Economical, double digestion process for Ta / Nb-containing raw materials with recycling of the used hydrofluoric acid

Abstract

The present invention describes a process for the dual digestion of Ta / Nb-containing raw materials, achieving total tantalum and niobium yields of over 99% by weight. Recycling of the hydrofluoric acid media used minimizes chemical waste and optimizes the process economically. Since no steam is introduced and the process is carried out at low temperatures, cost-effective plastic containers can be used for the digestion reactions.

Description

[0001] The production of Ta 2 O 5 , Nb 2 O 5 and K 2 TaF 7is usually carried out by digesting Ta / Nb-containing raw materials with 40-80% hydrofluoric acid (GL Miller, Tantalum and Niobium, London 1959, p. 71 ff). Due to the similarity of the two elements tantalum and niobium, tantalum ores always contain niobium and vice versa (association). One such raw material is coltan. In the digestion reaction, the tantalum and niobium oxides present in the ore are essentially converted into Ta and Nb heptafluoride complexes. The proportion of tantalum and niobium that dissolves depends on the amount of hydrofluoric acid used, the reaction time and the temperature, and increases with increasing temperatures. The digestion solution is then separated from the insoluble residue by filtration, and the dissolved tantalum and niobium compounds are extracted using a water-immiscible organic solvent.Methyl isobutyl ketone is usually used, but other solvents can also be used (US 3,117,833, US 2,767,047).

[0002] A problem with the described process is that the sludge residues still contain relatively large amounts of tantalum and / or niobium compounds (dissolved and / or undissolved), which are difficult to recover by washing. Large quantities of washing solution are required to wash out the final residues, making the process uneconomical. However, for occupational safety and environmental reasons, it is necessary to wash the residues with water until at least they are free of HF. This naturally generates large quantities of wastewater containing only very low concentrations of valuable substances (Ta and Nb complexes). Therefore, these solutions are often not recycled.

[0003] Various methods have been developed in the past to maximize yields and thus make the process more economically advantageous. For example, US 3,712,939 describes how, after extraction of the tantalum and / or niobium complexes, the remaining aqueous acidic phase is brought into contact again with the digestion residue, thus effectively conducting a second digestion reaction. In US 3,712,939, the residue from the first digestion, the aqueous acidic phase after the first extraction, and fresh organic solvent are directly mixed together. After an adequate contact time, the organic phase is separated, from which additional tantalum and / or niobium is subsequently recovered. The overall yield was thus increased from 80% to 90%.

[0004] Another way to increase the yield is to introduce steam during the digestion reaction (GL Miller, Tantalum and Niobium, London 1959). This method is problematic in that the condensation of water vapor in the digestion solution leads to uncontrollable dilution of the solution, which can impair further processing in the extraction (failure to meet the minimum acidity requirements, dilution of the components). Especially at temperatures above 100°C, boiling delays occur in this process, which generate uncontrolled HF emissions and thus can lead to environmental and occupational safety problems. A further disadvantage of this method is the high maintenance costs of the reaction vessels, which are usually made of rubber-lined metal and require frequent maintenance.

[0005] US Patent 5,437,848 describes how residues from hydrofluoric acid digestions are treated with concentrated sulfuric acid to dissolve Ta and Nb residues not removed in the initial digestion. The invention in US Patent 5,437,848 takes advantage of the fact that fluoride is already present in the residues used as starting materials, as this is needed to dissolve Ta and Nb. In this process, the raw material is treated sequentially in two separate processes with different digestion media, which requires an entire additional production facility; it is therefore not economical.

[0006] It is also known that residues containing Ta / Nb can be subjected to further digestion using hydrofluoric acid (Bull. Inst. Min. Metall., London, No. 458, 1943). During the second digestion, steam is applied. The combined digestion solutions from both steps are then further processed. A disadvantage of the second digestion, in particular, is that large excesses of hydrofluoric acid are used, and only small amounts of valuable materials are extracted (the loading of the solutions with valuable materials is low). This means that this process is also relatively uneconomical and generates excessive amounts of waste.

[0007] The object of the present invention was to develop an economical process for the digestion of Ta- and / or Nb-containing raw materials that optimizes the yields of the process and uses the chemicals in a sustainable manner so that process costs and environmentally harmful influences are minimized.

[0008] This problem was solved by the process disclosed in the claims. In particular, the process according to the invention has the advantage that it works exclusively with hydrofluoric acid of various concentrations, which makes recycling easy.

