GB2268484A

GB2268484A - Improving the brightness of white minerals

Info

Publication number: GB2268484A
Application number: GB9214319A
Authority: GB
Inventors: Jacek Antoni Kostuch; Darren John Craddock
Original assignee: ECC International Ltd
Current assignee: Imerys Minerals Ltd
Priority date: 1992-07-06
Filing date: 1992-07-06
Publication date: 1994-01-12
Anticipated expiration: 2012-07-06
Also published as: GB2268484B; GB9214319D0

Abstract

There is disclosed a process for improving the whiteness and brightness of a natural mineral material for the purpose of improving its usefulness as a pigment or filler, which process comprises treating the mineral material in aqueous suspension with a combination of a low oxidation state transition metal ion and a complexing agent.

Description

IMPROVING THE BRIGHTNESS OF WHITE MINERALS This invention relates to a process for improving the whiteness and brightness of natural white minerals in order to improve their usefulness as pigments or fillers.

The commonest cause of a natural white mineral having a poor colour is the presence of ferric ions on the mineral surface which impart a yellowish or orange colouration to the mineral. A further cause of discolouration is the presence of organic material associated with the mineral.

It has been known for many years that the colour and brightness of white minerals may be improved by treating the mineral with a reducing bleaching agent such as, for example, a salt of sulphurous acid or, especially, a salt of hydrosulphurous acid, which is also known as dithionous acid. In particular, processes of the type in which kaolinitic clay is treated with sodium or zinc dithionite are very widely used. The effect of this treatment is to reduce ferric ions on the surface of the mineral, and possibly, to a lesser extent, ferric ions in the crystal lattice of the mineral, to the less coloured ferrous state.The ferrous ions are also more soluble in water than the ferric ions and it is desirable that the ferrous ions should, as far as possible, be removed from the mineral crystals, because those remaining in situ would tend to be reoxidised to the ferric state on exposure to air.

The well known process using a dithionite salt is effective if the mineral is only relatively lightly stained with iron and has only a slight discolouration, but, if the amount of staining is severe and the mineral has a pronounced yellow or orange colouration, it is found that a very large quantity of the dithionite salt is required to improve the whiteness of the mineral to an acceptable level. The bleaching effect of a dithionite salt is believed to be due to a chemical reaction of the type:

Thus, one dithionite ion is required to donate one electron to each of two separate ferric ions in order to effect the reduction of the ferric ions to ferrous ions.

Reducing agents which comprise a complex formed between a low oxidation state transition metal ion and a complexing agent are often referred to by specialists in this field as "LOMI" reagents (low oxidation-state metal ion reagents), but it is generally understood that a LOMI reagent contains not only a transition metal ion in its low oxidation state, but also a suitable complexing agent to increase the solubility in water of the reduced metal ion which it is desired to remove.

According to the present invention, there is provided a process for improving the whiteness and brightness of a natural mineral material for the purpose of improving its usefulness as a pigment or filler, which process comprises treating the mineral material in aqueous suspension with a combination of a low oxidation state transition metal ion and a complexing agent.

The low oxidation state transition metal ion and the complexing agent form a complex reducing agent which is capable of effecting a reduction of components on the surface of the mineral affecting the colour of the mineral. It is believed that the complexing agent also maintains in solution (in the form of a complex) the reduced components (such as metal ions) newly dissolved from the surface of the mineral material.

The mineral material may be a white mineral, by which we mean a mineral which, in its pure state is substantially free from any non-white hues. For example, the mineral material may be a clay mineral such as a clay mineral of the kandite group, which comprises kaolinite, dickite, nacrite and halloysite, or a clay mineral of the smectite group, which comprises bentonite, montmorillonite, hectorite, beidellite and saponite. Alternatively the mineral material may be an alkaline earth metal carbonate or sulphate mineral, such as chalk, marble, dolomite, gypsum or barytes, or a silicate mineral such as talc, mica or wollastonite.

The process of the invention is especially useful when the natural mineral material is heavily stained with coloured compounds of iron or other transition metals.

By "low oxidation state transition metal ion" we mean a transition metal ion which is in an oxidation state lower than that which is normally stable with respect to oxidation by air in aqueous solution.

Although in theory any low oxidation state transition metal ion would be suitable, in practice it has been found that the preferred ions are Fe2+, Cr2+ and V2+.

The low oxidation state transition metal ion is used in tandem with a suitable complexing agent in order more effectively to bring into aqueous solution the ions, which in their higher oxidation state, are responsible for the colour of the discolouring impurities in the mineral material.

