Sintering Behavior and Hardness of Tungsten Prepared by Hard Metal Sludge Recycling Process without Ammonium Paratungstate

In this paper, we suggest a novel recycling process for hard metal sludge that does not use ammonium paratungstate. Ammonia, which in the conventional recycling process is essential for removing sodium and crystallized tungstate, was not used in the novel process. Instead of ammonia, acid was used to remove the sodium and crystallized tungstate resulting in the formation of tungstic acid (H2WO4). Tungsten powders were successfully synthesized by hydrogen reduction of the tungstic acid through H2O decomposition, WO3 to WO2 reduction, and tungsten metal formation. The tungsten powders prepared from tungstic acid were spherical in shape and had a higher sintering density than the facet-shaped tungsten powders prepared from tungsten oxide. The spherical shape of the tungsten powders enhanced their sinterability and resulted in an increase in the size of grains. This is a result of the high diffusion rate of the atoms along the particle surfaces. Despite having a higher density, the hardness of the sintered tungsten was lower than that of tungsten from tungsten oxide. High energy milling effectively reduced grain size and improved hardness. The hardness of the tungsten prepared from milled tungstic acid was enhanced to a value (max. 471 HV) higher than the best previously reported value (389 HV). In sum, tungsten can be hardened, thereby improving its sinterability and reducing grain size, with tungstic acid prepared using the proposed recycling process.

Facile Synthesis of Single-Phase Alpha-Tungsten Nanopowders from Ammonium Paratungstate by RF Induction Thermal Plasma and Thermochemical Reduction

Facile, economic methods of preparing tungsten (W) nanopowder are critically needed to meet industrial demand. Herein, we report a method of preparing single-phase alpha-W (α-W) nanopowders using ammonium paratungstate (APT) as a starting material and the optimum synthesis conditions. The process involves two stages: i) the radio-frequency (RF) induction thermal plasma treatment of APT, followed by ii) thermochemical reduction at 600-900 ^oC. The crystallographic phase and morphological evolution of all products were systematically investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM), and the effects of the annealing temperature on the phase and particle size of the obtained powders were also evaluated. When the RF induction thermal plasma treatment was conducted with and without H₂, the XRD and FESEM results showed the formation of mixed-phase α- and beta-W (β-W) nanopowder and WO₃ nanopowder, respectively. Single-phase α-W nanopowder was achieved by annealing the WO₃ nanopowder in an H₂ reductive atmosphere at 700 ^oC for 10 min, resulting in homogenous nanoparticles with a small particle size (d50) of 21.16 nm without any aggregation.

Correction to: Synthesis and Characterization of Nanoscale Tungsten Particles with Hollow Superstructure Using Spray Drying Combined with Calcination Process

In the article [1], the use of the formula (NH4)6W7O24·6H2O to represent the starting material ammonium paratungstate (APT) is outdated and incorrect.