[0009] The weaknesses of the prior art process, namely relatively large losses of tantalum and niobium, as well as relatively large amounts of wastewater produced, are minimized by the present invention, as will be described below.

[0010] The method according to the invention relates in particular to- a process for the double digestion of raw materials containing tantalum and / or niobium, wherein in a first digestion a digestion medium comprising aqueous hydrofluoric acid is brought into contact with a raw material containing tantalum and / or niobium, after the reaction has taken place the resulting hydrofluoric acid digestion solution, which contains dissolved tantalum and / or niobium compounds, is separated from the insoluble residue, the residue is subjected to a second digestion using a digestion medium comprising aqueous hydrofluoric acid, after the second digestion has taken place the resulting hydrofluoric acid digestion solution, which contains further dissolved tantalum and / or niobium compounds, is separated from the insoluble residue, the residue is washed with water, the washing water is collected, and wherein the digestion medium for the first and / or second digestion is prepared by mixing collected washing water and concentrated and / or diluted hydrofluoric acid; - a process in which the residues from several initial digestions are collected unwashed and subjected together to a second digestion; - a process in which the hydrofluoric acid digestion solution containing tantalum and / or niobium compounds separated from the insoluble residue after the second digestion is used as a digestion medium for a first digestion; - a process in which the hydrofluoric acid digestion solution containing tantalum and / or niobium compounds separated from the insoluble residue after the first and / or second digestion is subjected to extraction by an organic, water-immiscible solvent, preferably methyl isobutyl ketone, in order to extract the tantalum and / or niobium compounds present in the digestion solution; - a process wherein the digestion medium for the first and / or second digestion has a hydrogen fluoride content of at least 36% by weight, or at least 37% by weight, or at least 38% by weight, or at least 39% by weight, preferably at least 40% by weight, based on the total mass of the digestion medium; - a process that does not use steam or sulphuric acid; - a process wherein the digestion reactor in which the first and / or second digestion is carried out is made of plastic, preferably polypropylene; - a process wherein the reaction temperature during the first and / or the second digestion can be controlled by appropriate devices, preferably wherein the first digestion is carried out at a maximum temperature of 80°C and / or the second digestion is carried out at temperatures greater than 70°C; - a process wherein the total yield of a double digestion of a raw material containing tantalum and / or niobium is at least 95% by weight, preferably at least 96% by weight, preferably at least 97% by weight, further preferably at least 98% by weight, further preferably at least 99% by weight, based on the mass of the tantalum and / or niobium originally contained in the raw material.

[0011] The term "digestion" in the context of the present invention refers to a chemical reaction (digestion reaction) in which valuable materials are to be extracted from a solid mixture of substances. The reaction takes place in a "digestion reactor." In this case, the valuable materials are the metals niobium and tantalum, which appear neither native in the solid mixture nor in the target product, but have formed chemical compounds with other elements in their oxidized state.

[0012] To effect the digestion, a "digestion medium" is brought into contact with the solid mixture, and the resulting mixture is stirred and heated for several hours. Typically, the digestion medium is added, and the solid, preferably already crushed, mixture is added.

[0013] According to the invention, the digestion medium comprises or consists of hydrofluoric acid, i.e., aqueous solutions of hydrogen fluoride (HF), whereby the HF concentrations of these solutions must be suitably selected to effect an effective digestion reaction. The hydrogen fluoride content in a digestion medium should be greater than 36% by weight, or greater than 37% by weight, or greater than 38% by weight, or greater than 39% by weight, preferably about 40% by weight, based on the total mass of the digestion medium. 38% aqueous hydrogen fluoride is a negative azeotrope that boils at 112°C. In mixtures with a much higher HF content, HF losses can occur during digestion reactions conducted at temperatures of 100°C or more.

[0014] After digestion, the resulting "digestion solution," which contains dissolved tantalum and / or niobium in the form of their heptafluoride complexes, is separated from the insoluble "residue." This is preferably done by positive or negative pressure filtration. Depending on the quality of the phase separation, the remaining residue is more or less sludgy, thus still containing a liquid component, although the proportion of liquid component should be as low as possible.

[0015] Typically, the residue is then washed with water to recover traces of hydrogen fluoride and tantalum and / or niobium-containing valuables still present in the liquid component of the residue. The liquid produced during the washing process is collected separately and referred to as "wash water."