Examples of suitable complexing agents are ethylenediamine tetra-acetic acid, citric acid, picolinic acid, nitrilotriacetic acid, 2-6- dicarboxypyridine and 2,2'-bipyridyl. It is found in practice that certain complexing agents are most effective when used in combination with particular low oxidation state metal ions. For example, it has been found that the complexes formed between Fe2+ and ethylenediamine tetra-acetic acid, between Cr2+ and 2,2'- bipyridyl and between V2+ and picolinic acid are especially effective.

The low oxidation state transition metal ion and the complexing agent should be chosen so as to be capable of forming a combination which is sufficiently effective to perform the desired reduction relatively quickly, for instance in about F to about 3 hours and at room temperature and about 1 to about 30 minutes at elevated temperatures. The complex reducing agent should preferably be a "one electron" reducing agent i.e. a reducing agent which can transfer a single electron, rather than multiple electrons. In this connection, the pH is also important and, to this end, the pH of the aqueous medium in which the mineral material is suspended during treatment with the low oxidation metal ion complex is preferably maintained at a value within the range from about 1 to about 3.

The amount of the low oxidation state metal ion complex required to effect the desired improvement in the whiteness of the mineral material is generally in the range of from 0.1 to 2.0 mol. of the low oxidation state metal ion per kilogram of the dry mineral material. In most cases little advantage is to be gained from using more than the amount of the low oxidation state metal ion per kilogram of the dry mineral material which is given by the formula: wzlO.P M where W is the number of moles of the low oxidation state metal ion required per kilogram of the dry mineral material, M is the molecular weight of the discolouring oxide or oxyhydroxide present on the mineral surface and P is the percentage by weight of the oxide or oxyhydroxide which is present on the mineral surface.For example, if the mineral has on its surface 5% by weight of Fe203 of molecular weight 160g, the number of moles of the low oxidation state metal ion required per kilogram of the dry mineral would be 0.32.

In the case of the complex formed between the Fe2+ ion and ethylenediamine tetra-acetic acid it has been found that the optimum reducing effect on the Fe3+ ions contained in the discolouring impurities in the mineral material is obtained when the molar ratio of Fe2+ to ethylenediamine tetra-acetic acid is in the range from 1.25:1 to 2.75:1, and especially when the molar ratio is in the range from 1.5:1 to 2.0:1.

After treatment in accordance with the present invention, the mineral material may be dewatered, for instance by filtration, and dried. The water recovered containing the complexing agent and transition metal ion may be recycled although it may be necessary to supply additional amounts of the components for acceptable results to be obtained.

The invention will now be illustrated by the following examples: EXAMPLE 1 A sample of a kaolinitic clay which was heavily stained with ferric oxide and had a pronounced yellow hue was suspended in water containing a dispersing agent for the clay and was subjected to gravitational sedimentation for a time sufficient to give a fine fraction comprising particles substantially all of which had an equivalent spherical diameter smaller than 5pm. A portion of this fine fraction was filtered and dried and the dry material milled to a fine powder.

The fine fraction was found by X-Ray fluorescence analysis to have an iron content, as measured as Fe203, of 5.2t by weight and it was also determined that the fine powder had reflectance to light of 457 nm wavelength of 43.0% and to light of wavelength 570nm of 69.1%.

A complex reducing agent was prepared by reducing vanadyl sulphate (VOS04.H20) in solution in dilute sulphuric acid using the quantity of zinc powder required by the stoichiometry of the chemical reaction.

The reduction was carried out under non-oxidising conditions by bubbling a gentle stream of nitrogen gas through the suspension. The reduction from V4+ to V2+ was considered to be complete when a light violet coloured solution was obtained. A suspension containing 30g of the fine fraction of the clay in a solution containing three times the weight of picolinic acid required by the quantity of the V2+ ion to be used was transferred to a reaction vessel, and a gentle stream of nitrogen gas was bubbled through the suspension. The solution containing the V2+ ion was then added to the suspension, the pH was adjusted to 2.8 with 10% sodium hydroxide solution and the bleaching reaction was allowed to proceed for 30 minutes with continuous passage of nitrogen gas.The mixture was then filtered and the cake washed twice with 50cm3 of 0.3M picolinic acid and then with 100cm3 of deionised water. The washed clay was then filtered, dried at 80"C and finely milled.

