Recovery of tungsten trioxide from waste diamond core drilling crowns by nitric acid leaching

Highlights

• A cost-effective hydrometallurgy route for recycling diamond core crowns with a cemented carbide blade has been developed.

• The tungsten recovery rate of 95.6% was achieved.

• Obtained tungsten carbide and diamonds have been applied in practice for further use for the sintering of new heads.

• The main novelty was the use of LDI-MS to identify and characterise tungsten compounds in the recycling process.

Abstract

A cost-effective and simple hydrometallurgy procedure for recycling the diamond core drilling crowns by nitric acid leaching, followed by the sodium hydroxide leaching, has been developed. The effect of temperature, nitric acid/sodium hydroxide concentration, leaching time and stirring rate were studied. For the acid leaching process the optimal values for parameters were 60 °C, 1.0 M HNO₃, 2.5 h, and 800 rpm. It has been noted that the key factors in the alkaline leaching process were temperature and NaOH concentration. For the alkaline leaching, the optimal parameters were 100 °C, 40% concentration of NaOH solution, and leaching time of 4 h. Identification and characterisation of products in all technological processes were performed by X-Ray Diffraction (XRD), scanning electron microscope using energy-dispersive X-ray spectroscopy (SEM-EDS), and laser desorption ionisation mass spectrometry (LDI-MS). The analysis of solutions in all phases was performed by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The use of LDI-MS as a new approach to identify the products after phases in the recycling process was an important part of the study, and the results were consistent with the XRD analysis. The total tungsten recovery was over 95.6%, with a high purity of the obtained tungsten trioxide.

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.