Method for producing alkali metal silicates with reduced metal content

Abstract

This invention relates to a method for producing alkali metal silicates in which heavy metals are reduced in a high-temperature furnace melting process by using sustainable waste sand or recycled sand as raw material. This invention also relates to the use of waste sand, spent sand, or recycled sand (e.g., foundry waste sand) alone or in blend with virgin sand for the production of alkali metal silicates.

Description

Technical Field

[0001] The present invention relates to a method for producing alkali metal silicates by removing heavy metals from alkali metal silicates during a high-temperature furnace melting process by using sustainable silica raw materials. The present invention also relates to the use of waste sand, used sand, or recycled sand (e.g., foundry waste sand) alone or blended with virgin sand for producing alkali metal silicates.

Background Art

[0002] The present invention relates to the field of production of alkali metal silicates by using waste sand or recycled sand under a high-temperature furnace melting process.

[0003] In recent years, there has been an increasing interest in using sustainable raw materials in the production of alkali metal silicates. Those sustainable silica raw materials protect the ecosystem, environmental assets, and sustainable supply.

[0004] Therefore, an object of the present invention is to provide a sustainable silica raw material for producing alkali metal silicates.

[0005] Precipitated silica is typically produced by the reaction of an alkali metal silicate solution, such as sodium silicate, with a mineral acid, such as sulfuric acid. The acid and sodium silicate are added to water while stirring both together. Precipitation occurs, and the precipitated silica is recovered by filtration, and the remaining sodium sulfate is washed and dried.

[0006] The main material used in the production of precipitated silica is an alkali metal silicate, such as sodium silicate. Sodium silicate can be produced by either the reaction of sand with a caustic alkali aqueous solution (hydrothermal) or a furnace process in which sand and sodium carbonate are reacted at a temperature above 1000°C. Both processes use sand as the main raw material. Although sand is one of the most abundant materials on earth, it is mined and is not considered a sustainable material.

[0007] Furthermore, it is known that significant amounts of such heavy metals must be removed in order to produce alkali metal silicates (e.g., sodium silicate) in order to meet specific regulatory requirements for the final precipitated silica. Alkali silicate materials are required to exhibit low levels of heavy metal contamination.

[0008] This invention provides a novel sustainable raw material and manufacturing method for producing alkali metal silicates from raw materials known to contain high amounts of heavy metals.

[0009] It has been found that recycled sand or waste sand can be used as a sustainable raw material in high-temperature furnace processes to produce alkali metal silicates with reduced metal content.

[0010] Sustainable waste foundry sand (WFS) is a by-product of the foundry and metal casting industries.

[0011] Approximately 100 million tons of WFS (Whole Fibre Sand) are generated worldwide each year in the foundry industry. According to EC regulations, as stated in "Hinghofer-Szalkay D., Koch BA European Tort Law 2008. Springer; Berlin / Heidelberg, Germany: 2009, European Union, pp. 647-657," WFS is classified as harmless waste because, even if the total metal concentration in waste sand increases compared to virgin sand, it generally remains low.

[0012] Scientific research has shown and proven that WFS (Wood Freeze) is classified as a sustainable raw material that can replace natural river sand in concrete production.

[0013] In a typical foundry process, foundry sand refers to clean, uniformly sized, high-quality silica sand used in the casting process. The sand is bound together to form a mold, core, or pattern, which is then used to manufacture castings of ferrous or non-ferrous metals.

[0014] Foundries have successfully recycled and reused sand multiple times. When sand can no longer be reused in a foundry, it is removed from the foundry and considered waste foundry sand (WFS), used foundry sand (UFS), spent foundry sand (SFS), or even recycled foundry sand. In this invention, the term waste sand (particularly WFS) is used for consistency, but is not specifically limited to sustainably recycled sand, waste sand, or used sand.

[0015] In particular, there are studies characterizing the physical and chemical composition, fresh properties, mechanical performance, and durability of WFS concrete in its use. These characteristics can vary considerably from one foundry to another.

[0016] Over the past several decades, several studies have been conducted to investigate the effects of adding foundry waste sand as a partial or complete replacement for conventional sand in concrete.

[0017] In the art, it is well known that the use of furnace processes works well in the production of alkali metal silicates, particularly sodium silicate, by using high temperatures and controlling the stoichiometric amounts of sand and caustic alkali. One well-known example is described in European Patent Application Publication No. 1787960 (EP 1 787 960 A1) by HUBE.

[0018] There are publications on the properties and uses of WFS, but most of them concern the preparation and processing of mortar, cement, concrete, or other construction materials.

[0019] A method for co-treating granular waste incinerated as fly ash in a cement kiln is proposed in Chinese Patent Application Publication No. 111732353 (CN 111732353 A).

[0020] Other applications, as described in Chinese Patent Application Publication No. 103143672 (CN 103143672 A), refer to the use of used water glass for casting purposes. The said application discloses a method for manufacturing water glass for use in casting purposes.