The experiment was repeated using a range of different quantities of V2+ ion per unit weight of dry clay and in each case the finely milled, dry, bleached product was tested for reflectance to light of wavelengths 457nm and 570nm respectively.

The results obtained are set forth in Table I below: - Table 1 Wt. of V2+ ion % of reflectance to light of wavelength (mollKg dry clay) 457nm 570nm 0 43.0 69.1 0.02 44.6 69.5 0.14 51.3 71.4 0.28 60.8 74.6 0.55 68.2 75.6 1.96 70.0 76.9 As the dose of the V2+ ion was increased the bleached clay became progressively brighter and less yellow in colour, EXAMPLE 2 A sample of kaolinitic clay which was heavily stained with ferric oxide and had a pronounced orange hue was suspended in water containing a dispersing agent for the clay and was subjected to gravitational sedimentation for a time sufficient to give a fine fraction comprising particles substantially all of which had a equivalent spherical diameter smaller than 5pm. A portion of this fine fraction was filtered and dried and the dry material milled to a fine powder.

The fine powder was found to have an iron content, as measured by X-ray fluorescence as Fe203, of 1.9% by weight and a reflectance to light of wavelength 457 nm of 59.0% and a reflectance to light of wavelength 570nm of 78.5%.

50g of the fine fraction of the clay was suspended in 400cm3 of deionised water. A solution containing a known quantity of the sodium salt of ethylenediamine tetra-acetic acid (EDTA) was added to the suspension and allowed to equilibrate therewith for a period of 15 minutes while a gentle stream of nitrogen gas was bubbled through the suspension. A solution containing a known quantity of ferrous sulphate (FeS04.7H20) was then added to the suspension and the bleaching process was allowed to continue for 2 hours with the stream of nitrogen gas being maintained throughout. During the bleaching process the pH of the suspension was maintained at 2.0 by adding dilute sodium hydroxide solution when necessary.On the completion of the 2 hour period the suspension was filtered and the cake washed twice with 50cm3 of 1% by weight solution of potash alum and then twice with 50cm3 of deionised water. The washed clay was then filtered, dried at 80"C and finely milled.

The experiment was performed a total of 25 times with different quantities of EDTA equivalent to 20, 50, 75, 100 and 200 Kg, respectively, per tonne of dry clay, each being combined with quantities of ferrous sulphate equivalent to 10, 20, 50, 100 and 200 Kg, respectively, per tonne of dry clay to give different molar ratios of Fe2+ ion to EDTA.

In each case the reflectance to light of wavelengths 457nm and 570nm respectively was measured for each of the dry, milled products and the results obtained are set forth in Table II below: Table II Wt. of Wt. of ferrous No. of No. of Molar Ratio % reflectance to EDTA used sulphate used moles moles Fe2+: :EDTA light of wavelength (Kg/tonne) (Kg/tonne) EDTA Fe2+ 457nm 570nm 20 10 0.0538 0.0360 0.67 64.5 82.5 20 0.0719 1.34 65.0 82.6 50 0.1799 3.34 65.0 82.5 100 0.3597 6.68 65.0 83.0 200 0.7194 13.37 65.0 82.5 50 10 0.1344 0.0360 0.27 70.8 85.8 20 0.0719 0.54 75.6 87.6 50 0.1799 1.33 79.2 88.8 100 0.3597 2.68 80.4 89.4 200 0.7194 5.35 80.4 89.2 75 10 0.2016 0.0360 0.18 71.6 85.1 20 0.0719 0.36 77.5 88.4 50 0.1799 0.89 80.4 89.2 100 0.3597 1.78 82.8 89.8 200 0.7197 3.57 83.4 89.4 100 10 0.2688 0.0360 0.13 68.2 84.7 20 0.0719 0.27 72.0 86.3 50 0.1799 0.67 79.0 88.9 100 0.3597 1.34 83.0 89.3 200 0.7194 2.66 84.0 90.6 200 10 0.5376 0.0360 0.07 68.6 84.6 20 0.0719 0.13 70.4 85.4 50 0.1799 0.33 80.0 89.2 100 0.3597 0.67 84.5 90.3 200 0.7194 1.34 83.6 89.5 The bleached clay having the best combination of high brightness (% reflectance to violet light of wavelength 457nm) and low yellow colouration (difference between % reflectance to yellow light of wavelength 570nm and % reflectance to light of wavelength 457nm) was obtained when the number of moles of Fe2+ per kilogram of dry clay was about 0.36 and the molar ratio of Fe2+:EDTA was approximately in the range from 1.5:1 to 2:1.

