Stable solution of sodium and iron silicate, process for preparing said solution and uses thereof

Abstract

The present invention refers to a stable sodium and iron silicate solution that has a weight ratio of SiO.sub.2 to Na.sub.2O from 1.5 to 2.5 and a total percentage of solids, expressed by the sum of SiO.sub.2 and Na.sub.2O, from 20% to 55%. Said solution also has a soluble iron content, expressed by Fe, from 0.1% to 7%, and a water content from 38% to 79.9%. The present invention also refers to the process for preparing said stable solution of sodium and iron silicate, which comprises the steps of: (a) providing a siliceous material containing iron; (b) submitting said siliceous material containing iron to a hydrothermal treatment with caustic soda under high temperature and controlled pressure; and (c) filtering said reacted solution to separate the reacted portion of the hydrothermal treatment from the unreacted portion. Additionally, the present invention refers to the uses of said stable sodium and iron silicate solution.

Claims

1. A process for preparing a stable sodium and iron silicate solution comprising the steps of: a. providing a crystallized siliceous material containing iron; b. submitting said crystallized siliceous material containing iron to a hydrothermal treatment with caustic soda under high temperature and controlled pressure; and c. filtering said reacted solution to separate the reacted portion of the hydrothermal treatment from the unreacted portion.

2. A process for preparing a stable sodium and iron silicate solution comprising the steps of: a. providing a siliceous material containing iron, the siliceous material has having a free humidity of less than 10%, and introducing the siliceous material containing iron into a batch tank with water and caustic soda at room temperature to form a mixture; b. submitting the mixture to a hydrothermal treatment under high temperature and controlled pressure transferring the mixture to a rotative autoclave or to a stirring vertical container, increasing the temperature to the range of 120° C. to 220° C. under saturated vapor pressure and hydrothermally treating the mixture for 3 to 6 hours under controlled pressure; and c. transferring the mixture to a retention tank to relieve pressure, cooling the batch to less than 90° C. and filtering the reacted portion of the mixture from the unreacted portion in a press filter using unreacted material as precoat.

3. The process of claim 2 wherein said precoat is washed later to recover Na.sub.2O, which is then recycled back into the treatment tank as a source of caustic soda, thereby reducing the amount of fresh caustic soda added to the next batch.

4. A process for preparing a stable sodium and iron silicate solution comprising the steps of: a. providing a siliceous material containing iron; b. submitting said siliceous material containing iron to a hydrothermal treatment with caustic soda under high temperature and controlled pressure; and c. filtering said reacted solution to separate the reacted portion of the hydrothermal treatment from the unreacted portion, wherein the stable sodium and iron silicate solution has a weight ratio of SiO.sub.2 to Na.sub.2O from 1.5 to 2.5 and a total percentage of solids, expressed by the sum of SiO.sub.2 and Na.sub.2O, from 20% to 55%, said solution having a soluble iron content, expressed by Fe, from 0.1% to 1.0%, and a water content from 38% to 79.9%.

5. The process of claim 4, wherein: a. the siliceous material has a free humidity of less than 10% and is introduced into a batch tank with water and caustic soda at room temperature; b. the mixture of the batch tank containing the siliceous material, water and caustic soda is transferred to a rotative autoclave or to a stirring vertical container and the temperature is increased to 120° C. to 220° C. under saturated vapor pressure and the batch is hydrothermally treated for 3 to 6 hours under controlled pressure; and c. the batch is transferred to a retention tank to relieve pressure, cooled to less than 90° C. and filtered into a press filter using unreacted material as precoat.

6. The process of claim 5 wherein said precoat is washed later to recover Na.sub.2O, which is then recycled back into the treatment tank as a source of caustic soda, thereby reducing the amount of fresh caustic soda added to the next batch.