[0016] Solid mixtures of materials to be digested within the meaning of the invention can be the raw material mined in nature (the Ta / Nb-bearing ore), residues from previous digestions, or combinations thereof. Solid raw materials to be digested are preferably ground and / or comminuted in another form before the digestion reaction in order to increase the effectiveness of the digestion reaction by increasing the surface area. It is also possible to combine the residues from several previous digestion reactions and subject them to a new digestion process. Repeated digestion of a material containing valuable materials leads to successively reduced valuable material contents in the material and to successively increased overall yields if the valuable material yields are cumulated across all digestions.

[0017] If a material is digested several times in succession, a "first digestion" occurs when the solid mixture to be digested is a "raw material," i.e., a material that has never been subjected to a digestion reaction. The residue from a first digestion is a "first residue," and the digestion solution from a first digestion is a "first digestion solution." A "second digestion" occurs when the solid mixture to be digested consists of one or more combined first residues. The residue from a second digestion is a "second residue," and the digestion solution from a second digestion is a "second digestion solution."

[0018] The process according to the invention consists in collecting the residues generated during the first digestion step, rather than washing them, during a double digestion of tantalum- and / or niobium-containing raw materials and feeding them together to a second digestion step. This allows the amount of wash water generated during the overall process to be significantly reduced.

[0019] The double digestion of the process according to the invention also makes it possible to forgo maximizing the yield during the first digestion and instead configure the reaction parameters during the first digestion in a particularly economically advantageous manner. State-of-the-art digestions are usually carried out at temperatures above 100 °C and, including the heating and cooling phases, last up to 60 hours. Depending on the origin of the raw material, the first digestion of the process according to the invention only takes between 12 and 20 hours, since the maximum temperature of the first digestion is 80 °C, and thus the cooling process is significantly shorter. This allows more digestions to be carried out in the same time.

[0020] The low temperature during the initial digestion also allows the digestion reaction to be carried out in plastic containers, which are not only cheaper to purchase than traditional rubber-lined metal containers, but also cheaper to maintain.

[0021] The relatively low yields of the first digestion are compensated for by a second digestion according to the process of the invention. In a preferred variant of the invention, several residues from the first digestions are accumulated until the weight of a raw material quantity typically used in a first digestion is reached, and then subjected together to a second digestion.

[0022] If raw material has been digested in the process according to the invention, the mass of the insoluble residue is typically 10 to 30 percent by weight of the mass of the raw material used. This figure depends on the nature of the raw material used, as well as the reaction conditions (temperature, time, amount, and concentration of hydrofluoric acid). If residue from a first digestion has been digested a second time, the mass of the new, insoluble residue is typically 30 to 60 percent by weight of the mass of the residue used.

[0023] The digestion process can be optimized by heating / cooling the digestion reaction mixtures using heat exchangers, which can function as both heating and cooling elements.

[0024] The second digestion of the process according to the invention serves to maximize the yield. The temperature during a second digestion reaction should be at least 70 °C, preferably at least 80 °C, preferably at least 90 °C, preferably at least 100 °C. Preferably, heating is carried out to boiling. The boiling point of hydrofluoric acid with a 38 wt. percent HF content is 112 °C.

[0025] A second aspect of the present invention consists in collecting the wash water resulting from the second digestion and phase separation and using it to produce new digestion medium. For this purpose, the hydrofluoric acid wash water is mixed with concentrated and / or dilute hydrofluoric acid to achieve the acidity desired for digestion and subsequent extraction. High acidity of the digestion medium is important not only during the digestion reaction to break the oxide bonds in the ore, but also during the extraction of the digestion solution using an organic solvent that absorbs predominantly neutral compounds, i.e., the protonated fluoride complexes. For good yields in the extraction step, the normality of the digestion solution, based on hydrogen fluoride, should be at least 16, or 17, or 18, or 19, preferably at least 20. A 40% hydrofluoric acid solution has a normality of approximately 22.By recycling the wash water produced after the second digestion of the process according to the invention, the amount of waste generated in the overall process is reduced and the economic efficiency is increased. In a particularly preferred variant of the invention, the new digestion medium thus produced is used only for a second digestion of the process according to the invention.

