Stable solutions of sodium and iron silicates, methods for preparing such solutions and uses thereof

Abstract

The present invention relates to stable sodium silicate and iron silicate solutions having a SiO2 to Na2O weight ratio of 1.5 to 2.5 and a total percentage of solids, expressed as the sum of SiO2 and Na2O, of 20% to 55%. The solutions also have a soluble iron content, expressed as Fe, of 0.1% to 7% and a water content of 38% to 79.9%. The present invention also relates to a method for preparing stable solutions of sodium silicate and iron silicate solutions, comprising the steps of: (a) providing an iron-containing siliceous material; (b) subjecting the iron-containing siliceous material to hydrothermal treatment with caustic soda at elevated temperature and controlled pressure; and (c) filtering the reaction solution to separate the reacted portion of the hydrothermal treatment from the unreacted portion. Furthermore, the present invention relates to uses of the stable sodium silicate and iron silicate solutions.

Description

[Technical Field]

[0001] The disclosed and claimed invention relates to stable solutions of sodium and iron silicates. The long-term stability of the solutions is achieved by introducing soluble iron into the silicates by extracting silica from a source that contains some iron in its composition. The invention also relates to methods for preparing the stable solutions of sodium and iron silicates, their uses, and applications. [Background technology]

[0002] Iron ore and kaolin processing processes result in large amounts of tailings. These tailings essentially contain ore, sand, and water and are a concern for the mining industry due to the significant impact they can have on the environment if not properly treated. Therefore, it is of utmost importance to efficiently reuse these tailings to reduce their environmental impact as much as possible. One possible effective reuse of the above-mentioned tailings is the production of a stable solution of sodium silicate and iron silicate, which is the subject of this patent application.

[0003] Sodium silicate solutions are known in the industry to be highly versatile products with multiple uses. They can be produced as clear, low-turbidity solutions by applying a filtration step. Other forms of silicate are unfiltered and therefore exhibit some turbidity, presenting as cloudy solutions. Long-term stability is typically a potential issue for all silicate products. Impurities in silicate solutions, such as calcium and / or magnesium, can act as seeds leading to the formation of polymeric micelles and polysilicates. Such forms of polysilicate are long polymer chains that can precipitate in solution, causing solid settling over time, while polymeric micelles remain suspended but can increase turbidity. Storage temperatures above 40°C can also contribute to the accelerated formation of polysilicate or polymeric micelles. It is known in the industry that clear (filtered) sodium silicate solutions have higher storage stability than cloudy (unfiltered) solutions.

[0004] Turbidity is a measure of water clarity and is related to the amount of suspended solids present in the water. Clear water has a low turbidity value, while murky or polluted water has a high turbidity value. For example, a river may have highly colored water at certain times of the year, but this should not be confused with turbidity. Water may contain a significant amount of color that appears brown based on measurements from Nephelometric Turbidity Units (NTU) and still have very good clarity or low turbidity. NTU is a well-known unit of measurement in the state of the art and accepted by industry for turbidity, determined by an optical measurement of water's ability to scatter and absorb light rather than transmit it linearly.

[0005] Sodium silicate solution can be produced by directly fusing a sand source with sodium carbonate in an oven at 1,500°C. The molten sodium silicate is cooled in the oven, and the resulting glass is subsequently dissolved with steam and temperature / pressure to obtain a sodium silicate solution. This solution can be filtered using a filter aid to produce a transparent silicate, or it can be used without filtration due to the inherent turbidity of the solution. The typical SiO2 to Na2O mass ratio through the oven process is in the range of 1.6 to 3.5.

[0006] Sodium silicate solutions can also be produced by hydrothermal dissolution of a sand source with caustic soda at temperature and pressure using a rotary or stirred static autoclave. This process is usually carried out with excess sand, followed by a final filtration step to recover and reuse the excess sand, removing unreacted sand to produce a clear solution. In this case, the unreacted sand acts as a filter medium.

