metal-powder.net October 2005 MPR 23

ShowpieceAPT plantrides the tungsten wave

Figure 1. Bird's eye view of the Chenzhou APT powder plant.

China produces approximately 70 per cent of the world's tungsten raw materials and possesses nearly 45 per cent of the world's known tungsten ore reserves. Over the past five years the rapid growth inChina's economy and increasing global demand for tungsten has drivenup prices of high quality intermediate tungsten products such as ammonium paratungstate (APT) and tungsten oxides. In the second oftwo articles, Bernard Williams reports from a new 10 000 tonnes/yearAPT powder plant at Chenzhou in Hunan Province.

The recent and con-

tinuing boom in the

Chinese economy

has resulted in a

surge in demand for tungsten

raw materials. To meet increas-

ing demand from both domes-

tic producers and expanding

export markets, production of

tungsten concentrate grew in

China from an estimated

31,100 metric tonnes (65 per

cent WO3 content) in 1999 to a

Government-controlled quota

of 52,000 metric tonnes in

2004. At the same time some

of the larger Chinese tungsten

enterprises consolidated into

vertically integrated groups

involved in mining, ore pro-

cessing, and production of

downstream products such as

cutting tools.

One example is the

Chenzhou Diamond Tungsten

Products Co Ltd, now one of

China's largest producers of

ammonium paratungstate

(APT) and tungsten oxide

powders. Chenzhou Diamond

Tungsten's 10 000 tonnes/year

capacity powder plant was set

up in 2003 on an 110,000

square metre site some 12 km

from Chenzhou City in Hunan

Province. It is the result of a

joint venture involving an

investment of RMB 150 mil-

lion (approximately $20 mil-

lion) by carbide producer

Zhuzhou Cemented Carbide

Group (ZCC) and mining

company Hunan Shi Zhu Yuan

Non-ferrous Metals Ltd . ZCC

has a 60 per cent controlling

interest.

In 2004 the plant, which

employs just over 200 people,

produced a total of 9000

tonnes of intermediate tung-

sten powders having a sales

value of RMB 498 million. In

2005 the company expects sales

to double to more than RMB 1

billion mainly as a result of

increases in the selling price of

APT over the past couple of

years.

According to Mr Zhengang

Yao, deputy general manager

at Chenzhou Diamond

Tungsten, and Metal Powder

Report's guide on the tour of

the impressive facility, 70 per

cent of current production is

consumed by ZCC with some

20 per cent being directly

exported and 10 per cent sold

to domestic producers. Mr

Zhengang told Metal Powder

Report that prices for APT

powder have trebled over the

past two years from RMB

50/kg to RMB 160/kg. He

attributed the escalation in

prices to three main factors:

• A severe shortage of raw

materials from China's tung-

sten mines resulting in higher

tungsten ore concentrate

prices;

• China's surging domestic

demand for tungsten from its

producers of tungsten lamp fil-

aments, carbide tools, etc; and

• Growth in demand from

export markets.

Mr Zhengang said his com-

pany processes mainly two

types of scheelite concentrates

to produce its tungsten inter-

mediate powders - one having

65 per cent WO3 content and

another with 35 per cent WO3.

Some wolframite is also

processed, he said. While

Hunan Province is well

endowed with its own reserves

of tungsten ore, Chenzhou

Diamond Tungsten derives

only around 30 per cent of its

24 MPR October 2005 metal-powder.net

Figure 2. Hall showing part of the chemical purification process to produce APT powders.

page 26

26 MPR October 2005 metal-powder.net

raw materials from local mines

with the rest having to be pur-

chased elsewhere in China.

A tour of the plant included

a well-equipped Analysis and

Test Centre where the latest

array of spectrometers and

measuring instruments are in

use. The chemical plant being

relatively new also uses the lat-

est equipment including a com-

puter control centre which

oversees the whole process.

Individual parts of the process

are also computer controlled in

the different workshops.

The first stage of the chem-

ical process involves ball

milling the incoming tungsten

(scheelite or wolframite) con-

centrate to a powder having

less than 45 mesh particle size.

This ball-milled powder is

then charged under pressure of

10 bar into a tank containing a

sodium hydroxide solution

maintained at around 200°C

where the milled powder is

decomposed. Impurities such

as silica, arsenic and phospho-

rus are removed from this hot

sodium tungstate solution by

chemical purification to pro-

duce a pure sodium tungstate

(Na2WO4) solution. The same

basic process is used when

wolframite ore concentrates

are processed but the tempera-

ture during decomposition

under pressure is lower at

around 180°C. The next stage

involves treating the pure

sodium tungstate with

hydrochloric acid to remove

any molybdenum through

filtration.

A mineral of many colours

The result is an ammoni-

um wolframite solution

which is finally evaporated

at 90°C to produce insoluble

a m m o n i u m

paratungstate in a crystalline

form with the complex

chemical formula

(NH4)10W12O41.5H2O that is

also known as APT - the main

tungsten raw material traded

in the market. These crystals

are washed, filtered, dried in

continuous rotary furnaces at

200°C. The APT powder is

then fed from the drying

ovens onto vibrating screens

to sieve out the required 100-

200 mesh size fraction and

filled into 600 kg drums. A

further stage beyond the APT

grade is the production of

tungsten oxides.

Here the APT powder is cal-

cined in rotary kilns in a reduc-

ing atmosphere at a temperature

of 600°C - 700°C to convert it to

WO3. The decomposition con-

ditions in this heating step

determine the type and quality

of the oxide formed. For exam-

ple, high oxygen content during

calcining results in a yellow

coloured WO3. With a slightly

reducing atmosphere a blue

tungsten oxide is formed, and in

a hydrogen atmosphere WO2 is

formed. The type of oxide

determines the properties of the

tungsten or tungsten carbide

metal powder formed during the

final stage of the process.

Chenzhou Diamond Tungsten

has, for example, developed its

own proprietary process for

producingb ”purple” WO3 for

further processing into ultrafine

W or WC powder.

Figure 3. Mr Zhengang Yao demonstrates Chenzhou DiamondTungsten's computer control system.

Figure 4. Mr Zhengang (right) explains the chemical process to pro-duce APT powder to Metal Powder Report representatives BernardWilliams, Xiao Yulin (left) and Tao Zhengji (second left).

THE AUTHOR would like to

extend his warmest thanks

to Mr Yang Bohua,

President of ZCC, and

Vice-President Mr Gao

Zairong and his colleagues

at ZCC for their hospitality

during his visit. In particu-

lar thanks are also due to

Mr Xiao Yulin, and Mr Tao

Zhengji for their efforts in

arranging the visits, and for

their input to the organisa-

tion of Metal Powder

Report's PMAsia2005

Conference in Shanghai.

Acknowledgements

page 24