[0026] Digestion solution obtained in a previous digestion reaction can also serve as the digestion medium. This is particularly useful if the digestion solution is a "second digestion solution", since in this case the valuable material loading is low and the overall digestion potential of the solution has not yet been exhausted. It is also possible to previously add additional pure hydrofluoric acid and / or diluted hydrofluoric acids to digestion solutions that are to be used as digestion media in subsequent digestions in order to increase the acidity of the solution. In a particularly preferred variant of the invention, the filtrate after a second digestion, i.e. a second digestion solution, is used as the digestion medium for a first digestion.

[0027] After completion of a digestion reaction, the mixture is cooled to room temperature, and then the digestion solution is separated from the residue. This is preferably done by positive or negative pressure filtration. In the process according to the invention, the sludge residue is washed with water after a second digestion until the residue is largely free of hydrogen fluoride. The wash water is not combined with the filtrate, the digestion solution, but is collected separately. If the solid mixture that was digested was raw material, the residues can be collected and subjected together to a second digestion reaction to recover any remaining valuable substances.If the solid mixture of substances that was digested consisted of one or more residues from previous digestions, the residues can now be disposed of because they contain only negligible amounts of valuable substances and are essentially HF-free.

[0028] The digestion solution obtained in a digestion reaction can, after additional acidification by adding further acid, such as hydrogen fluoride, nitric acid, or hydrochloric acid, preferably HF, be subjected to extraction, in which the desired tantalum and niobium complexes are isolated. Organic, water-immiscible solvents are typically used for the extraction. Methyl isobutyl ketone is particularly preferred, but other ketones, alcohols, esters, or aldehydes can also be used. The aqueous phase obtained after the extraction is usually discarded. Fig.is intended to illustrate the underlying process flows.

[0029] The combination of the above-described measures of the inventive process—that is, the double digestion of a raw material with collection of the first residues and their combined feeding to a second digestion, the recycling of the hydrofluoric acid wash water to generate a new digestion medium, and the use of the second digestion solution as the first digestion medium—can save considerable labor and energy and create additional dissolving capacity. This combination of process steps ensures that the large excess of hydrofluoric acid used to extract valuable materials from the residues is not lost but is advantageously reintegrated into the process.

[0030] By the double digestion of tantalum and / or niobium-containing raw materials described in this invention, at least 95% by weight, preferably at least 96% by weight, preferably at least 97% by weight, further preferably at least 98% by weight, further preferably at least 99% by weight of the tantalum and / or niobium can be dissolved out of the raw material.

[0031] In addition to improving the yield and recycling of diluted hydrofluoric acids, the process according to the invention also succeeds in increasing the throughput of raw materials with the same plant size, thus reducing production costs. The experimental examples show that conventional dissolution processes require long reaction times and energy input to achieve the best possible degree of dissolution.

[0032] An additional, not to be overlooked advantage is the control and reduction of the resulting exhaust gases, which require complex purification systems for their removal. The process according to the invention can significantly reduce these investments. The invention is explained below by means of examples, which are not limiting. Example 1 (Preparation of a digestion medium using recycled dilute hydrofluoric acid)

[0033] 500 mL of 6% hydrofluoric acid were mixed with 300 g of 100% HF in a PP container equipped with flow-redirecting heating / cooling elements, while stirring. The dosing rate was adjusted so that the resulting heat of dilution was dissipated through the heat exchangers and the temperature did not exceed 60 °C. The resulting acid had an HF content of approximately 37%. Example 2 (First digestion of a fresh raw material batch)

[0034] 300 g of a raw material were added to 800 mL of an HF solution prepared according to Example 1. The suspension was stirred at 100 °C for 20 h, then cooled, and the insoluble residue was filtered off and washed with water. Analysis of the solid before dissolution: 29.26% Ta 2 O 5 , 27.93% Nb 2 O 5 . Remaining residue (based on raw material used): 15%. Analysis of the residue (based on the total mass of the residue): 11.35% Ta 2 O 5 , 7.39% Nb 2 O 5 This results in the following yields: 94.18% Ta 2 O 5 , 96.03% Nb 2 O 5 . Example 3 (Second digestion of a cumulative residue)

[0035] The collected residues from several digestions according to Example 2, a total of 300 g (calculated on the dried substance), were added to 800 mL of an HF solution prepared according to Example 1. The mixture was heated to boiling (approx. 112 °C) with stirring for 20 hours. The reaction mixture was then cooled, filtered, and the solid washed with water to remove any valuable substances. Analysis of the solid before dissolution: 11.35% Ta 2 O 5 , 7.39% Nb 2 O 5 . Remaining residue (based on mass before digestion): 45%. Analysis of the residue after digestion: 0.3% Ta 2 O 5 , 0.76% Nb 2 O 5 This results in the following yields: 99.9% Ta 2 O 5 , 99.9% Nb 2 O 5 . Example 4 (Second digestion of a cumulative residue)