[0021] Another patent application, International Publication 2015 / 187778 (WO 2015 / 187778 A1), also discloses the stabilization of at least one heavy metal contained in sodium-containing fly ash using a water-soluble silicate raw material and a material containing calcium and / or magnesium.

[0022] Another paper by Mohamed Ismail Abdul Aleem et al. (published in the Journal of Cleaner Production, Elsevier Amsterdam NL, vol. 264, April 18, 2020, XP086164598, ISSN: 0959-6526, DOI: 10.1016 / J.JCLEPRO.2020.121689) deals with the treatment of consumable foundry sand used as a raw material for the synthesis of sodium silicate. However, this paper specifically focuses on removing phenolic carbon and metallic impurities from consumable foundry sand before the hydrothermal process by calcining and subsequently bleaching it, which can require a lot of additional energy.

[0023] Indian Patent Application No. 201941026558 (IN 201941026558) discloses a method for producing alkali metal silicates from consumable foundry sand via a hydrothermal method at temperatures of 150 to 300°C.

[0024] None of the prior art disclosures disclose the use of WFS to produce alkali metal silicates by using a furnace melting process that utilizes high temperatures, and that as a result, the silica content is low, the molar ratio is high, and filtration to remove insoluble metal impurities is easy.

Prior Art Documents

Patent Documents

[0025]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Non-Patent Documents

[0026]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0027] Therefore, there is a need to find a sustainable raw material for producing alkali metal silicate under suitable process conditions that lead to less waste and a high molar ratio of silica / alkali metal.

Means for Solving the Problems

[0028] Summary of the Invention An advantage of the present invention is to provide a sustainable silica raw material for producing an alkali metal silicate with a reduced metal content that can be easily filtered.

[0029] After a thorough investigation, the inventors of the present invention unexpectedly found that the use of waste sand or recycled sand (e.g., foundry waste sand) in the furnace process for the production of alkali metal silicates as defined in claim 1 solves the above technical problems because it provides a sustainable silica raw material for the production of alkali silicates and enables the production of silica / alkali metal with a higher molar ratio, less waste, and reduced heavy metal content.

[0030] Advantageously, the use of waste sand or recycled sand offers a favorable environmental impact in the sustainable reuse of materials.

[0031] According to a first aspect, the present invention relates to a method for producing alkali metal silicate material (water glass) by using waste sand alone or in combination with virgin sand (e.g., foundry waste sand or a mixture thereof with virgin sand) via a furnace process defined in claim 1.

[0032] A second aspect of the present invention is the use of waste sand or recycled sand as a sustainable silica raw material (or raw material) for producing alkali metal silicates with reduced heavy metal content.

[0033] A third aspect of the present invention is the use of alkali metal silicates (water glass) produced by using waste sand in a furnace process defined in claim 1, in which sand is reacted with sodium carbonate at a temperature exceeding 1000°C, for the production of precipitated silica.

[0034] The present invention provides a direct process using a high-temperature furnace method in a solid state to produce alkali metal silicates, particularly sodium silicate, which can be dissolved and filtered without problems, resulting in an aqueous solution of sodium silicate with improved color and reduced metal content.

[0035] The advantage of the present invention is to provide a sustainable raw material for producing alkali metal silicates with reduced heavy metal content. It has been found that by using phosphate-containing waste sand alone or in a blend with virgin sand, and then reacting it with sodium carbonate in a high-temperature furnace melting process, dissolving it in water, and filtering it, an aqueous sodium silicate solution with reduced metal content is produced. Alternatively, phosphates, such as sodium phosphate or sodium tripolyphosphate, can be added to foundry waste sand or virgin sand to produce silicon dioxide raw materials, such as WFS with a phosphorus content exceeding 30 ppm.

[0036] In this invention, a furnace melting process is used to convert phosphate-containing waste sand or recycled sand into a solid alkali silicate material. During this high-temperature reaction between the sand and the caustic alkali material (e.g., sodium hydroxide, sodium carbonate), the phosphate reacts with various metal impurities to form insoluble metal phosphate complexes, which are thought to be easily filtered out when solid sodium silicate aqueous glass is dissolved in water to produce an aqueous sodium silicate solution.

[0037] Therefore, the advantage of the present invention is to provide a sustainable silica raw material (e.g., foundry waste sand) in a significantly improved manner for removing substantial amounts of undesirable heavy metal contaminants from alkali metal silicate materials.

[0038] A further advantage of the present invention is that it is easier to filter than known methods for metal removal, including the method first disclosed in U.S. Patent No. 7,297,318 B2, in which calcium phosphate is used to treat an aqueous solution of metal silicate. While the aforementioned process is effective in compounding metals, the insoluble heavy metal complexes formed are difficult to filter. The present invention, in which phosphate reacts with heavy metal impurities in sand during a solid-phase furnace process, provides a solution that is easily filtered once dissolved in water.