EXAMPLE 3 A further lOg portion of the fine fraction of orange-stained, kaolinitic clay, prepared as described in Example 2, was suspended in 500cm3 of deionised water. A solution containing 1.34g of the disodium salt of ethylenediamine tetra-acetic acid was added to the suspension and allowed to equilibrate therewith for a period of 15 minutes while a gentle stream of nitrogen gas was bubbled through the suspension. A solution containing 2g of ferrous sulphate (i.e. the amount which would provide 2 mols of Fe2+ ion per mol.

of EDTA) was then added to the suspension and the bleaching process was allowed to continue for 2 hours with the stream of nitrogen gas being maintained throughout. During the bleaching process the pH of the suspension was maintained at 1.5. On the completion of the 2 hour period the suspension was filtered and the cake washed twice with 50cm3 of 1% by weight solution of potash alum and then twice with 50cm3 of deionised water. The washed clay was then filtered, dried at 80"C and finely milled.

The experiment was then repeated three times but with the pH maintained at 2.0, 2.5 and 3.1 respectively.

In each case the reflectance to light of wavelengths 457nm and 570nm respectively was measured for each of the dry, milled products and the results are set forth in Table III below: Table III pH reflectance to light of wavelength 457nm 570nm 1.5 79.4 88.7 2.0 85.5 90.7 2.5 83.2 89.6 3.1 70.3 85.3 The bleached clay having the best combination of high brightness and low yellow colouration was obtained when the pH was maintained at 2.0.

EXAMPLE 4 A 50g portion of the fine fraction of the orangestained, kaolinitic clay, prepared as described in Example 2, was suspended in 400cm3 of deionised water.

The pH of the suspension was adjusted to pH 2.0 by adding an appropriate amount of a 5% by weight solution of sulphuric acid. The suspension was then stirred continuously in a reaction vessel for 15 minutes while a gentle stream of nitrogen was bubbled through the suspension. The reaction vessel was then placed in a thermostatically controlled tank at 200C, a solution containing 3.75g of the disodium salt of ethylenediamine tetra-acetic acid was added to the suspension and the pH was readjusted to 2.0. After the suspension had been stirred for a further 15 minutes, a solution containing 5.0g of ferrous sulphate was added to the reaction vessel and the pH was again adjusted to 2.0. At various intervals after the addition of the ferrous sulphate, samples of the suspension were removed and the bleached kaolinitic clay was removed by filtration.In each case the filter cake was washed twice with 50cm3 of a 1% by weight solution of potash alum and twice with 50cm3 of deionised water. The washed clay was then filtered, dried in an oven at 800C and finally milled. The experiment was repeated with the suspension at a series of different temperatures by placing the reaction vessel on a hot plate until the temperature of the suspension reached the required level.

The reflectance to light of wavelengths 457nm and 570nm respectively was measured for each of the dry milled products, and the results are set forth in Table IV below: Table IV Temperature Time % reflectance to light of wavelength ( C) (mins) 457nm 570nm 20 0 59.0 78.5 1 64.2 82.0 60 78.0 87.5 43 0 59.0 78.5 1 79.5 88.3 5 80.6 88.6 30 82.0 88.0 60 83.0 88.8 60 0 59.0 78.5 1 82.2 88.2 5 83.5 88.6 60 84.0 87.7 75 0 59.0 78.5 1 83.0 88.9 60 85.0 89.0 EXAMPLE 5 A 50g portion of the fine fraction of the same orange-stained kaolinitic clay as was used in Example 4 was suspended in 400cm3 of deionised water. The pH of the suspension was adjusted to pH 2.0 by adding an appropriate amount of a 5% by weight solution of sulphuric acid.The suspension was then stirred continuously in a reaction vessel for 15 minutes while a gentle stream of nitrogen was bubbled through the suspension. The reaction vessel was then placed on a hot plate until the temperature of the suspension reached 750C. A solution containing 3.75g of EDTA was then added to the suspension and the pH was readjusted -to 2.0. After. the suspension had been stirred for a further 15 minutes a solution containing 5.0g of ferrous sulphate was added to the reaction vessel, and the pH was again adjusted to 2.0. The suspension was stirred continuously for a period of 1 minute after which the bleached kaolinitic clay was separated by filtration. The filtrate was retained and the cake was washed, filtered, dried and milled as described in Example 4 above.

The reflectance to light of wavelengths 457nm and 570nm, respectively, was measured and the results are reported for "Portion 1" in Table V below.