[0036] Example 3 is repeated, but this time the mixture is stirred and heated to boiling for only 8 h. Analysis of the solid before dissolution: 11.35% Ta 2 O 5 , 7.39% Nb 2 O 5 . Remaining residue (based on mass before digestion): 51.5%. Analysis of the residue after digestion: 1.77% Ta 2 O 5 , 1.26% Nb 2 O 5 This results in the following yields: 99.4% Ta 2 O 5 , 99.5% Nb 2 O 5 . Example 5 (Second digestion of a cumulative residue)

[0037] Example 4 is repeated, but this time the mixture is stirred at room temperature. Analysis of the solid before dissolution: 11.35% Ta 2 O 5 , 7.39% Nb 2 O 5 . Remaining residue (based on mass before digestion): 65%. Analysis of the residue after digestion: 5.76% Ta 2 O 5 , 3.77% Nb 2 O 5This results in the following yields: 97.4% Ta 2 O 5 , 98.2% Nb 2 O 5 . QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] US 3,117,833

[0001] US 2,767,047

[0001] US 3,712,939

[0003] US 5,437,848

[0005] Cited non-patent literature

[0000] GL Miller, Tantalum and Niobium, London 1959, p.71 ff

[0001] Bull. Inst. Min. Metal., London, No. 458, 1943

[0006]

Claims

[1] Process for the double digestion of tantalum and / or niobium-containing raw materials, whereby - in a first digestion, a digestion medium comprising aqueous hydrofluoric acid is brought into contact with a raw material containing tantalum and / or niobium, - after the reaction, the resulting hydrofluoric acid digestion solution, which contains dissolved tantalum and / or niobium compounds, is separated from the insoluble residue, - the residue obtained is subjected to a second digestion using a digestion medium containing aqueous hydrofluoric acid, - after the second digestion, the resulting hydrofluoric acid digestion solution, which contains further dissolved tantalum and / or niobium compounds, is separated from the insoluble residue, - the residue is washed with water, - the wash water is collected, and wherein the digestion medium for the first and / or second digestion is prepared by mixing collected wash water and concentrated and / or diluted hydrofluoric acid. [2] A process according to claim 1, wherein the residues from several first digestions are collected and jointly subjected to a second digestion. [3] A process according to claim 1 or 2, wherein the residue(s) is / are not washed after the first digestion has taken place. [4] Process according to at least one of claims 1 to 3, wherein the hydrofluoric acid digestion solution containing tantalum and / or niobium compounds separated from the insoluble residue after the second digestion is used as a digestion medium for a first digestion. [5] Process according to at least one of claims 1 to 4, wherein the hydrofluoric acid digestion solution containing tantalum and / or niobium compounds separated from the insoluble residue after the first and / or second digestion is subjected to extraction by an organic, water-immiscible solvent, preferably methyl isobutyl ketone, in order to extract the tantalum and / or niobium compounds present in the digestion solution. [6] Process according to at least one of claims 1 to 5, wherein the digestion medium for the first and / or second digestion has a hydrogen fluoride content of at least 36% by weight, or at least 37% by weight, or at least 38% by weight, or at least 39% by weight, preferably at least 40% by weight, based on the total mass of the digestion medium. [7] Process according to at least one of claims 1 to 6, wherein neither steam nor sulphuric acid are used during the entire process. [8] Method according to at least one of claims 1 to 7, wherein the digestion reactor in which the first and / or the second digestion is carried out is made of plastic, preferably of polypropylene. [9] Process according to at least one of claims 1 to 8, wherein the reaction temperature during the first and / or the second digestion can be controlled by appropriate devices, preferably wherein the first digestion is carried out at a temperature of at most 80°C, and / or the second digestion is carried out at temperatures greater than 70°C. [10] Process according to at least one of claims 1 to 9, wherein the total yield of a double digestion of a raw material containing tantalum and / or niobium is at least 95% by weight, preferably at least 96% by weight, preferably at least 97% by weight, further preferably at least 98% by weight, further preferably at least 99% by weight, based on the mass of the tantalum and / or niobium originally contained in the raw material.

Images