[0007] The production of sodium silicate solutions by the hydrothermal reaction of sand with caustic soda is known from the state of the art to be limited by the sand silica source. Typically, sand sources that are mostly crystalline in nature allow an operating window of SiO2 to Na2O mass ratios in the range of 1.5 to 2.5. The introduction of silica sources from cristobalite or heat-treated sand can extend the operating window of SiO2 to Na2O mass ratios up to 3.7.

[0008] U.S. Pat. No. 4,770,866 describes a process for the production of a clear sodium silicate solution by hydrothermal synthesis having a SiO2 to Na2O weight ratio of 1.9 to 2.1, which involves reacting 5 to 10% excess sand with at least 99% by weight of SiO2 and 25 to 35% sodium hydroxide solution in a rotating cylindrical pressure reactor at a temperature of 200 to 250°C and saturated vapor pressure, followed by filtration to separate the excess sand from the silicate using perlite as a filter aid.

[0009] EP 0033108 describes a process for producing sodium silicate solutions having a SiO2 to Na2O weight ratio of 1.0 to 2.8 by reacting sand with 20 to 30% sodium hydroxide solution in a rotating cylindrical pressure reactor at temperatures of 170 to 250 °C and saturated vapor pressure. Excess sand, up to 300% based on the SiO2 to Na2O molar ratio in the filtered mixture, is present; unreacted sand is recycled to the process. The excess sand acts as a filter medium.

[0010] U.S. Pat. No. 5,000,933 describes a method for producing aqueous sodium silicate solutions by hydrothermal treatment in a sealed pressure reactor at a temperature of 150°C to 300°C and saturated vapor pressure, using a silica source containing at least 50% cristobalite or a silica source produced by sand heat treatment at a temperature of at least 1,100°C but below the melting point of silica, with a weight ratio of SiO2 to Na2O of 2.9 to 3.7.

[0011] Sodium silicate solutions can also be produced from siliceous sources that contain sufficient silica to react with alkali (caustic soda or sodium carbonate). As previously mentioned, one of these sources may be iron-containing siliceous materials such as kaolin processing tailings or flotation tailings for iron ore processing. These siliceous materials are typically provided as water pastes in settling ponds and contain a reasonable amount of crystalline silica that can be digested with an alkali treatment.

[0012] U.S. Patent No. 3,163,518 describes a method for liberating silica from siliceous iron ore to obtain a more refined iron ore by exposing the ore to sodium hydroxide in an amount of 1 to 14% by weight and subjecting the mixture to temperatures of 260°C to 400°C and pressures of 7 bar to atmospheric pressure. The solution is hydrothermally treated under these conditions, maintaining a steam atmosphere under saturated or superheated conditions, followed by cooling to reduce the temperature and further separating the solution and unreacted materials. Summary of the Invention

[0013] Surprisingly, sodium silicate solutions made from iron-containing siliceous materials have been found to have long-term stability. Digestion of this siliceous material produces sodium silicate and iron silicate with a brownish-red color due to the presence of soluble iron compounds and turbidity. Due to these characteristics, sodium silicate and iron silicate solutions would not be expected to be stable under long-term storage conditions. However, stability tests conducted on static samples maintained at a temperature of 60°C for 8 weeks showed no apparent change in initial turbidity or the formation of settled solid deposits. The same stability study conducted on silicate made with crystalline silica from a refined sand source with a SiO2 content greater than 98% by weight resulted in a significant increase in turbidity over the same period. The same stability study conducted on silicate made with iron-containing siliceous materials pretreated to remove iron prior to hydrothermal digestion also showed a reasonable increase in turbidity over the same period (see Table 4 below).

[0014] The long-term stability of iron-containing siliceous materials can be attributed to the presence of solution-soluble iron compounds that prevent the formation of polymeric micelles and polysilicates over time; this type of silicate solution differs from other types of crystalline sand sources with low iron content. Thus, the presence of iron is an inhibitor of the formation of polymeric compounds from calcium and magnesium.

