 632-651; Fig 16(A) on p. 650 and "The Zero Point of Charge of Calcite" ; P. Somasundaran and G. E. Agar; J. of Colloid and Interface Science 24, 433-440 (1967)). In the latter reference the PZC (Potential of Zero Charge) of calcite is determined at pH values between 8-9 'by adsorption of suitable positively and negatively charged collectors and frothing the resulting product by known methods.

Therefore, this working pH range, below 8, is not broad enough to produce calcite of prime properties, as specified above. Specifically, the electric charges on its surface are not sufficient to prevent the flocculation of the fine particles and also the tendency of the calcite particles to adsorb negatively charged species such as cellulose and fatty acid salts onto their surfaces is quite limited or non-existent.

Obtaining highly positively charged calcite particles that do not flocculate and also strongly attract carboxylic acid salts and cellulose is, as mentioned above, of prime industrial importance.

Turning the calcite highly positively charged at the pH range above 6, especially at above 7, is also important in the beneficiation of phosphate rocks, in which the calcite can be coated, selectively, with inexpensive carboxylic acid salts and be separated from the apatite, francolite, crandalite, etc.; in the polymers and plastics industry, where the calcite is coated with fatty acids prior to its use as a filler; in the paper industry, at which the attachment of the relatively expensive cellulose to the maximum amount of the relatively inexpensive calcite may give rise to outstanding economical benefits; in the production of highly positively charged fine powders (regular and PCC) in stable and transportable slurries; in the production of highly positively charged PCC in which operation the following advantages can be achieved: a) Operating under an additional controlling parameter (temperature, pH, stirring, etc.) over the crystalline size distribution, b) Obtaining, mainly, single crystals that are already coated with the suitable reagent(s). c) Operating with Ca(OH)2 of lower purities, as the PCC surface can be coated with inexpensive fatty acid anions, e.g. tall oil, and be subjected to flotation, leaving the impurities in the liquid phase; and d) Separating PCC with much lower water content by either filtration or flotation.

Dolomite Powders

Dolomite (CaMg(CO3)2) and partially calcined dolomite at ~900°C that gives rise to

MgO CaCO3, are ideal candidates for treatment according to the present invention, as they can be subjected to fast grinding/milling by using suitable surface modification methods. The brightness of the MgO improves the overall properties of the final fine powder.

Talc Powders

This material has many commercial uses. The exact chemical composition of talc depends on its origin and the kind of processing it undergoes. However, the following common formula is usually assigned to talc - 3MgO ^' 4SiO ₂ ^' H ₂ O (cf. "Ullmann's Encyclopedia of Industrial Chemistry"; Fifth, Completely Revised Edition; B. Elvers, et al; Vol. 26; page 47; VCH publishers, 220 East 23rd St., New York, N.Y. 10010- 4606(USA); 1995). Due to its structure and the variety of impurities that talc contains, usually its PZC varies quite considerably over the pH range between ~2 (SiO ₂ ) and ~12 (MgO) (cf. "The Isoelectric Points of Solid Oxides, Solid Hydroxides and Aqueous Hydroxo Complex Systems"; G. A. Parks; Chem. Rev. £5_, 177 (1965); particularly: "Effects of Impurities" on pages 186,7). Therefore, being able to effect the surface charge of talc is particularly important.

The desire to turn powder minerals, that are regularly weakly charged with negative charges, into highly positively charged materials is a general phenomenon. As will be detailed below, the surface charges of other minerals such as CaMg(CO3)2, MgCO3, Ca(OH)2 and CaSO4 can be turned highly positive by applying the present invention. ^' Surprisingly, it was found that the surface charges of natural and artificial minerals containing Ca and/or Mg like MgO, talc (3MgO-4SiO2-H20), Mg(OH) , CaCO3, CaMg(CO3)2, MgCO3, Ca(OH)2 and CaSO4 can be modified quite easily to carry high positive surface charges by producing them in the presence of minute concentrations of water soluble salts of bivalent and trivalent cations like Mg, Ca and Al at the pH range above their PZC or at a pH of at least 5.5.

It is a purpose of the present invention to provide an inexpensive and simple method to produce fine powders that are charged with high and positive electric charges.

It is a further purpose of the invention to provide methods to produce these fine powders using common and inexpensive raw materials.

It is a further purpose ofthe invention to provide methods to improve these fine powders by coating them with suitable carboxylic acid salts and other additives using common and inexpensive raw materials.

