Process for removing iron from kaolin, quartz and other mineral concentrates of industrial interest

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 in a sulphuric acid medium (pH < 3). The Fe⁺⁺⁺ is reduced to Fe⁺⁺ and hence solubilized as FeSO₄, which is then removed by pressure filtration (see in this respect FR-A-2030730; US-A-3528759; RO-A-49334; SU-A-485093; SU-A-628087; "Formation of aggressive substance in East German caolins", 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%. DE-A-2338937 discloses a process for bleaching ceramic minerals such as corondum and garnet, by suspending them in a stirred, hydrous chloride acid solution at a temperature up to 70°C, followed by collecting the deferrized residue by filtration. Said document however neither teach nor suggest the use of sugar in the bleaching of minerals. Detailed description of the invention The present invention enables the iron present as ferric iron (Fe³⁺) 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) 2/3 A[Fe₃(SO₄)₂(OH)₆] + 2/3 SO₄2- + H⁺ ----> ----> Fe(OH)₃ + Fe(SO₄)₂- + H₂O + 2/3 A⁺ where A⁺ can be K⁺, Na⁺, NH₄⁺ and H⁺. 2) 24 Fe³⁺ + C₆H₁₂O₆ + 6 H₂O ----> 24 Fe²⁺ + 6 CO₂ + 24 H⁺ 3) C₆H₁₂O₆ + 12 Fe₂O₃ + 48 H⁺ ----> 24 Fe²⁺ + 30 H₂O + 6 CO₂ 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 inerting 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 4 M 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, H₂SO₄, H₃PO₄ 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. EXAMPLE 1 Three aqueous suspensions (0.5 l) made acid by sulphuric acid (concentration 1 M) 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. Iron solubilization (ppm) Time (h) 6 24 48 40°C solution 79.8 148 370 50°C solution 430 707 687 60°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. 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 sulphuric 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. Iron in solution (ppm) System Ferrous iron Ferric iron Total iron HCl 0.1 M 53 81 134 HCl 0.1 M + 0.2 g saccharose 204 44 248 HCl 0.1 M + 1 g saccharose 414 40 454 HCl 1 M - 3487 3434 HCl 1 M + 0.2 g saccharose 2438 596 3034 HCl 1 M + 1 g saccharose 2954 46 3000 HCl 5 M 41 3342 3383 HCl 5 M + 0.2 g saccharose 2058 720 2778 HCl 5 M + 1 g saccharose 2124 54 2178 H₂SO₄ 0.05 M 40 66 106 H₂SO₄ 0.05 M + 0.2 g saccharose 113 45 158 H₂SO₄ 0.05 M + 1 g saccharose 232 45 277 H₂SO₄ 0.5 M - 1840 1781 H₂SO₄ 0.5 M + 0.2 g saccharose 885 1182 2067 H₂SO₄ 0.5 M + 1 g saccharose 2339 69 2408 H₂SO₄ 2,5 M - 2459 2458 H₂SO₄ 2,5 M + 0.2 g saccharose 1962 607 2569 H₂SO₄ 2.5 M + 1 g saccharose 2425 65 2490 As can be seen, complete solubilization of the awaited iron resulted from treatment with 1 M HCl. 2.5 M 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 H₂SO₄ 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). Iron in solution (ppm) Time (minutes) 20 40 60 80 100 H₂SO₄ 9.06 mM 20 35 45 54 61 H₂SO₄ 9.06 mM + 0.22 g sacch. 33 65 101 128 153 H₂SO₄ 44.5 mM 81 119 157 189 206 H₂SO₄ 44.5 mM + 0.22 g sacch. 106 210 335 451 558 H₂SO₄ 227.3 mM 551 869 1048 1207 1291 H₂SO₄ 227.3 mM + 0.22 g sacch. 672 1227 1794 2046 2194 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 0.25 M 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 (H₂SO₄ 0.2 M, 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. Time (h) 3 4 20 23 Fe total 568 - 983 1081 Fe 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) System Ferrous iron Ferric iron Total iron HCl 1 M 200 3100 3300 HCl 1 M + 0.2 g molasses 2400 700 3100 HCl 1 M + 1 g molasses 3130 20 3150 H₂SO₄ 0.5 M 200 1700 1900 H₂SO₄ 0.5 M + 0.2 g molasses 1150 1050 2200 H₂SO₄ 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.

Claims: A process for bleaching kaolin, quartz, titanium minerals, ceramic minerals for glass, paper and electronics use and other like materials, in which the suitably ground material is suspended in an acid solution , said suspension is kept stirred with or without heating, and the deferrized residue is finally collected by filtration, characterized by the fact that said acid solution contains a low sugar concentration. A process as claimed in claim 1, wherein the acid concentration is between 0.1 and 4 M. A process as claimed in claim 2, wherein the concentration of the material to be bleached is up to 35% by weight of the solution volume. A process as claimed in claim 3, wherein the reaction temperature is between 20°C and 120°C. A process as claimed in claim 4, wherein the reaction time is up to 48 hours. A process as claimed in claims 1 to 5, wherein the acid is HCl. A process as claimed in claim 6, wherein HCl is present in a concentration of between 0.1 M and 5 M. A process as claimed in claims 1 to 5, wherein the acid is H₂SO₄. A process as claimed in claim 8, wherein H₂SO₄ is present in a concentration of between 0.1 M and 4 M. A process as claimed in claims 1 to 9, wherein the sugar is a sugar containing 5 or more carbon atoms, spent milk whey or beet molasses. A process as claimed in claim 10, wherein the sugar with 5 or more carbon atoms is glucose, fructose or saccharose. A process as claimed in claim 11, wherein the sugar is saccharose. A process as claimed in claims 1 to 12, wherein the sugar is present in a concentration of 0.5-4 g/l. A process as claimed in claims 1 to 13, wherein the acid used is sulphuric acid in a concentration of 0.1-4 M, the sugar is beet molasses, the extraction temperature is 120°C and the time employed for the extractive process is 30 minutes. A process as claimed in claims 1 to 13, wherein the acid used is hydrochloric acid in a concentration of 0.1-5 M, the sugar is beet molasses, the extraction temperature is 120°C and the time employed for the extractive process is 30 minutes.

Inventors: MARABINI ANNA MARIA, PAPONETTI BRUNO, PASSARIELLO BRUNO, TORO LUIGI, MARABINI, ANNA MARIA, PAPONETTI, BRUNO, PASSARIELLO, BRUNO, TORO, LUIGI
Applicants: CONSIGLIO NAZIONALE RICERCHE, CONSIGLIO NAZIONALE DELLE RICERCHE
Application year: 1991
Main classification: C04B33
IPC codes: C04B33, C09C1