[0039] To better illustrate the advantages and properties of the claimed alkali metal silicate composition, which is the subject of this invention, the following figures are disclosed together with the present invention. [Brief explanation of the drawing]

[0040] [Figure 1] Figure 1 is a photograph showing a sodium silicate solution produced via a hydrothermal process using a mixture of waste sand and virgin sand. [Figure 2] Figure 2 is a photograph showing a sodium silicate solution produced via a furnace process using a mixture of waste sand and virgin sand. [Figure 3] Figure 3 provides analytical data showing the visible range absorbance of a sodium silicate solution produced via a furnace process compared to a sodium silicate solution produced via a hydrothermal process. [Modes for carrying out the invention]

[0041] Detailed description of the invention Commercially, alkali metal silicates are produced by two common methods: the hydrothermal method (liquid) and the furnace method (solid water glass). The silica raw material used (quartz) is typically small-grained sand. The alkali metal can be sodium, potassium, or lithium, with sodium being preferred.

[0042] In the hydrothermal method, a hydrothermal reaction occurs between sand (quartz) and a caustic alkali solution (e.g., sodium hydroxide or potassium hydroxide) at a temperature of 150°C to 250°C. In this method, excess sand, caustic alkali, and water are stirred in a pressure reactor, and then unreacted sand is separated from the resulting sodium silicate solution by filtration. The resulting molar ratio is generally 2.5 to 2.8 (SiO2 / Na).

[0043] In the furnace method, sand (quartz) and a base (e.g., sodium carbonate or sodium hydroxide) react together at high temperatures. In this method, stoichiometric amounts of sand and sodium carbonate (Na2CO3) are heated to 1000°C to approximately 1400°C in a gas-fired furnace or an electric or plasma furnace to produce solid silicate water glass. The formed water glass is cooled, crushed, and dissolved in water to form a liquid sodium silicate solution with a molar ratio of 3.0 to 3.8 (SiO2 / Na). The dissolved silicate solution is then filtered to remove insoluble heavy metal complexes and unreacted sand.

[0044] In both methods, the mixture is filtered to remove unreacted sand and other insoluble species. Insoluble heavy metal complexes and / or salts are removed from the alkali metal silicate mixture by filtration, centrifugation, or vibrating sieve, preferably by filtration, such as pressurized foliar filtration. Pressurized foliar filtration is a preferred filtration method.

[0045] According to the present invention, alkali metal silicates (water glass) are preferably produced using a furnace method due to the advantages of providing a higher molar ratio (SiO2 / Na) and ease of filtration. At high temperatures (temperatures above 1000°C), it is thought that phosphates react with metal impurities to form metal phosphate composites that are not water-soluble and are easily removed by filtration.

[0046] Therefore, the present invention is as follows: a) Steps to prepare silicon dioxide raw materials or mixtures thereof, b) The step of mixing the silicon dioxide raw material with an alkali base, c) The mixture is heated at a temperature above 1000°C for 1 to 5 hours. d) The step of dissolving furnace water glass in water, and e) Removing the resulting insoluble heavy metal complexes and / or salts from the alkali metal silicate mixture. The method for producing alkali metal silicate material includes a method in which the silicon dioxide raw material is waste sand or a mixture thereof, or a mixture of waste sand and virgin sand.

[0047] Another aspect of the present invention is the use of waste sand as a sustainable raw material for producing alkali metal silicates.

[0048] A further aspect of the present invention is the use of alkali metal silicates produced by using waste sand in a furnace process in which sand is reacted with sodium carbonate at a temperature exceeding 1000°C, for the production of precipitated silica.

[0049] According to the present invention, the waste sand is preferably phosphate-containing waste sand, and more preferably foundry waste sand (WFS) containing phosphate.

[0050] According to the present invention, the base is preferably a metal carbonate, such as sodium carbonate, potassium carbonate, or lithium carbonate, preferably sodium carbonate (referred to as soda ash), or a hydroxide, preferably sodium hydroxide.

[0051] According to the present invention, it is preferable that the waste sand is introduced into the base such that solid sodium silicate is produced at a molar ratio of 3.0 to 3.8.

[0052] According to a preferred embodiment, waste sand / recycled sand (e.g., WFS) or a combination thereof is used alone as the silicon dioxide raw material.

[0053] According to a preferred embodiment, waste sand / recycled sand (e.g., WFS) is mixed with virgin sand.

[0054] According to a preferred embodiment, waste sand is introduced into the virgin sand in an amount of 20% to 80% by mass relative to the total amount of sand.

[0055] According to the present invention, silicon dioxide raw materials with a phosphorus content exceeding 30 ppm can be produced by adding additional phosphate raw materials to foundry waste sand or virgin sand. The phosphate raw materials are selected from the group consisting of hydroxyapatite, dicalcium phosphate, tricalcium phosphate, STTP hydroxyapatite, preferably sodium phosphate or sodium tripolyphosphate, and any mixture thereof.