The filtrate retained from the experiment described above was heated to 750C on the hot plate, and there was then mixed therewith a further 50g portion of the same orange-stained kaolinitic clay, and the pH was adjusted to 2.0. The suspension was stirred continuously for a period of 15 minutes after which a sample was taken, the bleached kaolinitic clay was separated therefrom by filtration and the cake was washed, filtered, dried and milled as described in Example 4 above.

The reflectance to light of wavelengths 457nm and 570nm, respectively, was measured and the results are reported for "Portion 2" in Table V below.

The suspension remaining in the reaction vessel after the sample had been taken for the reflectance measurements for Portion 2 was divided into two approximately equal portions. The first portion was heated to 750C on the hot plate and there was added thereto a solution containing 2.5g of ferrous sulphate and the pH was adjusted to 2.0. The suspension was stirred continuously for a period of 15 minutes after which the bleached kaolinitic clay was separated therefrom by filtration and the cake was washed, filtered, dried and milled as described in Example 4 above.

The reflectance to light of wavelengths 457nm and 570nm, respectively, was measured and the results are reported for "Portion 2A" in Table V below.

The second portion of the suspension was then heated to 750C on the hot plate and there was added thereto a solution containing 1.875g of EDTA and the pH was adjusted to 2.0. The suspension was stirred continuously for a period of 1 minute after which the bleached kaolinitic clay was separated therefrom by filtration and the cake was washed, filtered, dried and milled as described in Example 4 above.

The reflectance to light of wavelengths 457nm and 570nm, respectively, was measured and the results are reported for "Portion 2B" in Table V below.

Table V % reflectance to light of wavelength 457nm 570nm Unbleached clay 59.0 78.5 Portion 1, clay bleached with fresh reagents 86.4 90.6 Portion 2, clay bleached with recycled reagents .59.0 78.5 Portion 2A, ferrous sulphate added 62.7 80.6 Portion 2B, EDTA added 85.3 90.2 These results show that, if clay is to be bleached with recycled filtrate containing recovered ferrous sulphate and EDTA, it is necessary to supply additional EDTA if acceptable results are to be obtained.

Claims

CLAIMS:

1. A process for improving the whiteness and brightness of a natural mineral material for the purpose of improving its usefulness as a pigment or filler, which process comprises treating the mineral material in aqueous suspension with a combination of a -low oxidation state transition metal ion and a complexing agent.

2. A process according to claim 1, wherein the mineral material may be a white mineral

3. A process according to claim 1 or 2, wherein the mineral material is a clay mineral, an alkaline earth metal carbonate or sulphate mineral or a silicate mineral.

4. A process according to claim 1, 2 or 3, wherein the mineral material is heavily stained with coloured compounds of iron or other transition metals.

5. A process according to any one of claims 1 to 4, wherein the low oxidation state transition metal ion is chosen from Fe2+, Cr2+ and V2+.

6. A process according to any one of claims 1 to 5, wherein the complexing agent is chosen from ethylenediamine tetra-acetic acid, citric acid, picolinic acid, nitrilotriacetic acid, 2,6-dicarboxypyridine and 2,2'-bipyridyl.

7. A process according to any one of claims 1 to 4, wherein the low oxidation state transition metal ion is Fe2+ and the complexing agent is ethylenediamine tetra-acetic acid.

8. A process according to claim 7, wherein the molar ratio of Fe2+ to ethylenediamine tetra-acetic acid is in the range from 1.25:1 to 2.75:1.

9. A process according to claim 7, wherein the molar ratio of Fe2+ to ethylenediamine tetra-acetic acid is in the range from 1.5:1 to 2.0:1.

10. A process according to any one of claims 1 to 4, wherein the low oxidation state transition metal ion is Cr2+ and the complexing agent is 2,2'- bipyridyl.

11. A process according to any one of claims 1 to 4, wherein the low oxidation state transition metal ion is V2+ and the complexing agent is picolinic acid.

12. A process for improving the whiteness and brightness of a natural mineral material for the purpose of improving its usefulness as a pigment or filler, substantially as hereinbefore described, with reference to the Examples

12. A process according to any preceding claim, wherein the pH of the aqueous medium in which the mineral material is suspended during treatment with the.

low oxidation metal ion complex is maintained at a value within the range of from about 1 to about 3.

13. A process according to any preceding claim, wherein the low oxidation state metal ion is employed in an amount in the range of from 0.1 to 2.0 mol. per kilogram of the dry mineral material.