Chemicals Based Technologies - Ultra-Purification of Arsenic

Introduction

Ultrapure arsenic (6N) finds extensive application in electronics industry as III-V compound semiconductors and as a dopant. Arsenic (6N pure) at present is not manufactured in India and needs import. BARC has standardized a protocol for producing 6N (99.9999%) pure arsenic from commercially available arsenic trioxide (99.9%). The product quality obtained is equal to or better than the imported product. The process developed is at a 200 gm batch scale.

Handling of ultra-pure arsenic is carried out under clean benches in clean room conditions as impurities may enter the product though ambient air and thus spoil the quality of product. Commercial arsenic trioxide (99.9%) is used as the starting material.

Ultrapurification of Arsenic

The main impurities present in this are antimony, selenium, tellurium, sulphur, phophorus, zinc, copper, cadmium which are elements having similar properties to arsenic or originated from the ore from which arsenic trioxide has been obtained.

Purification of arsenic is carried out in clean rooms as described below to bring impurities down to ppb level.

1. Commercial arsenic trioxide is first purified by fractional sublimation under static vacuum in specially designed quartz apparatus under gradient temperature. The gradient temperature has been optimized such that the volatile, non volatile impurities are separated and arsenic trioxide of 99.999% purity is obtained.
The reaction occurs as follows:

As2O3 (s) + 3 H2 O (g) -----> 2As (s)+ 3H2O (g)

Arsenic thus obtained is 99.9999% (6N) pure in which all the impurities are reduced to sub ppm levels.

2. This purified arsenic trioxide is reduced to ultrapure arsenic at high temperature under high purity hydrogen gas flow.

3. This ultra-pure arsenic is again heated under a hydrogen gas flow in order to remove any trace impurities and oxide layer , if any.

The ultrapure arsenic is sealed under high vacuum in previously degassed pyrex glass ampoules for storage. This ensures that purity of the product is maintained.

Applications

Ultrapure arsenic is used in the manufacture of semiconductor materials such as gallium arsenide, indium arsenide, etc. which are used in high speed opto-electronic devices (gun diodes, laser diodes, light emitting diodes, etc). These applications have created an increased demand for high purity arsenic in recent years.

Infrastuctural Requirements

A. Clean room area (Class 10000) 15ft (w) x 15ft (1)
B. Clean benches (Class 100 - 1 no.)
C. Fume hood (3 nos.)
D. Skilled Man Power: B.Sc. (chemistry) graduate from recognized university (4 nos.)

For further information please contact :

Head,
Technology Transfer & Collaboration Division,
Bhabha Atomic Research Centre, Trombay,
Mumbai - 400 085
Tel : 091-022-25505337/25593897
Fax : 091-022-25505151
E-mail : headttcd@barc.gov.in

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