7. A process for preparing a stable sodium and iron silicate solution comprising the steps of: a. providing a crystalline siliceous material containing iron; b. submitting said crystalline siliceous material containing iron to a hydrothermal treatment with caustic soda under high temperature and controlled pressure; and c. filtering said reacted solution to separate the reacted portion of the hydrothermal treatment from the unreacted portion, wherein the stable sodium and iron silicate solution has a weight ratio of SiO.sub.2 to Na.sub.2O from 1.5 to 2.5 and a total percentage of solids, expressed by the sum of SiO.sub.2 and Na.sub.2O, from 20% to 55%, said solution having a soluble iron content, expressed by Fe, from 0.1% to 1.0%, and a water content from 38% to 79.9%.

8. The process of claim 7, wherein: a. the siliceous material has a free humidity of less than 10% and is introduced into a batch tank with water and caustic soda at room temperature; b. the mixture of the batch tank containing the siliceous material, water and caustic soda is transferred to a rotative autoclave or to a stirring vertical container and the temperature is increased to 120° C. to 220° C. under saturated vapor pressure and the batch is hydrothermally treated for 3 to 6 hours under controlled pressure; and c. the batch is transferred to a retention tank to relieve pressure, cooled to less than 90° C. and filtered into a press filter using unreacted material as precoat.

9. The process of claim 8 wherein said precoat is washed later to recover Na.sub.2O, which is then recycled back into the treatment tank as a source of caustic soda, thereby reducing the amount of fresh caustic soda added to the next batch.

Description

PREFERRED EMBODIMENT OF THE INVENTION

(1) Although the present invention is described with reference to preferred embodiments, it will be understood by those skilled in the art that several changes can be made and the equivalents can be replaced by elements thereof.

(2) Sodium and iron silicate solutions can be produced through a hydrothermal treatment of siliceous material containing iron with caustic soda in a rotative or static autoclave stirred under high temperature and controlled pressure.

(3) In a preferred embodiment of the present invention, the siliceous material containing iron must be separated from water and have at least 65% crystalline silica. Some residual moisture may be present in the material if it can be handled by a bulk solids handling system.

(4) Preferably, the siliceous material containing iron should be mixed with caustic soda and water and hydrothermally treated at temperatures of 120° C. to 220° C. under saturated vapor pressure. The weight ratio of SiO.sub.2 to Na.sub.2O must be between 1.5 and 2.5 and any material that has not reacted must be filtered to produce a sodium and iron silicate solution. The solution should have a total percentage of solids, expressed by the sum of SiO.sub.2 and Na.sub.2O, between 20% and 55%, with a soluble iron content, expressed in Fe, from 0.1% to 7%, and the mass balance being water.

ADDITIONAL DESCRIPTION OF PROCESS CONDITIONS

Experiment 1

(5) Sodium silicate standards were stored at 80° C. in polypropylene vials. The formation of polysilicate was followed in time by measuring the turbidity (NTU) of silicate solutions after 0, 12, 23 and 40 days. After 40 days, silicate solutions were centrifuged. The solid polysilicate material was washed 3 times with demineralized water and was subsequently filtered. The solid polysilicate material was dried at 120° C. for 15 hours and the amount of polysilicate was weighed. The results of the experiment are shown in table 1.

(6) TABLE-US-00001 TABLE 1 Sodium Polysilicate silicate grade Turbidity (NTU) concentration (R = 3.3) Start 12 days 23 days 40 days after 40 days Filtered 1.4 36.4 78.3 1788 0.61% Unfiltered 15.4 47.3 238 >4000 1.95%

(7) Table 1 shows increased turbidity as measured by NTU over time. The results show that the unfiltered sodium silicate standard has a higher initial and final turbidity compared to the filtered sodium silicate standard. The degree of turbidity is correlated with the degree of polysilicate formation.

(8) It is possible to observe very fine flakes (polysilicates) above 40 NTU. After 40 days, visible solid polysilicates were measured and expressed. Unfiltered sodium silicate contained 1.95% polysilicate. Filtered sodium silicate contained 0.61% polysilicate. These results show that regardless of the type of sodium silicate and of whether sodium silicate has been filtered, the turbidity of a sodium silicate solution will significantly increase over time.