[0015] The stable solutions of sodium and iron silicates that are the object of the present invention are preferably prepared by the following steps: a. providing an iron-containing siliceous material; b. subjecting the iron-containing siliceous material to hydrothermal treatment with caustic soda at elevated temperature and controlled pressure; c. filtering the reaction solution to separate the reacted portion of the hydrothermal treatment from the unreacted portion; The compound is obtained from a method comprising:

[0016] More specifically, the method comprises the following steps: a. introducing a siliceous material containing iron and having a free moisture content of less than 10% into a batch tank together with water and caustic soda at room temperature; b. transferring the mixture of the siliceous material, water and caustic soda in the batch tank into a rotary autoclave or a stirred vertical vessel, raising the temperature to 120°C-220°C under saturated steam pressure, and hydrothermally treating the batch under controlled pressure for 3-6 hours; c. transferring the batch to a holding tank, releasing the pressure, cooling to below 90°C, and filtering through a press filter using the unreacted material as a precoat; Includes:

[0017] The precoat can later be washed to recover the Na2O, which can then be recycled to the treatment tank as a source of caustic soda, thereby reducing the amount of fresh caustic soda added to the next batch.

[0018] Although they have some color and turbidity, those skilled in the art will find many potential uses for the sodium silicate and iron silicate solutions of the present invention. One such use may be as a binder for fines where color is not a disqualifying condition. The binder can be used in pelletizing, briquetting, or similar techniques.

[0019] Another use of sodium silicate and iron silicate is in the production of precipitated silica, where color is not a disqualifying condition. One of the most important markets for precipitated silica is in rubber reinforcement, where it is widely used in green tires.

[0020] Another use of sodium silicate and iron silicate can be in the foundry industry as a binder for sand molds, in the ceramic industry as a deflocculating agent, and in refractories as a binder for clays and adhesives.

[0021] Another use of sodium silicate and iron silicate could be in the production of silica gel, where color is not a disqualifying condition. Some potential markets for this type of silica gel could be in biodiesel processing and as a flow promoter. DETAILED DESCRIPTION OF THE INVENTION

[0022] Although the present invention has been described with reference to preferred embodiments, those skilled in the art will recognize that certain modifications may be made and equivalents may be substituted for elements thereof.

[0023] Sodium silicate and iron silicate solutions can be produced by hydrothermal treatment of iron-containing siliceous materials with caustic soda in stirred rotary or static autoclaves under elevated temperature and controlled pressure.

[0024] In a preferred embodiment of the present invention, the iron-containing siliceous material is separated from water and must have at least 65% crystalline silica. Some residual moisture may be present in the material if it can be handled by bulk solids handling systems.

[0025] Preferably, the iron-containing siliceous material is mixed with caustic soda and water and hydrothermally treated at temperatures between 120°C and 220°C under saturated vapor pressure. The weight ratio of SiO2 to Na2O should be between 1.5 and 2.5, and unreacted material should be filtered to produce a sodium silicate and iron silicate solution. The solution should have a total percentage of solids, expressed as the sum of SiO2 and Na2O, between 20% and 55%, a soluble iron content, expressed as Fe, between 0.1% and 7%, and the mass balance should be water.

[0026] Additional explanation of processing conditions Experiment 1 Sodium silicate standards were stored in polypropylene vials at 80°C. The formation of polysilicate was followed over time by measuring the turbidity (NTU) of the silicate solution after 0, 12, 23, and 40 days. After 40 days, the silicate solution was centrifuged. The solid polysilicate material was washed three times with demineralized water and 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. [Table 1]

[0027] Table 1 shows the increase in turbidity as measured by NTU over time. The results show that the unfiltered sodium silicate standard has higher initial and final turbidity compared to the filtered sodium silicate standard. Turbidity correlates with the degree of polysilicate formation.

[0028] Very fine flakes (polysilicate) of over 40 NTU can be observed. After 40 days, the visible solid polysilicate was measured and shown to be: Unfiltered sodium silicate contained 1.95% polysilicate; filtered sodium silicate contained 0.61% polysilicate. These results show that the turbidity of sodium silicate solutions increases significantly over time, regardless of the type of sodium silicate and whether the sodium silicate was filtered or not.

