Study On One-step Preparation Of High-concentration Ammonium Metatungstate Solution From Tungsten Trioxide Monohydrate

Ammonium metatungstate is highly water-soluble and has an irreplaceable advantage in catalysis,which is important for making catalysts and ultrafine tungsten powder.The current industrial preparation of ammonium metatungstate mainly includes the preparation of highly concentrated ammonium metatungstate solution from ammonium paratungstate and spray pyrolysis,which has the problems of high energy consumption,long process and heavy pollution.In this paper,we propose a new process to prepare high-concentration ammonium metatungstate solution directly by the reaction of tungsten trioxide monohydrate and ammonium-containing compounds in one step.The reaction process of wet tungsten trioxide monohydrate with ammonium bicarbonate,ammonium paratungstate decahydrate,and ammonium-containing compound C was studied.The effects of the reaction temperature,time,liquid-solid mass ratio and other major factors on the transformation rate of tungsten and the concentration of ammonium metatungstate solution were investigated.The main research works and conclusions are as follows.(1)A new method for the direct preparation of highly concentrated ammonium metatungstate solution by one-step hydrothermal reaction between tungsten trioxide monohydrate and ammonium source was proposed.The thermodynamic analysis and preliminary experiments show that: when ammonium bicarbonate was used as the ammonium source,the concentration of ammonium metatungstate solution was up to 599.16 g/L,the tungsten transformation rate was 70.29%.When ammonium paratungstate decahydrate was used as the ammonium source,the concentration of ammonium metatungstate solution could reach 711.61 g/L.The hydrothermal reaction of ammonium source C with tungsten trioxide monohydrate was carried out by adding 500 g/L of priming solution,the tungsten transformation rate was 60.5% after the reaction,and the concentration of ammonium metatungstate solution was 1015.92 g/L.(2)The influence laws of temperature,liquid-solid mass ratio,and reaction time on the transformation of tungsten trioxide monohydrate to ammonium metatungstate were clarified.The transformation rate of tungsten showed a trend of increasing and then decreasing when the temperature was increased and reached the highest at 110-130℃.The solid-liquid mass ratio had an obvious influence on the hydrothermal reaction: the environment with less water was not conducive to the reaction;the transformation of tungsten trioxide monohydrate increased with the increase of the liquidsolid mass ratio and stabilized when the liquid-solid mass ratio was greater than 3.The hydrothermal reaction of tungsten trioxide monohydrate and ammonium bicarbonate could be completed within 10 minutes,the insoluble material is only a small amount of tungsten trioxide monohydrate that has not been completely reacted.(3)The intrinsic mechanism that limits the increase of tungsten transformation in tungsten trioxide monohydrate was initially revealed.At90°C,the insoluble product formed by the reaction between tungsten trioxide monohydrate and ammonium bicarbonate was ammonium paratungstate tetrahydrate,which was difficult to continue the reaction with tungsten trioxide monohydrate to form ammonium metatungstate;and during the reaction of tungsten trioxide monohydrate with ammonium paratungstate decahydrate,the dissolved amount of ammonium paratungstate decahydrate decreased significantly with the increase of the concentration of ammonium metatungstate solution in the system,resulting in the remaining part of ammonium paratungstate decahydrate could not be dissolved and participate in the reaction.When ammonium bicarbonate or ammonium paratungstate decahydrate reacts with tungsten trioxide monohydrate at higher than 100°C,another reaction slag such as pyrochlore-type tungsten trioxide and ammonium tungsten bronze was formed in the late stage of the reaction,which increased the loss of tungsten.