[0056] According to the present invention, a mixture of silicon dioxide raw material and caustic alkali is heated to a temperature of 1000°C to 1400°C, preferably 1100°C to 1400°C, and most preferably 1150°C to 1200°C for 1 to 5 hours, preferably 1 to 3 hours, and more preferably 1.5 to 2 hours.

[0057] According to the present invention, water glass is produced through a melting process at a very high temperature (1100-1200°C, preferably 1150°C), and the reaction is carried out by melting together solid sand and soda ash, as shown in the following chemical formula: SiO2(s)+Na2CO3(s)Δ → Na2SiO3(s)+CO2(g) The main advantage of this invention is the use of sustainable silica raw materials to produce alkali metal silicates.

[0058] Another advantage of the present invention is that by applying a furnace process at temperatures exceeding 1000°C and using phosphate-containing waste sand, the metal content in alkali metal silicates (e.g., sodium silicate) can be reduced.

[0059] Another advantage of the present invention over conventionally known methods is that filtration is easy. After the alkali metal silicates produced via the furnace process are dissolved in water, they can be easily filtered to remove insoluble heavy metal complexes and / or slag from the alkali metal silicate mixture.

[0060] Alkali metal silicates (water glass) can be used as is, as an ingredient in the final formulation, or as a raw material for producing precipitated silica, silica gel, calcium silicate, magnesium silicate, sodium aluminosilicate, and sodium magnesium aluminosilicate, etc., and it meets the requirements for food, cosmetics, and pharmaceuticals regarding the amount of metal contaminants present.

[0061] The alkali metal silicate produced by this invention is the main raw material for producing precipitated silica.

[0062] Precipitated silica is used in important applications such as toothpaste, cosmetics, food, and rubber tires. In these applications, silica offers unique and significant benefits and cannot be easily replaced.

[0063] End-use formulations include a variety of personal care preparations, such as toothpaste, cosmetic compositions, such as body powder, and other similar applications, such as anti-caking agents and / or free-flowing agents, such as fillers for paper, plastics, and rubber, where human consumption or food contact is inevitable, and pharmaceutical excipients for appropriate use. Many of these end-use formulations are based on materials produced from silicon dioxide raw materials with varying levels of heavy metal contamination. The most expensive silicon dioxide raw materials are considered the purest in terms of the presence of such heavy metals and therefore generally do not require modification to reduce the potential toxic effects that may arise from them. However, as the need to reduce the cost of raw materials increases and / or as pure silicon dioxide raw materials become more scarce, it has become important to provide some method for reducing the amount of heavy metals present in such starting materials, or more specifically in the intermediate materials produced therefrom.

[0064] Precipitated silica is basically produced by first manufacturing a metal silicate (e.g., sodium silicate, without limitation), then exposing such material to a mineral acid (e.g., sulfuric acid), and subsequently allowing the resulting silica to precipitate. Silica gel can also be formed through the reaction of sodium silicate with an acid under various conditions. Essentially, the ability to reduce the amount of heavy metals in such a final product is recognized as being addressed during one of the process steps for producing the required alkali silicate intermediate material.

[0065] Metals, such as lead, calcium, nickel, manganese, chromium, arsenic, cadmium, copper, iron, and zinc, when present as impurities in precipitated silica, can not only pose potential toxicity in food applications but can also interfere with the hardening chemical process when silica is used in the vulcanization process of rubber tires.

[0066] Heavy metals, such as lead, arsenic, cadmium, copper, and zinc, exhibit varying levels of toxicity in mammals. In particular, when ingested and metabolized and absorbed in the body—that is, when ingested in a biologically usable form—such metals, even in small amounts, tend to accumulate in the body and exhibit toxic effects. As a result, regulatory agencies at both the state and federal levels have established regulations regarding the maximum generally acceptable amounts of such heavy metals in materials that humans may consume. Therefore, it is crucial to develop methods to minimize the amount of such heavy metals in materials that can be utilized and ingested by humans (and other mammals).

[0067] Phosphate materials have been proposed as possible additives for removing certain heavy metals, such as lead, from aqueous solutions, as disclosed in U.S. Patent No. 7,297,318 (US7,297,318 B2), where virgin sand is used as the silica source, but not from solid materials. Thus, while the use of phosphate and calcium phosphate materials for immobilizing lead and heavy metals has been proposed in the prior art, no method has been disclosed to carry out the immobilization of lead or other heavy metals from the high-temperature reaction of sand with sodium carbonate in a furnace melting process where waste sand is used as the silicon dioxide source.

[0068] Preferred Embodiment of the Invention The present invention progresses through the following steps: a) Steps to prepare silicon dioxide raw materials or mixtures thereof, b) The step of mixing the silicon dioxide raw material with an alkali base, c) The mixture is heated at a temperature above 1000°C for 1 to 5 hours. d) Dissolving the resulting solid sodium silicate glass in water, and e) Removing the resulting insoluble heavy metal complexes and / or salts from the alkali metal silicate mixture. The present invention provides a method for producing an alkali metal silicate material, wherein the silicon dioxide raw material is sustainable waste sand or a mixture thereof, or a mixture of waste sand and virgin sand.

