CVD technology since the late sixties, the coated tungsten carbide cutter has been very widely used. In the CVD technique, the preparation of the required metal source vapor deposition is relatively easy to achieve TiN, TiC, TiCN, TiBN, TiB2, Al2O3, such as single and multi-multi-layer composite carbide coating, the coating and substrate high strength, a film thickness of up to 7 ~ 9μm, CVD coating has good abrasion resistance.

In the late eighties, the United States 85% of tungsten carbide cutting tools with a coating, wherein the CVD coated tungsten carbide cutting tools accounted for 99%.

The mid-nineties, CVD coated tungsten carbide coated tungsten carbide inserts in the tool still account for more than 80%. For example, using the CVD method TiC single coat of WC-based cemented carbide substrate YW1 / TiC, and uncoated WC-based cemented carbide turning YT15 contrast. TiC single coated blade wear is much higher than YT15. CVD technology but also has its congenital defects, one process treatment temperature is high, the tool is easy to cause a decline in the flexural strength of the material; the second is the internal film tensile stress state, the use of micro-cracks easily lead; third CVD technology emissions, waste can cause industrial pollution, greater impact on the environment, and the current conflict to promote green industry, so the mid-nineties the development of high-temperature CVD technology subject to certain constraints.




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The ammonium paratungstate is present in the ammoniated aqueous strip solution in an amount of from about 200 to about 300 grams per liter. Further, the ammonium paratungstate is substantially free of the impurities originally present in the brine.

The concentrated, purified ammonium paratungstate can be recovered from the ammoniated aqueous strip solution by any technique known in the art, such as evaporative crystallization, cooling the hot strip solution to allow the ammonium paratungstate to crystallize and the like. The crystallized ammonium paratungstate then can be recovered by filtration.

The filtrate of the filtration can be recycled to the two stage strip circuit of the secondary solvent extraction system where, after pH adjustment, the filtrate is reloaded with ammonium paratungstate at a temperature of from about 45C.to about 55° C. This procedure eliminates the need for additional evaporation stages and thus results in a substantial savings in equipment costs.

The ammonium paratungstate product recovered from the filtration of the strip solution can be dried using any suitable technique known in the art. Further, the ammonium paratungstate will be substantially free of impurities originally present in the brine, namely arsenic, boron, phosphorus, silica and other heavy metals.

The above described process provides an efficient and effective method for recovering concentrated, purified tungsten values (such as ammonium paratungstate) from a brine containing minor amounts of tungsten values (in the form of tungstic oxide) and substantial amounts of impurities without the use of substantial chemical costs, processing costs, or equipment modifications to obtain the desired result.

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Ammonium paratungstate was prepared from tungsten alloy swarf employing anodic dissolution in sodium hydroxide using a packed bed configuration followed by chemical conversion into ammonium tungstate through tungstic acid. The alkaline tungstate solution was dialysed to recover sodium hydroxide, which could be recycled to the electrolytic dissolution. The influence of anodic current density and other parameters on dissolution/recovery was studied and discussed. It was observed that a current density of 600 mA/100 g scrap could be applied when the concentration of NaOH was maintained around 2.5 M. The energy consumption for anodic dissolution was 2.14 kWh/kg W with more than 90% recovery achieved. The exhausted scrap contained an alloy of nickel and iron with a very low percentage of tungsten. APT sample prepared through anodic dissolution in NaOH was characterized.

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Within the framework of a review of the fundamental processes applied for the manufacture of ammonium paratungstate (APT) the main byproducts and wastes generated in these processes and their environmental impact are summarized.

A subsidiary process based on the electrodialytic recovery of NaOH for the elimination of the salt byproduct, which is generated in the largest quantities, is outlined together with the main aspects to be taken into account for large-scale industrial implementation.

The present stage of development and implementation of an APT production line for the processing of typical scrap material from the lighting filament industry is shown, as well as the oxidizing and dissolving process of compact and lumpy tungsten alloy scrap (Cu-W, Fe-Ni-W) and hard metal scrap (Co-WC, Co-WC-Ta(Nb)C-TiC).

Several as yet unexploited potentials are sketched which could, from scientific and engineering standpoints, be interesting for the development of more efficient intermediate manufacturing processes for APT and tungsten or process operations for a wide variety of raw materials.

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