Abstract:
The present invention relates to a process for selectively purifying ceramic materials for glass, paper and electronics use contaminated with iron compounds, the process using ascorbic acid or its derivatives as the iron reducing agent.

Description:
This is a continuation of application Ser. No. 07/702,294 filed May 20, 1991, now abandoned. 
    
    
     STATE OF THE ART 
     After undergoing other purification processes, materials for ceramics, glass, paper and electronics use are finally bleached by eliminating iron compounds by a process of classical reduction with sodium hydrosulphite (Na 2  S 2  O 4 ) in a sulphuric medium (pH&lt;3). 
     By this method the iron is dissolved in the form of FeSO 4 , which is then removed by filtration under pressure (see in this respect FR-A-2030730; U.S. Pat. No. 3,528,769; RO-A-49334; SU-A-485093; SU-A-628087; &#34;Formation of aggressive substances in East German kaolins&#34;, Silikattechnik (81) p. 262-5, Vol. 32, No. 9). 
     However with these processes in most cases it is not possible to obtain high-quality products (for example the degree of whiteness obtainable in a kaolin by these methods is about 85-88%). 
     This is because the hydrosulphite is unable to reduce all the iron, which is present in the form of various chemical compositions. 
     With the treatment according to the present invention, using ascorbic, oxalic, citric or tannic acid in the presence of H 2  SO 4  (pH&lt;3), it is possible to reduce the iron present in its various compounds by making it totally soluble and hence removable by washing. In this manner a degree of whiteness of 89-94% is obtained, this representing a considerable improvement over the results obtained by traditional methods. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The mineral, such as kaolin, quartzose sand, or pumice, is suspended in water and the ascorbic acid or its derivatives is suspended in this suspension in a quantity of between 0.1% and 5%. The suspension pH is then adjusted to below 3 (preferably between 2 and 3) with H 2  SO 4 . 
     The suspension is heated to a temperature of 65°-85° C. for 1-3 hours under stirring. The deferrized product is then collected by filtration, washed with water and dried. 
     The tests for determining the degree of whiteness were carried out by firing the sample in a Seger No. 8 cone and determining the whiteness index with a Photovolt Reflection Meter 670, using a green filter (λ=550 μm). Two buttons of each sample were obtained by casting in plaster moulds. 
     The chemical, mineralogical and morphological characteristics of the kaolin and quartzose sand used in the subsequently described examples are given below. 
     
         ______________________________________KAOLINDegree of whiteness: 85%Average chemical analysis           Average particle size distribution______________________________________STANDARD        ISO 565     CumulativeL.O.I.          microns     residue______________________________________(LOSS OFIGNITION)  8.5 + 0.1SiO.sub.2  64.7 + 0.8                60           1%Al.sub.2 O.sub.3      24.5 + 0.2                30           5%Fe.sub.2 O.sub.3      0.8 + 0.1 10          22%TiO.sub.2  0.34 + 0.05                 5          40%CaO        0.08 + 0.02                 2          75%K.sub.2 O  0.65 + 0.05Na.sub.2 O 0.14 + 0.05MgO         0.1 + 0.03Rational analysisKaolinite  60Quartz     33Vitreous    7substancesMoisture     Raw shrinkage Whiteness15%          4.6%          85%Flexural strength        Shrinkage on firing                      Porosity20.2 kg/cm.sup.2        9.2%            27.9%______________________________________QUARTZOSE SANDSample separated by treating kaolin exceedng 71 μmDegree of whiteness: 44%Chemical analysis______________________________________Al.sub.2 O.sub.3  5.09%P.C.   3.04%SiO.sub.2 88.31%Fe.sub.2 O.sub.3  0.81%______________________________________ 
    