It is a further purpose of the invention to provide methods to carry out the beneficiation of minerals and to obtain purer materials at lower costs.

It is a further purpose of the invention considerably to enhance the production of certain minerals and to modify their crystallographic nature.

Other purposes and advantages of the invention will become apparent as the description proceeds.

Summary of the Invention

Surprisingly, it has been found that the addition of water-soluble salts of bi- and tri¬ valent cations to insoluble or sparingly soluble natural or artificial minerals such as MgO, Mg(OH) ₂ , CaCO ₃ , CaMg(CO ₃ )2, MgCO ₃ , 3MgO-4SiO ₂ -H ₂ O, Ca(OH) ₂ and CaSO at the pH range above their PZC or above pH = 5.5., leads to a dramatic change of their electric surface charges. Specifically, the electric surface charges turn from weakly positive or negative to strongly positive. Moreover, in cases at which certain materials are formed in the presence of various such water soluble salts, their crystallographic nature is substantially changed. This in turn gives rise to the following additional phenomena:

a. The resulting surface-modified minerals are faster and more easily size reduced by grinding/milling and the fine particles, which are produced, do no coalesce during long periods of time. This applies to fine powders in slurries, which exhibit outstanding stabilities. This effect is also manifested in cases where the fine mineral powders are obtained by recrystallization or precipitation caused by reacting suitable reagents (e.g. formation of PCC, formation of brucite, etc.)

b. The resulting surface-modified minerals react faster. For instance, when MgO is hydrolyzed in water its transformation into brucite is substantially faster in the presence of surface modifying agents than in plain solutions.

c. The resulting surface-modified minerals strongly absorb cellulose, which is negatively charged. Thus, two benefits results in the paper industry, in which the inexpensive calcite is added to the expensive cellulose. First, larger quantities of calcite can be loaded in the paper and, second, the calcite is adsorbed from the slurries used more efficiently, leaving less calcite in the effluents.

d. The resulting surface-modified minerals strongly absorb carboxylic acid salts, which are negatively charged. This leads to substantial advantages in a large variety of applications. For instance, grinding/milling calcite or precipitating calcium carbonate by reacting Ca(OH)2 and CO2 produce products that are usually charged by weak negative electric charges, as described above. The addition of carboxylic acid salts results in a very inefficient adsorption of the carboxylates onto the surface of the fine particles. However, this is changed dramatically when the adsorption of the carboxylates is done in the presence water-soluble salts of bi- and tri-valent cations, as described in the examples to follow.

A similar behavior is manifested by brucite. The addition of water-soluble salts of bi- and tri-valent cations greatly enhances the adsorption of the carboxylates onto the surface of the product, allowing the use of substantially lower quantities of fatty acids than those required in regular prior art operations ( >0.5% wt).

e. The crystallographic nature and the surface properties of the various minerals can be affected selectively. This is the basis for producing variety of materials and for applying the present invention in the beneficiation processes of minerals like calcite, brucite, etc. using inexpensive fatty acids as collectors and frothing the selected materials in the regular flotation processes. The resulting procedures are substantially more economic than the prior art processes, due to the simplicity of operation, the cost of reagents and the selectivity achieved.

Detailed Description of Preferred Embodiments

Water insoluble or sparingly soluble natural and/or artificial mineral powders, which are not produced in brine solutions, based on Ca and/or Mg that are coated with quite small amounts of water-soluble salts (of which concentrations are: >100ppm for CaCO3 and >500ppm for other minerals, based on the aqueous solutions) of bivalent and trivalent metal cations in the pH range beginning at lower ofthe PZC of the respective mineral or pH=5.5 - are termed hereinafter "surface-modified minerals". Examples of such "surface-modified minerals" are Mg(OH)2, MgO, 3MgO SiO2-H2O, CaCO3, CaMg(CO3)2, MgCO3, Ca(OH)2, CaSO4 and mixtures thereof.

Generally, the concentrations of the water soluble salts, mentioned above, should, preferably, be as low as possible, in order to avoid the intensive and expensive washing ofthe final products (the fine powders), which is one ofthe drawbacks ofthe present art, in which processes the minerals are produced in brines. The optimal concentration ofthe

salts may vary from case to another, but the tendency is to minimize it as much as possible.

