Abstract:
A chemical process is described for removing ferric iron present in submarginal quantities in concentrates of kaolin, quartz, titanium minerals, ceramic minerals for glass, paper and electronics use and other materials of industrial interest by which ferric iron is reduced to ferrous iron using acid solutions, possibly in the presence of low concentrations of sugars containing 5 or more carbon atoms.

Description:
FIELD OF THE INVENTION 
     A process for bleaching kaolin, quartz, titanium minerals, ceramic minerals for glass, paper and electronics use and other like materials containing ferric iron is described in which the mineral is suspended in water at acid pH, a sugar is possibly added to the suspension, the suspension is kept stirred with or without heating, and the residue is finally collected by filtration and dried. 
     STATE OF THE ART 
     The removal of ferric iron contained in small quantities in materials such as kaolin, quartz, titanium minerals, ceramic minerals for glass, paper and electronics use and other like materials, with their consequent bleaching, is commonly achieved either by physical separations such as magnetic separation, flotation etc., or by redox chemical processes. The reagent commonly used for chemical bleaching of kaolins and other stated like materials is sodium hydrosulphite (Na 2  S 2  O 4 ) in a sulphuric acid medium (pH&lt;3). 
     The Fe +++  is reduced to Fe ++  and hence solubilized as FeSO 4 , which is then removed by pressure filtration (see in this respect FR-A-2030730; U.S. Pat. No. 3,528,759; RO-A-49334; SU-A-485093; SU-A-628087; &#34;Formation of aggressive substance in East German caolins&#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 because the hydrosulphite is unable to reduce all the iron, which is present in the form of various chemical compositions so that, for example, the degree of whiteness obtainable with a kaolin by these methods is about 85-88%. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention enables the iron present as ferric iron (Fe 3+ ) in the material for bleaching to be eliminated by making it totally soluble and hence removable by washing, to enable a degree of whiteness of greater than 90% and up to 96% to be obtained, this result being clearly superior to those obtainable with the aforesaid methods. This result is obtained by treating in an acid environment those minerals coloured with small quantities of iron-containing minerals in which the iron oxidation level is 3+. The possible addition of sugars to the solution facilitates said reduction process by allowing any ferric iron present either as iron sesquioxide or as jarosite to be reduced, and by maintaining the iron in ferrous form and hence more soluble by virtue of its greater solubility compared with the corresponding oxidized metal form, this being a very interesting aspect in the liquid/solid separation stage. 
     The mechanism of the process according to the invention can be schematized by the following reactions: 
     1) 2/3A[Fe 3  (SO 4 ) 2  (OH) 6  ]+2/3SO 4  2-+H +  →→Fe(OH) 3  +Fe(SO 4 ) 2  -+H 2  O+2/3A +  where A +  can be K + , Na + , NH 4   +  and H + . 
     2) 24 Fe 3+  +C 6  H 12  O 6  +6H 2  O→24 Fe 2+  +6 CO 2  +24H +   
     3) C 6  H 12  O 6  +12 Fe 2  O 3  +48H +  →24 Fe 2+  +30H 2  O+6 CO 2   
     The process according to the invention also enables industrial wastes containing sugar, such as spent milk whey, beet molasses etc., to be used as reagents, these not only being of low cost but often representing undesirable and/or dangerous waste products which would otherwise have to be subjected to complex inserting processes. 
     EXPERIMENTAL PART 
     The process according to the present invention is conducted by treating the material to be bleached with an aqueous acid solution in which the acid concentration is between 0.1 and 4M at a temperature of between 20° C. and 120° C. for a time of up to 48 hours. The content of the material to be bleached is between 4 and 35%. The acids usable in the process of the invention are HCl, H 2  SO 4 , H 3  PO 4  etc. 
     In a preferred version of the process according to the present invention a sugar is added to the acid solution in a concentration of 0.5-4 g/l. 
     Sugars usable in the process of the present invention include for example: saccharose, fructose, arabinose etc. In addition, as stated, sugar mixtures such as those represented by beet molasses, spent milk whey etc. are of interest provided they enable a sugar concentration of the aforesaid value to be obtained. 
     The liquid phase, separated by centrifuge, is analyzed for iron content by ICP. The solid residue, suitably dried, is tested to determine the degree of whiteness by firing the sample in a Seger No. 8 cone; two buttons of each sample were obtained by casting in plaster moulds, the whiteness index is determined with a Photovolt Reflection Meter 670, using a green filter (λ=550 μm). 
     Details and advantages of the process according to the present invention will be more apparent from the examples given hereinafter for the purposes of non-limiting illustration. 
     Table 1 shows the characteristics of the kaolin treated by the process of the present invention. 
     
                       TABLE 1______________________________________Characteristics of the kaolin used in the bleaching processDegree of whiteness: 85%            Average particle            size compositionAverage chemical analysis              ISO 565  CumulativeSTANDARD               microns  residue______________________________________L.O.C.     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 substances          7Moisture      Reversion     Whiteness15%           4.6%          85%Flexural strength         Shrinkage on firing                       Porosity20.2 kg/cm.sup.2         9.2%          27.9%______________________________________ 
    
    
    
     EXAMPLE 1 
     Three aqueous suspensions (0.5 l) made acid by sulphuric acid (concentration 1M) and each containing 40 g/l of suitably pulverized kaolin and 20 g/l of saccharose were heated while stirring to temperatures of 40° C. 50° C. and 60° C. respectively. Three samples were taken from each solution after 6, 24 and 48 hours, the results indicating the solution iron concentrations at the time of taking the respective samples being given in Table 2. 
     