[0029] Experiment 2 Three different silicate sources were used to conduct experiments to establish a weight ratio (SiO:NaO) of 2.0–2.2 for sodium silicate solutions. All silicate sources were hydrothermally treated at 200 °C and saturated vapor pressure for 4 h in a rotating autoclave containing 30% caustic soda. When more than 10% of the silicate source was used, the resulting solution was filtered to obtain impurity-free silicate solutions.

[0030] A siliceous material containing approximately 92% SiO2, 3% Fe, 0.6% Al2O2, and 4% H2O as moisture was used (Sample 1).

[0031] The same siliceous material was first treated with a 50% HCl solution and washed. The pretreatment aimed to solubilize and remove iron from the sand source, resulting in a colorless silicate solution (Sample 2). A crystalline sand source (Beaujean) containing more than 98% SiO2 (Sample 3) was used as a control source.

[0032] The products obtained in Samples 1, 2 and 3 were analyzed to determine their components. Trace amounts of undissolved metals in the sand source and sodium silicate solution are listed in Table 2 below. [Table 2]

[0033] Table 2 shows that the sodium silicate solutions from the untreated siliceous source had a high content of soluble iron. Pretreatment with HCl washed out a significant portion of the iron, limiting the amount of iron in the silicate solution. However, the iron content of the solutions produced in Experiments 1 and 2 was higher than that of the control sand. The concentrations of calcium and magnesium, which are precursors for polysilicate and polymeric micelle formation, were comparable in the three silicate solutions. The concentrations of calcium and magnesium in the control sand (Beaujean) are considered moderate to high, suggesting that they are potential precursors for polymeric micelle and polysilicate formation.

[0034] The silicate solution obtained from each experiment was measured to determine its turbidity and color. The results of these measurements are shown in Table 3 below. [Table 3]

[0035] Table 3 shows that the sodium silicate and iron silicate solutions had a distinct brownish-red color. The solutions of the treated siliceous materials had a light pink color. The control sand had a watery appearance.

[0036] The silicate solutions obtained from each experiment were stored under static conditions at a temperature of 60° C. to collect data on long-term storage stability. The results are listed in Table 4 below. [Table 4]

[0037] Table 4 shows that the sodium silicate solution obtained from untreated iron ore is stable for 8 weeks at 60°C under static storage conditions, i.e., the turbidity is similar before and after the static storage stability test. Treated iron ore shows a significant increase in turbidity. The control solution of silica sand shows a significant increase in turbidity, indicating long-term storage instability.

[0038] The above experiments demonstrate that the presence of iron in sodium silicate solutions provides long-term stability. Such sodium silicate and iron silicate solutions can provide stable, shelf-life solutions for applications where the color of the sodium silicate solution is not an issue.