[0069] According to a preferred embodiment, the waste sand is phosphate-containing waste sand, preferably foundry waste sand having a phosphorus content of 20 ppm or more, preferably 25 ppm or more.

[0070] According to a preferred embodiment, the alkali base is a metal carbonate selected from sodium carbonate, potassium carbonate, or lithium carbonate, preferably sodium carbonate, or a hydroxide, preferably sodium hydroxide.

[0071] According to a preferred embodiment, the waste sand is introduced into the alkali base to produce solid sodium silicate at a molar ratio of 3.0 to 3.8.

[0072] According to another preferred embodiment, the waste sand is introduced into the virgin sand in an amount of 20% to 80% by mass relative to the total mass of the sand, where the sand mixture comprises 20% to 50% by mass of phosphate-containing waste sand and 50% to 80% by mass of virgin sand relative to the total mass of the sand.

[0073] In a preferred embodiment, the mixture of silicon dioxide raw material and base is heated to a temperature of 1000°C to 1400°C for 1 to 3 hours.

[0074] According to a preferred embodiment, the present invention progresses through the following steps: a) The step of preparing phosphate-containing waste sand or a mixture thereof as a silicon dioxide raw material, or a combination thereof with virgin sand, b) The step of mixing the waste sand and sodium carbonate, c) The mixture is heated at a temperature of 1150°C for 1.5 to 2 hours. d) Dissolving the resulting solid sodium silicate glass in water, and e) Removing the resulting insoluble heavy metal complexes and / or salts from the alkali metal silicate mixture. The present invention provides a method for producing an alkali metal silicate material, wherein the phosphate-containing waste sand is foundry waste sand (WFS).

[0075] According to a preferred embodiment, the present invention provides the use of waste sand, particularly foundry waste sand, as a sustainable raw material for producing alkali metal silicates.

[0076] According to a preferred embodiment, the present invention provides a method for producing precipitated silica by using an alkali metal silicate material produced by using foundry waste sand alone or in combination with virgin sand via a furnace process.

[0077] According to other embodiments, the present invention progresses to the following steps: a) The step of preparing an alkali silicate solution containing alkali silicate and water, b) Mixing the alkali silicate solution with mineral acid and stirring, c) The process involves sedimentation, recovery of the settled silica by filtration, washing away insoluble residues and / or salts, and then drying. The present invention provides a method for producing precipitated silica, wherein the alkali silicate is produced via a furnace process using foundry waste sand.

[0078] According to a more preferred embodiment of the present invention, the alkali silicate used to produce precipitated silica is sodium silicate, and the mineral acid is sulfuric acid.

[0079] According to another embodiment of the present invention, the following steps: a) The step of preparing an alkaline silicate solution containing sodium silicate and water, b) Adding sulfuric acid and sodium silicate solution while stirring together, c) After sedimentation, the settled silica is recovered by filtration, the salt is washed off, and then the mixture is dried. The present invention provides a method for producing precipitated silica, wherein the sodium silicate material is produced via a furnace process and using foundry waste sand (as described in the present invention and in accordance with the above embodiments).

[0080] According to the present invention, an appropriate amount of foundry waste sand is mixed with an appropriate amount of sodium carbonate via a laboratory-sized grinder (e.g., a coffee grinder) to homogenize the mixture. Next, the mixture is charged into a graphite crucible and heated in a melting furnace. The mixture is then heated to 1150°C for 1 to 3 hours. Next, the molten mixture (water glass) is poured into a receiving vessel and cooled. The cooled water glass is crushed into small pieces by pressure and charged into a pressure reactor with an appropriate amount of distilled water, where the reactor is heated to 185°C for 1 to 2 hours with a stirring speed of 300 rpm. Finally, the molten water glass is filtered to remove insoluble metal composites and unreacted sand.

[0081] In a preferred embodiment, a phosphate material is added to a mixture of sand and an alkaline base. The phosphate material is selected from the group consisting of, but is not limited to, sodium phosphate, sodium tripolyphosphate (STTP), calcium phosphate, preferably sodium phosphate or sodium tripolyphosphate, and any mixture thereof, where the phosphate material reaches a phosphorus content of more than 30 ppm.

[0082] The present invention preferably relates to the use of waste sand as a sustainable raw material for producing alkali metal silicates, more preferably to the use of foundry waste sand.

[0083] The present invention preferably relates to the use of alkali metal silicate materials produced from waste sand or foundry waste sand for producing precipitated silica.

[0084] Methods for measuring metals and phosphorus Metal and phosphorus concentrations are measured using Thermo i-Cap Q ICP / MS and Perkin Elmer Optima 8300 ICP / OES.