                       TABLE 2______________________________________Iron solubilization (ppm)      Time (h)      6         24     48______________________________________40° C. solution          79.8      148    37050° C. solution        430         707    68760° C. solution        289         650    590______________________________________ 
    
     As can be seen from the Table 2 data for 50° C. and 60° C., iron solubilization is complete after 24 hours. 
     The whiteness index of the solid residues from this treatment was then measured by the aforedescribed method, the results obtained being shown in Table 3. 
     
                       TABLE 3______________________________________Whiteness index of solid residues after treatment              Time (h)              6     24      48______________________________________Residue from solution treated at 40° C.                90%     94%     94%Residue from solution treated at 50° C.                95%     95%     96%Residue from solution treated at 60° C.                94%     N.D.    96%______________________________________ N.D. = not determined 
    
     As can be seen from the data of Table 3, virtually complete decolorization is achieved after 24 hours, treatment at 50° C. having proved the quickest. 
     EXAMPLE 2 
     Example 1 was repeated using suspensions containing 300 g/l of suitably pulverized kaolin and different concentrations of hydrochloric or sullphuric acid, in the presence or absence of saccharose, at a temperature of 120° C. for 30 minutes. The results obtained are shown in Table 4, the awaited iron content being 3429 ppm. 
     
                       TABLE 4______________________________________Iron in solution (ppm)               Ferrous  Ferric  TotalSystem              iron     iron    iron______________________________________HCl 0.1 M            53      81       134HCl 0.1 M + 0.2 g saccharose                204     44       248HCl 0.1 M + 1 g saccharose                414     40       454HCl 1 M             --       3487    3434HCl 1 M + 0.2 g saccharose               2438     596     3034HCl 1 M + 1 g saccharose               2954     46      3000HCl 5 M              41      3342    3383HCl 5 M + 0.2 g saccharose               2058     720     2778HCl 5 M + 1 g saccharose               2124     54      2178H.sub.2 SO.sub.4 0.05 M                40      66       106H.sub.2 SO.sub.4 0.05 M + 0.2 g saccharose                113     45       158H.sub.2 SO.sub.4 0.05 M + 1 g saccharose                232     45       277H.sub.2 SO.sub.4 0.5 M               --       1840    1781H.sub.2 SO.sub.4 0.5 M + 0.2 g saccharose                885     1182    2067H.sub.2 SO.sub.4 0.5 M + 1 g saccharose               2339     69      2408H.sub.2 SO.sub.4 2.5 M               --       2459    2458H.sub.2 SO.sub.4 2.5 M + 0.2 g saccharose               1962     607     2569H.sub.2 SO.sub.4 2.5 M + 1 g saccharose               2425     65      2490______________________________________ 
    
     As can be seen, complete solubilization of the awaited iron resulted from treatment with 1M HCl. 2.5M sulphuric acid also gave very high solubilization. It is interesting to note the reducing action of the sugar, which transforms the ferric iron into ferrous, whereas in its absence the iron is present almost entirely in the form of ferric iron. As stated, converting the ferric iron into ferrous iron considerably facilitates the subsequent separation stages as the ferrous form is more stable in solution than the corresponding oxidized form. 
     EXAMPLE 3 
     100 ml of a 30% kaolin solution were treated with different H 2  SO 4  concentrations in an autoclave at 120° C. in the presence or absence of saccharose (concentration 2%), samples being taken every 20 minutes to measure the presence of iron in solution. The results obtained are given in Table 5 (awaited concentration for total solubilization 3450 ppm). 
     
                       TABLE 5______________________________________Iron in solution (ppm)       Time (minutes)       20   40      60      80    100______________________________________H.sub.2 SO.sub.4 9.06 mM          20     35      45    54    61H.sub.2 SO.sub.4 9.06 mM +          33     65     101   128   1530.22 g sacch.H.sub.2 SO.sub.4 44.5 mM          81    119     157   189   206H.sub.2 SO.sub.4 44.5 mM +         106    210     335   451   5580.22 g sacch.H.sub.2 SO.sub.4 227.3 mM         551    869     1048  1207  1291H.sub.2 SO.sub.4 227.3 mM +         672    1227    1794  2046  21940.22 g sacch.______________________________________ 
    
     As can be seen from the table, with sugar present and other conditions being equal, the iron concentrations in solution are higher. It can however be seen that at this temperature sulphuric acid concentrations of less than 0.25M were ineffective in promoting significant iron solubilization. 
     EXAMPLE 4 
     An examination was made of iron solubilization and total iron/ferric iron ratios in the presence of a reactive system of low acidity and low sugar concentration (H 2  SO 4  0.2M, saccharose 2 g/l), operating for different times on 30% kaolin suspensions at a temperature of 85° C. The results obtained are shown in Table 6. 
     
                       TABLE 6______________________________________     Time (h)     3    4          20     23______________________________________Fe total    568    --         983  1081Fe 3+       451    496        541   508______________________________________ 
    
     As can be seen, at this temperature the extractive process is considerably retarded. 
     EXAMPLE 5 
     The extractive process described in the preceding examples was conducted with different reactive systems and using beet molasses as reagent instead of saccharose, at a temperature of 120° C. for a reaction time of 30 minutes. The awaited iron concentration in solution is 3429 ppm. 
     
         ______________________________________Iron in solution (ppm)            Ferrous   Ferric   TotalSystem           iron      iron     iron______________________________________HCl 1 M           200      3100     3300HCl 1 M + 0.2 g molasses            2400       700     3100HCl 1 M + 1 g molasses            3130       20      3150H.sub.2 SO.sub.4 0.5 M             200      1700     1900H.sub.2 SO.sub.4 0.5 M + 0.2 g molasses            1150      1050     2200H.sub.2 SO.sub.4 0.5 M + 1 g molasses            2570       30      2600______________________________________ 
    
     As can be seen, the results obtained with molasses, a waste product, are equal to or even better than those obtained with saccharose.