Recovery of tungsten trioxide from waste diamond core drilling crowns by nitric acid leaching
Introduction
Diamond core drilling crowns with a cemented carbide blade are used for drilling in soft and medium-hard rocks. These crowns are made by inserting the cemented carbide blades into a steel body using special procedures, usually, in the form of octagonal prisms or plates. Cemented carbide blades are produced by the technological processes of powder metallurgy. The blades consist of cemented carbide grains, size of 0.3 to 2 μm, and cobalt metal powder (5–30%) as a binder. The mixed metal powder is pressed into moulds to shape the produced blade. Then, by heating the mould below the melting point of the powder mixture (1400–1500 °C), the metal powder is combined by melting the cobalt grains [1,2]. The cemented carbide provides a high hardness (approaching the diamond hardness and wear-resistance. Acting as a binder, cobalt adds a strength to the cemented carbide mixture [2]. About 60% of current tungsten consumption is used to produce cemented carbides, with the major applications of cutting tools, mining, oil and gas drilling, and other machine tools [3,4].Therefore, the recycling of cemented carbides (WC-Co-(NiFe) hard metals) can be considered as a significant secondary resource of tungsten and cobalt with significantly lower production costs than the primary sources [4]. Previous studies have shown that the recycling of tungsten, containing the cemented carbide scrap, uses up to 79% less energy and causes 40% less CO2 emissions compared to the tungsten recovery from ore. A high concentration of tungsten is one of the main advantages of using secondary raw materials, such as the cement carbide. The content is 40 to 95%, while in the primary raw materials it is lower and ranges from 7 to 60% [5].
Many researchers have investigated the possibilities of tungsten carbide recycling and production of precious metals such as W, Co, Ni, and others. Different methods for recycling the cemented carbide waste materials are available: hydrometallurgy, pyrometallurgy, or a combination thereof [6,7]. The pyrometallurgical processes have disadvantages such as higher energy consumption and emission of toxic gases that cause pollution [8]. Contrary to that, due to a lower energy consumption and environmental impact, the hydrometallurgical processes are being increasingly used for tungsten and cobalt recovery from cemented carbide.
In the hydrometallurgical recycling processes, the cemented tungsten carbides are immersed in leachant to dissolve the matrix or binder material and leave a tungsten carbide residue. The recycling processes are based on alkaline or acid treatment [[8], [9], [10], [11], [12], [13], [14], [15]]. Renee and others have developed the recycling technology for production of the bulk WC-Co cemented carbides from a mixture of WO3, CoWO4, and graphite powders using the carbothermal reduction combined with reactive sintering [16].
On the other hand, the identification and characterisation of products in each phase of the recycling process is an important task. An essential novelty of this work is the use of LDI-MS as a complementary method to the XRD as the established method for identification of the crystallographic phases. Previous studies have shown that the matrix-assisted laser desorption ionisation mass spectrometry (MALDI-MS) can be applied in the chemistry of tungsten compounds. For example, polyoxometalates (POMs), built with ten tungsten-oxides, distributed around a central silicate nucleus, and grafted with two linear alkylphosphonium chains were effectively examined using the MALDI-MS in the negative ion mode [17]. However, the analyses of inorganic systems often use the MALDI method without a matrix, called LDI-MS [[18], [19], [20]]. This method was successfully applied to determine the different species of tungsten anions from authentic standard solutions of tungsten compounds such as sodium tungstate (Na2WO4x2H2O), sodium metatungstate (3Na2WO4x9WO3xH2O), and ammonium paratungstate ((NH4)10H2(W2O7)6xH2O), tungsten dioxide (WO2), and tungsten trioxide (WO3). In previous studies, the results revealed that the main characteristic of the LDI mass spectra for these compounds are two types of tungsten cluster anions [(WO3)n(WO3)−] and [(WO3)n(OH)−]. The LDI-MS provides conditions to detect their isotopic peaks, enabling the determination and differentiation of tungsten compounds. Therefore, LDI-MS is proposed as a useful screening method of synthetic procedures in the field of tungsten chemistry [21,22].
The present study has been focused on tungsten recycling from cemented carbide alloy by the use of leaching of diamond core drilling crowns. In order to obtain the most efficient tungsten recycling process, the optimal conditions of the two key stages, leaching with nitric acid and leaching with sodium hydroxide, were determined. The XRD, SEM-EDS, ICP-AES, and LDI-MS methods were applied to identify the tungsten compounds obtained in crucial phases in the recycling process. The LDI-MS method greatly reduced the analysis time and amount of the sample compared to the other methods such as XRD, hence the additional aim of this paper is the promotion of the use of the LDI-MS method in technological processes in the future.
Section snippets
Experimental the study was carried out on ten samples of waste diamond core drilling crowns with a total weight of 4524.45 g
The waste drilling crowns are made of two cemented carbide grades. The nominal composition of tungsten carbide and binder used for sintering, with the mechanical properties of the sintered grades, are given in Table 1. The manufacturer of cemented carbides was Boart HWF GmbH & Co. KG.
The samples were ground in a ball mill (ball diameter of 30 mm) for one hour. After grounding, samples were sieved with a mesh size of 2 mm, in order to remove the steel ring. The total mass of diamond core
Characterisation of waste diamond core drilling crowns
The average chemical composition of ten samples of waste diamond core drilling crowns obtained after grinding and using several analytical methods, is given in Table 2.
The Table 2 shows that the main elements of the samples were tungsten, cobalt, iron, and carbon. The main impurities were nickel, titanium, tantalum, chromium, niobium, and aluminium. Also, it can be concluded from this Table that the chemical analysis results, obtained by three different methods, are very similar. Deviation of
Conclusion
A simple and cost-effective hydrometallurgy recycling route of diamond core drilling crowns recycling has been developed. The complete process consists of two main phases, leaching in nitric acid in order to remove Ni, Co, Fe, and Cr, and obtain tungsten carbide, followed by another leaching with sodium hydroxide solution with sodium tungstate as the final product.
The optimal results for acid leaching were achieved with the following parameters: 1.0 M concentration of nitric acid, leaching time
Declaration of Competing Interest
None.
Acknowledgments
This work was financially supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia, Grant Nos. 451-03-9/2021-14/ 200052; 451-03-9/2021-14/ 200135; 451-03-9/2021-14/ 200017. This article is based on the work of COST Action CA115102, supported by the COST (European Cooperation in Science and Technology) and the ITHACA Project: Innovative and Sustainable Technologies to Reduce the Critical Dependence of Raw Materials for Cleaner Transport Applications.
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