[0085] The sand sample is stabilized by heating 1.0 g of the sample in a platinum dish with approximately 15 ml of HClO4 and 10 ml of HF until fumes are produced. The platinum dish is cooled, 5 ml of HF is added, and it is heated again to produce fumes. The dish is rinsed with DI water, and then heated again to produce fumes. This solution is quantitatively transferred to a 100 ml volumetric flask, 1.0 ml of scandium standard solution is added, and then diluted to the mark with deionized water. Then 2.0 ml of this sample solution and 100 μL of (Ga, Ir, In, Y) mixture standard solution are added to compensate for the transfer conditions, diluted with water to 50 ml, and drawn into an ICP.

[0086] The concentrations of metal and phosphorus in a sample are measured by comparing the sample's response to metal and phosphorus standards with known concentrations.

[0087] Method for measuring molar ratio The molar ratio of sodium silicate is measured according to the standard evaluation method described below.

[0088] The sodium oxide and silica content of sodium silicate is measured using a volumetric method in the same sample. First, the alkali in the sample is measured in the usual manner by titration with hydrochloric acid. Next, excess sodium fluoride is added, which reacts with the present silicic acid to form fluorosilicate and sodium hydroxide. The thus liberated alkali is titrated with a standard acid, and two titrations provide a basis for calculating the sodium oxide and silica in the original sample.

[0089] Methods for measuring color Color measurements were performed by scanning absorbance within the visible range (400 nm to 700 nm) using a HACH DR3900 spectrophotometer. [Examples]

[0090] The present invention will be described in more detail below with reference to examples and comparative examples, but this is not intended to limit the scope of the invention.

[0091] abbreviation WFS: Foundry waste sand WFS-CP WFS-WF CP and WF reflect the various raw materials available from a variety of casting materials.

[0092] STTP: Sodium tripolyphosphate DI: Distilled water In the following examples, various types of foundry waste sand (WFS), such as WFS-CP or WFS-WF (without limitation), were used. These sands may be used individually or in mixtures thereof.

[0093] Example Group 1: Reduction of metal impurities by using waste sand compared to virgin sand. Comparative Example 1: 1. Mix 100g of virgin sand with 53.3g of sodium carbonate. Grind the mixture in a pulverizer to homogenize it. Next, charge the mixture into a graphite crucible and place it in a melting furnace. Heat the mixture to 1150°C for 1.5 to 2 hours.

[0094] 2. After the molten water glass mixture has cooled, 251 g of water glass is crushed and dissolved in 700 g of distilled water in a pressure reactor. The mixture is heated to 185°C for 2 hours. The resulting mixture is then easily filtered to obtain liquid sodium silicate.

[0095] [Table 1]

[0096] Example 1(a) of the present invention: Production of water glass via a melting process (100% CP core blend) 3. Mix 101g of CP core sand with 50.9g of sodium carbonate. Grind the mixture in a pulverizer to homogenize it. Next, charge the mixture into a graphite crucible and place it in a melting furnace. Heat the mixture to 1150°C for 1.5 to 2 hours.

[0097] 4. After the molten water glass mixture has cooled, 254 g of water glass is crushed and dissolved in 624 g of distilled water in a pressure reactor. The mixture is heated to 185°C for 2 hours. The resulting mixture is then easily filtered to obtain liquid sodium silicate.

[0098] [Table 2]

[0099] Example 1(b) of the present invention: Production of water glass via a melting process (100% CP core blend) 1. Mix 101g of WFS-WF recycled sand with 51.3g of sodium carbonate. Grind the mixture in a pulverizer to homogenize it. Next, charge the mixture into a graphite crucible and place it in a melting furnace. Heat the mixture to 1150°C for 1.5 to 2 hours.

[0100] 2. After the molten water glass mixture has cooled, 258 g of water glass is crushed and dissolved in 634 g of distilled water in a pressure reactor. The mixture is heated to 185°C for 2 hours. The resulting mixture is then easily filtered to obtain liquid sodium silicate.

[0101] [Table 3]

[0102] Example 1(c) of the present invention: Production of water glass via a melting process (100% WFS-CP core scrap) 3. Mix 103g of WFS-CP scrap sand with 50.2g of sodium carbonate. Grind the mixture in a pulverizer to homogenize it. Next, charge the mixture into a graphite crucible and place it in a melting furnace. Heat the mixture to 1150°C for 1.5 to 2 hours.

[0103] 4. After the molten water glass mixture has cooled, 254 g of water glass is crushed and dissolved in 624 g of distilled water in a pressure reactor. The mixture is heated to 185°C for 2 hours. The resulting mixture is then easily filtered to obtain liquid sodium silicate.

[0104] [Table 4]

[0105] As shown in Tables 1(a) to (c) above, the residual amounts of metallic impurities present in the sodium silicate solution and insoluble metallic impurities were measured after filtration. Tables 1(a) to (c) above show that metallic impurities are dramatically reduced when foundry waste sand is used and a furnace process is applied. Given that virgin sand contains fewer metallic impurities than waste sand or WFS, the data cannot be compared accordingly. The metallic impurities remaining after filtration always depend on the sand (silicon dioxide raw material) used in the mixture, and foundry waste sand always initially contains more metallic impurities than virgin sand.