The water-soluble salts are selected from halides, sulfates, sulfites, nitrates, nitrites, organic sulfates and organic sulfonates of bivalent and trivalent cations. The halides and sulfates of Mg ^4-1" and Al ⁺⁺⁺ and the halides of Ca ^"1-1" are the preferable salts. The chloride and sulfate of Mg ^"1-1" are the most preferable salts. It should be noted that the water-soluble salts can be prepared, in situ, by reacting basic minerals with the required amounts of suitable acids and thereafter controlling the pH by using an appropriate base. The pH can be controlled by using common and inexpensive industrial bases, such as NaOH, MgO, Mg(OH)2, CaO, Ca(OH)2, ammonia, amines, etc.. However, the preferred base is the relatively inexpensive Ca(OH)2-

According to a preferred embodiment of the invention the surface-modification of the minerals is carried out in situ during the grinding/milling of the corresponding minerals with amounts of water of at least 0.5 wt%.

According to another preferred embodiment of the invention the surface-modified minerals are prepared in situ during the wet grinding/milling of the corresponding minerals.

Naturally the surface-modified minerals can be further treated with carboxylic (polycarboxylic) acids and/or carboxylic (polycarboxylic) acid salts and/or carboxylic (polycarboxylic) acid anhydrides. The carboxylic and polycarboxylic acid salts are selected from, Na ⁺ , Mg* ^4" , Al ^{" "1-} * ^" and Ca ^-1" salts. The carboxylic acids and/or their anhydrides and/or salts are added to the inorganic materials as such or together with other substances. Naturally, the optimum pH range for applying these carboxylic acids is above the pKa values ofthe respective acids.

The carboxylic acids and/or carboxylic acid salts and or carboxylic acid anhydrides may or may not have polymerized, in situ, during the production ofthe powders, or may have partially polymerized, in situ, during the production of the powders, and when it has polymerized it is dimerized and/or oligomerized and/or polymerized in the presence or the absence of any added polymerization initiators. The polymerization initiators, if added, are selected from among organic azo compounds and organic peroxide compounds, such as percarboxy lates; inorganic peroxides, such as. hydrogen peroxide, persulfates, percarbonates and perborates.

Other additives that are commonly used in the art are chosen from among long chain fatty acid esters, paraffins, polymer greases, waxes and silicone rubbers. Of course, other conventional additives can also be added. In a prefeπed mode of operation, these additives may be pre-mixed with suitable carboxylic acids.

The production of the surface-modified minerals and the beneficiation of the various minerals can be carried out in stiπed reactors and/or in flotation reactors, or in any other suitable equipment, as will be easily recognized by the skilled person.

Experimental Data

Raw Materials

- Oleic Acid of Aldrich

- Palmitic Acid of Aldrich

- Stearic Acid of Aldrich

- Tall oil of Arizona Chemical Co.

- NaOH of Frutarom - Ca(OH)2 of Frutarom

- Mg(OH)2 of Frutarom

- Al2(SO4)3 of Aldrich

- MgSO4 solution having a density of d=1.2 g/cm^ where the ratio H2O/MgSO4 = 3.1

- MgCl2 solution having a density of d= 1.267- 1.27 g/cm^ where the ratio H2O/MgCl2=2.61

- 10% (wt) Al2(SO4)3 solution

- CaCO3 powder (d5fj=18 microns) of Polychrom, Israel- "Girulite-40"

Example 1

Charging the CaCO ₃ With a Positive Charge

70% (wt) CaCO3 powder (d50=18 microns) of Polychrom, Israel- "Girulite-40" slurry in water was prepared by stirring the components with a RW20 IKA. stiπer for 30 mins. Various salts and alkalis were added during the stirring and the Zeta potential of the resulting slurries were measured. The results are given in Table 1 : ϊah l

Weight (g) Zeta

Test CaCO3 Water Alkali Salt Alkali Salt pH Potential Remarks

1 560 240 _ . _ _ 7.2 -18 mV Reference la 560 240 0.3 . Ca(OH) ₂ - 9.0 -30 mV Reference lb 560 240 _ 3.0 _ MgCl ₂ 7.1 + 5 mV Reference lc 560 240 . 3.0 _ Al ₂ (SO ₄ ) ₃ 7.2 + 4 mV Reference