[0039] Although the above description contains certain specificities, they should not be construed as limitations on the scope of the invention, but as examples of preferred embodiments of the invention. Accordingly, the scope of the invention should be determined not by the embodiments shown, but by the appended claims and their legal equivalents. Although overlapping with other descriptions, various aspects of the present invention are described below, however, the present invention is not limited to the following. [1] 1.5 to 2.5 SiO 2 Against Na 2 Weight ratio of SiO, and 2 and Na 2 A stable sodium silicate and iron silicate solution having a total percentage of solids, expressed as the sum of O, of 20% to 55%, a soluble iron content, expressed as Fe, of 0.1% to 7%, and a water content of 38% to 79.9%. [2] 10. A stable sodium silicate and iron silicate solution according to [1], wherein the solution exhibits relatively minimal growth of polysilicate deposits under long-term storage conditions. [3] The solution contains at least 65% SiO 2 and a stable sodium silicate and iron silicate solution according to [1], prepared from a siliceous source containing at least 1% Fe. [4] The stable sodium silicate and iron silicate solution according to [3], wherein the siliceous source is mixed with caustic soda and water and hydrothermally treated at a temperature in the range of 120°C to 220°C under saturated vapor pressure. [5] [4] The stable sodium silicate and iron silicate solution of [4], wherein the hydrothermally treated solution is filtered to remove unreacted materials, producing a sodium silicate solution that is generally free of impurities. [6] 1. A method for preparing stable sodium silicate and iron silicate solutions, comprising: a. providing an iron-containing siliceous material, said siliceous material having a free humidity of less than 10%; b. subjecting said siliceous material containing iron to a batch tank at room temperature for hydrothermal treatment with caustic soda under elevated temperature and controlled pressure to form a mixture; c. transferring the mixture from the batch tank to either a rotary autoclave or a stirred vertical vessel, raising the temperature to a range of 120°C to 220°C under saturated steam pressure, and hydrothermally treating the batch under controlled pressure for 3 to 6 hours; d. transferring the mixture to a holding tank, releasing the pressure, and cooling to below 90°C; e. filtering the reaction solution through a press filter using unreacted material as a precoat to separate the reacted portion of the hydrothermal treatment from the unreacted portion; Including, The precoat is then washed to remove Na 2 The method of claim 1, wherein the O is recovered and then recycled to the treatment tank as a source of caustic soda, thereby reducing the amount of fresh caustic soda added to the next batch. [7] 1. The stable sodium silicate and iron silicate solution according to [1], wherein the solution is used as a binder for fine powders. [8] 1. The stable sodium silicate and iron silicate solution according to [1], wherein the solution is used for the production of precipitated silica. [9] 1. The stable sodium silicate and iron silicate solution according to [1], wherein the solution is used as a binder for sand moulds, as a deflocculating agent in the ceramic industry, or as a binder for clays and adhesives.

[10] [1] A stable sodium silicate and iron silicate solution according to [1], wherein the solution is used for producing silica gel. 。

Claims

1. 1.5 to 2.5 SiO ２ Against Na ２ The weight ratio of O, and SiO ２ and Na ２ A crystalline stable sodium and iron silicate solution having a total percentage of solids, expressed as the sum of O, of 20% to 55%, a soluble iron content, expressed as Fe, of 0.1% to 7%, and a water content of 38% to 79.9%.

2. 10. The stable sodium silicate and iron silicate solutions of claim 1, wherein the solutions exhibit relative stability as measured by maintaining the same order of magnitude of turbidity under long-term storage conditions for at least 8 weeks.

3. 1. A method for preparing stable sodium silicate and iron silicate solutions, comprising: a. providing an iron-containing crystalline siliceous material, said siliceous material having a free humidity of less than 10%; b. subjecting said crystalline siliceous material containing iron to a batch tank at room temperature for hydrothermal treatment with caustic soda under elevated temperature and controlled pressure to form a mixture; c) transferring the mixture from the batch tank to either a rotary autoclave or a stirred vertical vessel, raising the temperature to a range of 120°C to 220°C under saturated steam pressure, and hydrothermally treating the mixture under controlled pressure for 3 to 6 hours; d. Transferring the mixture to a holding tank, releasing the pressure, and cooling to below 90°C; e. filtering the mixture through a press filter using unreacted material as a precoat to separate the reacted portion of the hydrothermal treatment from the unreacted portion; Including, The precoat is then washed to remove Na ２ The method of claim 1, wherein the O is recovered and then recycled to the batch tank as a source of caustic soda, thereby reducing the amount of fresh caustic soda added to the next batch of siliceous material to be treated.

4. 10. The stable sodium silicate and iron silicate solution of claim 1, wherein the solution is used as a binder for fine powders.

5. 10. The stable sodium silicate and iron silicate solution of claim 1, wherein the solution is used to produce precipitated silica.

6. 10. The stable sodium silicate and iron silicate solution of claim 1, wherein the solution is used as a binder for sand molds, as a deflocculating agent in the ceramic industry, or as a binder for clays and adhesives.

7. 10. The stable sodium silicate and iron silicate solution of claim 1, wherein the solution is used to produce silica gel.

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