[0106] Example Group 2: Comparison of Melting Process and Hydrothermal Process (Virgin Sand Mixed with WFS) In both the comparative example and the example of the present invention, 20% of the virgin sand was replaced with CP core foundry waste sand.

[0107] Comparative Example 2: Production of water glass via a hydrothermal process (20% CP core blend mixed with virgin sand relative to the total mass of sand) 1. Mix 378g of distilled water and 448g of 50% sodium hydroxide solution (Fisher Scientific) in a pressure reactor. Add 376g of virgin sand and 96g of CP core WFS to the liquid mixture. Set the stirrer to 300rpm and heat the mixture to 192°C (approximately 150psi) for 4 hours.

[0108] 2. After the molten water glass mixture has cooled, the resulting sodium silicate is filtered, and the filter is washed with 680 g of water.

[0109] Example 2 of the present invention: Production of water glass via a melting process (mixing virgin sand with a CP core blend at a ratio of 20% relative to the total mass of sand) 1. Mix 80g of virgin sand (Silex 325, Covia) with 20g of CP core WFS and 51.5g of sodium carbonate. Grind the mixture in a pulverizer to homogenize it. Next, charge the mixture into a graphite crucible and place it in a melting furnace. Heat the mixture to 1150°C for 1.5 to 2 hours.

[0110] 2. After the molten water glass mixture has cooled, 284 g of water glass is crushed and dissolved in 615 g of distilled water in a pressure reactor. The mixture is heated to 185°C for 2 hours. The resulting mixture is then easily filtered to obtain liquid sodium silicate.

[0111] [Table 5]

[0112] Foundry waste sand typically contains organic binders used in sand casting processes. As can be seen from Table 2, during hydrothermal processes, where the achievable steam temperature (in a hydrothermal pressure reactor) is below 200°C, the dissolution of sand (both virgin sand and foundry waste sand) is less than in melting processes. Therefore, hydrothermal processes yield silicates with a lower molar ratio compared to melting processes.

[0113] Furthermore, compared to the melting process, more carbon species from the foundry waste sand remain along with the liquid silicate. The resulting silicate contains less silica (lower molar ratio silicate) and is contaminated with carbon pollutants, which result in a brown coloration after filtration. Figure 1 shows the color of sodium silicate produced via the hydrothermal process under Comparative Example 2, and Figure 2 shows the color of sodium silicate produced via the melting process under Example 2 of the present invention. Figure 3, which shows absorbance analysis data in the visible range, also clearly shows color absorption for the product produced via the hydrothermal process.

[0114] Example 3 of the present invention: Use of a hydrothermal process (addition of phosphate to foundry waste sand mixed with virgin sand) As stated in the body of the disclosure, phosphate materials have been proposed (in hydrothermal processes) as possible additives for removing certain heavy metals, such as lead, from aqueous solutions, as disclosed in U.S. Patent No. 7,297,318, where virgin sand is used as the silica raw material. Thus, while the use of phosphate materials for the immobilization of lead and heavy metals has been proposed in the prior art, difficulties arise when WFS is used in hydrothermal processes during the filtration of silicate materials. Furthermore, the use of phosphates is not very effective in reducing various metal impurities from silicates in liquid state, as shown in Table 3 below.

[0115] Example 3(a): Production of water glass using a sand mixture via a hydrothermal process To produce water glass via a hydrothermal process, 20% CP core blend is mixed with 80% virgin sand relative to the total mass of sand.

[0116] Here, we follow the procedure defined in Example 2 of U.S. Patent No. 7,297,318. Two different phosphates (hydroxyapatite (Sigma-Aldrich) and sodium tripolyphosphate (STPP) (Fisher Scientific)) are added separately to a sodium silicate solution. The two samples are then tested for the effect of the silicates on reducing metallic impurities from the liquid process.

[0117] 100 g of silicate solution from the above mixture was added to three separate Teflon beakers. 1 g of hydroxyapatite or STPP was added separately to two of these Teflon beakers. The three samples were stirred with a magnetic stirring rod and heated and maintained at 70-75°C on a hot plate. After 60 minutes, the mixture was filtered using a warmed syringe (0.45 μm) filter, which proved to be very difficult. The resulting filtrate was analyzed for the target metal impurities.

[0118] Table 3.1 below clearly shows that neither hydroxyapatite nor STPP has a significant effect on reducing various metal impurities from liquid silicates, other than Ni.

[0119] [Table 6]

[0120] Example 3(b): Production of water glass via a hydrothermal process (addition of phosphate to WFS) A 100% CP core blend is used to manufacture water glass via a hydrothermal process.