2 560 240 0.3 3.0 Ca(OH) ₂ Al ₂ (SO ₄ )3 9.0 +18 mV

3 560 240 0.3 3.0 Ca(OH) ₂ CaCl 8.8 +17 mV

4 560 240 0.2 3.0 Na(OH) MgCl 8.8 +21 mV

5 560 240 0.8 3.0 Mg(OH) ₂ MgCl ₂ 8.6 +30 mV

6 560 240 0.05 3.0 Ca(OH) ₂ MgCl ₂ 7.7 +18 mV

7 560 240 0.3 3.0 Ca(OH) ₂ MgCl ₂ 8.6 +35 mV

8 560 240 0.4 3.0 Ca(OH) ₂ MgCl2 9.0 +38 mV

9 560 240 1.5 3.0 Ca(OH) ₂ MgCl2 11. +20 mV

Remarks;

1. The charge is inversed.

2. The optimal pH range is up to 11. At higher pH values the charges are reduced..

Example 2 Use nf "surface-modified CaCO ₂ " in the Paper Industry

"Surface-modified CaCO3" slurries were prepared in the laboratory by grinding CaCO3 in a 3L alumina ball mill ( 1.2L ceramic balls; density = 4 g/cm ³ ) during 24 hrs.. The particle size distribution ofthe products were measured after the grinding and after 10 days. The results are summarized in Table 2:

Ialι 2

Weigh (g) d50 (μm d50 (μm)

Test # CaCO3 Water P-20 Alkal Sal Alkali Salt pH 24 hrs 240 hrs

10 650 280 . _ . . _ 7.2 2.9 8.0

10a 650 280 . - 5.0 - MgCl 7.1 2.7 7.1

10b 650 280 - 0.5 _ Ca(OH)2 _ 8.6 3.0 8.2

11 650 280 2.0 0.9 5.0 Mg(OH) MgCl 8.8 1.4 1.5

12 650 280 - 0.5 5.0 Ca(OH) ₂ MgCb 8.5 2.2 3.5

Remarks:

1. Tests # 10, 10a and 10b are reference experiments.

2. The fine powder obtained in Tests # 10, 10a and 10b after 24 hrs. undergo coalescence and precipitate after 10 days.

The fine powders that were obtained in the above grinding process (after 24 hrs.) were tested in a laboratory paper making machine. The major parameters were determined:

% Load - The amount of CaCO3 in the cellulose.

% Retention (I) - The retained CaCO3 on the cellulose after the 1st pass.

% Brightness - Brightness compared to MgO powder.

The results are summarized in Table 3:

Test # % Load % Retention % Brightness

10 22.0 50.0 89.9

11 29.0 68 0 94.1

12 30.0 70.0 92.5

Remarks:

1. The "surface-modified CaCO3 " gives better results.

2. The "surface-modified CaCO3 " gives brighter products.

3. The commercial Albacar PCC has a brightness of 95.6.

Example 3

Production of Hydrophobic "Surface-Modified CaCO ₃ »>

50% (wt) CaCO3 powder (d5o=18 microns) of Polychrom, Israel- "Girulite-40" slurry in water were subjected to grinding for 30 mins. in a Retch model KM-1. During the grinding Ca(OH)2 was added to maintain the pH at 8.5. In addition, salts and fatty acids, preheated to 80°C, were added to the slurry.

The products were then filtered off, washed and dried at 100°C for 15 hrs. The dry products were subjected to the " Modified Hallimond Tube " test to check the quality of coating obtained. The results are given in Table 4:

Table 4

Weight (g) % wt

Test CaCO3 Water F.A. Salt F.A. Salt pH Float Remarks

13 300 300 - - - . 8.5 2 Reference

13a 300 300 0.2 - Oleic . 8.5 40 Reference

13b 300 300 0.2 3.0 Oleic MgCl ₂ 7.1 35 Reference

14 300 300 0.2 3.0 Oleic MgCl ₂ 8.5 100

15 300 300 0.2 3.0 Tall oil MgCl ₂ 8.5 100

16 300 300 0.2 3.0 Palmitic MgSO ₄ 8.5 100

17 300 300 0.2 3.0 Stearic MgSO4 8.5 100

Remarks:

1. F.A. - Fatty Acid used.

2. Test 13b was conducted without a base.

3. The " Modified Hallimond Tube " flotation test that is described in the experiment followed the well known floatability tests ( cf. "Mineral Processing"; E. J. Pryor; Third Ed.; Elsevier Publishing Co.; 1965; pp. 463-468).

All the above description and examples have been provided for the purpose of illustration and are not intended to limit the invention. Many modifications can be effected in the various procedures, processes and additives, to give a variety of surface- modified minerals, all without exceeding the scope of the invention.