     EXAMPLE 1 
     100 g of kaolin are made up to 400 cc with water, 4 g of ascorbic acid are added and the pH adjusted to 2.5 with H 2  SO 4 . The suspension is heated to 72° C. and left stirring for 2 h. The residue is filtered off and washed with hot water. 
     (Degree of whiteness after firing 91.5%). 
     EXAMPLE 2 
     100 g of kaolin are made up to 300 cc with water, 4 g of ascorbic acid are added and the pH adjusted to 2.5 with H 2  SO 4 . The suspension is heated to 72° C. and left stirring for 1 h. The residue is filtered off and washed with hot water. 
     (Degree of whiteness after firing 919). 
     EXAMPLE 3 
     100 g of kaolin are made up to 300 cc with the solution of the preceding test, and the pH adjusted to 2.3 with H 2  SO 4 . The suspension is heated to 72° C. and left stirring for 2 h. The residue is filtered off and washed with hot water. 
     (Degree of whiteness after firing 89%). 
     EXAMPLE 4 
     100 g of kaolin are made up to 300 cc with water, 4 g of ascorbic acid are added and the pH adjusted to 2.5 with H 2  SO 4 . The suspension is heated to 72° C. and left stirring for 3 h. A further 1 g of ascorbic acid is added, the pH readjusted to 2.5 with sulphuric acid the suspension left stirring for 1 h. The residue is filtered off and washed with hot water. 
     (Degree of whiteness after firing 93.5%). 
     EXAMPLE 5 
     100 g of kaolin are made up to 300 cc with water, 0.5 g of ascorbic acid are added and the pH adjusted to 2.5 with H 2  SO 4 . The suspension is heated to 85° C. and left stirring for 2 h. The residue is filtered off and washed with hot water. 
     (Degree of whiteness after firing 919). 
     EXAMPLE 6 
     100 g of kaolin are made up to 300 cc with water, 0.12 g of ascorbic acid are added and the pH adjusted to 2.5 with H 2  SO 4 . The suspension is heated to 85° C. and left stirring for 3 h. The residue is filtered off and washed with hot water. 
     (Degree of whiteness after firing 90.5%). 
     EXAMPLE 7 
     100 g of kaolin are made up to 300 cc with water, 0.5 g of ascorbic acid are added and the pH adjusted to 2.5 with H 2  SO 4 . The suspension is heated to 80° C. and left stirring for 1 h. A further 0.5 g of ascorbic acid are added and the suspension again heated to 80° C. for 1 h. The residue is filtered off and washed with hot water. 
     (Degree of whiteness after firing 89%). 
     EXAMPLE 8 
     100 g of kaolin are made up to 300 cc with water, the pH is adjusted to 2.5 with H 2  SO 4  and the suspension left stirring for 3 h. 0.5 g of ascorbic acid are added and the suspension heated to 80° C. for 1 h, a further 0.5 g of ascorbic acid are added and the suspension again heated to 80° C. for 1 h under stirring. The residue is filtered off and washed with hot water. 
     (Degree of whiteness after firing 93.5%). 
     EXAMPLE 9 
     100 g of quartzose sand are made up to 300 cc with water, 4 g of ascorbic acid are added to the suspension and the pH is adjusted to 2.3 with sulphuric acid. The suspension is heated to 60° C. and left stirring for 2 h. The residue is filtered off and washed with hot water. 
     (Degree of whiteness after firing 54%). 
     EXAMPLE 10 
     100 g of quartzose sand are made up to 300 cc with water, 4 g of oxalic acid are added to the suspension and the pH is adjusted to 2.3 with sulphuric acid. The suspension is heated to 60° C. and left stirring for 2 h. The residue is filtered off and washed with hot water. 
     (Degree of whiteness after firing 52%). 
     EXAMPLE 11 
     100 g of kaolin are made up to 300 cc with water, 1 g of K 2  S 2  O 7  are added to the suspension and the pH is adjusted to 2.3 with sulphuric acid. The suspension is heated to 75° C. under stirring for 2 h. 1 g of ascorbic acid are now added and the system left stirring for 1 h. The residue is filtered off and washed with hot water. 
     (Degree of whiteness after firing 89%). 
     EXAMPLE 12 
     100 g of mineral are made up to 300 cc with water, 4 g of oxalic acid are added to the suspension and the pH is adjusted to 2.4 with sulphuric acid. The suspension is heated to 80° C. and left stirring for 2 h. The residue is filtered off and washed with hot water. 
     (Degree of whiteness after firing 929). 
     EXAMPLE 13 
     100 g of mineral are made up to 300 cc with water, 4 g of citric acid are added to the suspension and the pH is adjusted to 2.5 with sulphuric acid. The suspension is heated to 80° C. and left stirring for 2 h. The residue is filtered off and washed with hot water. 
     (Degree of whiteness after firing 89%). 
     EXAMPLE 14 
     100 g of mineral are made up to 300 cc with water, 4 g of tannic acid are added to the suspension and the pH is adjusted to 2.3 with sulphuric acid. The suspension is heated to 80° C. and left stirring for 2 h. The residue is filtered off and washed with hot water. 
     (Degree of whiteness after firing 89%).