[0121] Here, the same procedure as defined in Example 3(a) was applied. 100 g of the silicate solution prepared above was added to three separate Teflon beakers. 1 g of hydroxyapatite or STPP was added separately to two of those Teflon beakers. The three samples were stirred with a magnetic stirring rod and heated and maintained at 70-75°C on a hot plate. After 60 minutes, the mixture was filtered using a warmed syringe (0.45 μm) filter, but this proved to be very difficult. The resulting filtrate was analyzed for the desired metal impurities.

[0122] Table 3.2 below clearly shows that neither hydroxyapatite nor STPP has a significant effect on reducing various metal impurities from liquid silicates, except for Mn.

[0123] [Table 7]

[0124] Examples 3(a and b) above and corresponding Tables 3.1 and 3.2 demonstrate that the use of phosphates to remove metal impurities from aqueous solutions (in hydrothermal processes) is ineffective in reducing the metal impurity content after filtration, even if it was a solution previously used to reduce metal impurities from aqueous solutions via hydrothermal processes. When using hydrothermal processes, the addition of phosphates to aqueous solutions is clearly ineffective for foundry waste sand.

Claims

1. A method for producing alkali metal silicate materials, comprising the following steps: a) Steps to prepare silicon dioxide raw materials or mixtures thereof, b) A step of mixing the silicon dioxide raw material with an alkali base selected from sodium carbonate, potassium carbonate, or lithium carbonate, or a metal hydroxide, c) The mixture is heated at a temperature exceeding 1000°C for 1 to 5 hours. d) The step of dissolving the resulting solid alkali metal silicate water glass in water, and e) Removing the resulting insoluble heavy metal complexes and / or salts from the alkali metal silicate mixture. The method comprising, wherein the silicon dioxide raw material is waste sand, a mixture thereof, or a mixture of waste sand and virgin sand.

2. The method according to claim 1, wherein the waste sand is phosphate-containing waste sand.

3. The method according to claim 1 or 2, wherein the waste sand is foundry waste sand.

4. The method according to any one of claims 1 to 3, wherein the alkali base is sodium carbonate or sodium hydroxide.

5. The method according to any one of claims 1 to 4, wherein the waste sand is introduced into the alkali base in an amount that produces a solid alkali metal silicate, preferably sodium silicate, with a molar ratio of 3.0 to 3.

8.

6. The method according to any one of claims 1 to 5, wherein the waste sand is introduced into the virgin sand in an amount of 20% to 80% by mass relative to the total amount of sand.

7. The method according to any one of claims 1 to 6, wherein a mixture of silicon dioxide raw material and an alkaline base is heated to a temperature of 1000°C to 1400°C for 1 to 3 hours.

8. The following stages: a) The step of preparing phosphate-containing waste sand or a mixture thereof as a silicon dioxide raw material, or a mixture of waste sand and virgin sand, b) The step of mixing the waste sand and sodium carbonate, c) The mixture is heated at a temperature of 1150°C for 1.5 to 2 hours. d) The step of dissolving the resulting solid sodium silicate water glass in water, and e) Removing the resulting insoluble heavy metal complexes and / or salts from the alkali metal silicate mixture. A method for producing an alkali metal silicate material according to any one of claims 1 to 7, wherein the phosphate-containing waste sand is foundry waste sand (WFS).

9. The following stages: a) In the step of preparing the sand mixture, b) The step of mixing the sand mixture with sodium carbonate, c) The mixture is heated at a temperature of 1150°C for 1.5 to 2 hours. d) The step of dissolving the resulting solid sodium silicate water glass in water, and e) Removing the resulting insoluble heavy metal complexes and / or salts from the alkali metal silicate mixture. A method for producing an alkali metal silicate material according to any one of claims 1 to 7, wherein the sand mixture comprises 20% to 50% by mass of phosphate-containing waste sand and 50% to 80% by mass of virgin sand based on the total mass of the sand.

10. The method according to any one of claims 1 to 9, wherein the phosphate-containing foundry waste sand has a phosphorus content of 20 ppm or more, preferably 25 ppm or more.

11. The method according to any one of claims 1 to 10, wherein a further phosphate material is added to the mixture of sand and alkali base to reach a phosphorus content of more than 30 ppm, the phosphate material being selected from the group consisting of sodium phosphate, sodium tripolyphosphate (STTP), calcium phosphate, preferably sodium phosphate or sodium tripolyphosphate, and any mixture thereof.

12. The use of waste sand, particularly foundry waste sand, as a sustainable raw material for the production of alkali metal silicates.

13. A method for producing precipitated silica, comprising the following steps: d) The step of preparing an alkali silicate solution containing alkali metal silicate and water, e) Adding sulfuric acid and alkali metal silicate solution while stirring together, f) After settling, the settled silica is recovered by filtration, the salt is washed off, and then the mixture is dried. The method comprising, wherein the alkali metal silicate material is produced by the method according to any one of claims 1 to 11, via a furnace process and using foundry waste sand.

Images