Experiment 2

(9) Three different siliceous sources were used to carry out an experiment to establish a weight ratio (SiO.sub.2:Na.sub.2O) from 2.0 to 2.2 of sodium silicate solution. All siliceous sources were hydrothermally treated in a rotative autoclave with 30% caustic soda at 200° C. and saturated vapor pressure for 4 hours. An excess of 10% of the siliceous source was used, and the resulting solution was filtered to obtain an impurity-free silicate solution.

(10) A siliceous material containing approximately 92% SiO.sub.2, 3% Fe, 0.6% Al2O.sub.2 and 4% H.sub.2O as moisture was used (Sample 1).

(11) The same siliceous material was first treated with a 50% HCl solution and washed. The pre-treatment was intended to solubilize and remove the iron from the sand source to obtain a colorless silicate solution (Sample 2). A source of crystalline sand (Beaujean) with >98% SiO.sub.2 (Sample 3) was used as a control source.

(12) The resulting products produced in samples 1, 2 and 3 were analyzed, and their constituents were determined. The traces of non-dissolved metals in sand sources and sodium silicate solutions are listed in table 2 below:

(13) TABLE-US-00002 TABLE 2 Metals Siliceous or Beaujean iron Sample Sample Sample oxides Sand source 1 2 3 Al.sub.2O.sub.3 [ppm] 937 6,000 1,905 956 446 CaO [ppm] 150 130 67 57 52 MgO [ppm] 81 83 33 30 27 Fe [ppm] 141 30,000 545 46 27

(14) Table 2 shows that the sodium silicate solution from untreated siliceous source had a high content of soluble iron. With pretreatment with HCl, a significant portion of the iron was washed, resulting in a limited amount of iron in the silicate solution. However, the iron content of the solutions produced in Experiments 1 and 2 was higher than in the control sand. The level of calcium and magnesium, precursors of formation of polysilicate and polymeric micelles, was equivalent in the three silicate solutions. Calcium and magnesium levels in the control sand (Beaujean) are considered medium to high as potential precursors for the formation of polymeric micelles and polysilicates.

(15) The silicate solution resulting from each experiment was measured for determining its turbidity and color. The results of these measurements are expressed in table 3 below:

(16) TABLE-US-00003 TABLE 3 Turbidity CIELAB CIELAB CIELAB Sample (NTU) Color L Color a Color b #1 Iron Ore 73.3 21.93 23.95 36.70 #2 Treated Iron Ore 29.8 80.11 6.24 5.87 #3 Beaujean Sand 5.8 89.83 2.61 10.03

(17) Table 3 shows that the sodium and iron silicate solution had a distinct brownish red tone. The solution of treated siliceous material had a light pink color. The control sand had a water appearance.

(18) The silicate solution resulting from each experiment was stored under static conditions, at a temperature of 60° C., to collect data on the long-term storage stability. The results are listed in table 4, below:

(19) TABLE-US-00004 TABLE 4 Sample Turbidity No. Silicate Solution Start of test (NTU) Week 8 (NTU) 1 Silicesous material 73.3 75.2 2 Treated siliceous 29.8 37.1 material 3 Sand (Beaujean) 5.8 38.3

(20) Table 4 shows that the sodium silicate solution obtained from untreated iron ore is stable at a temperature of 60° C. for 8 weeks under static storage conditions, i.e., turbidity is in the same order of magnitude before and after the static storage stability test. Treated iron ore has a considerable increase in turbidity. The control solution of sand silicate had a significant increase in turbidity, indicating long-term storage instability.

(21) The experiments above demonstrate that the presence of iron in a sodium silicate solution provides long-term stability. Such sodium and iron silicate solutions can provide a stable shelf-life solution for applications where the color of sodium silicate solution is not a concern.

(22) Although the description above contains certain specificities, they should not be interpreted as limitations to the scope of the invention, but as an example of a preferred embodiment of the same. Therefore, the scope of the present invention must not be determined by the embodiments illustrated, but by the attached set of claims and its legal equivalents.