Analytical Chemistry of Molybdenum and Tungsten. Including the Analysis of the Metals and Their Alloys

284 Pages·1971·8 MB·English

by  W. T. Elwell

ANALYTICAL
CHEMISTRY OF MOLYBDENUM
AND TUNGSTEN
(Including the Analysis of the Metals and their Alloys)
BY
W. T. ELWELL AND D. F. WOOD
Imperial Metal Industries Limited
(Kynoch Works)
Witton, Birmingham, B6 7BA
PERGAMON PRESS
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Copyright © 1971 W. T. Elwell and D. F. Wood
All Rights Reserved. No part of this publication may be repro­
duced, stored in a retrieval system, or transmitted, in any form
or by any means, electronic, mechanical, photocopying, record­
ing or otherwise, without the prior permission of Pergamon Press
Ltd.
First edition 1971
Library of Congress Catalog Card No. 79-163641
Printed in Great Britain by A. Wheaton & Co., Exeter
08 016673 3
PREFACE
THE increasing importance of producing materials for high-temperature applications in
supersonic aircraft, re-entry vehicles, and other space systems, has resulted in extensive
research and development programmes involving molybdenum, tungsten, and other
refractor}^ metals. These new ventures, and the sustained interests in molybdenum and
tungsten for other purposes, e.g. in the production of alloyed steels, have contributed to the
growing importance of the analytical chemistry of these metals and resulted in significant
advances in the principles and methods used for their analysis and determination.
Current trends in the analytical chemistry of molybdenum and tungsten are evident from
the numerous publications that have appeared during the last decade, and analysts, especially
those who have become actively engaged in this field for the first time, often find difficulty
in selecting the best methods to use from the abundance of scattered information.
With this background, and the authors' interests and practical experience in this area of
analytical chemistry, the vast amount of widespread information on this subject has been
collated, and because of the close chemical and physico-chemical similarity of molybdenum
and tungsten (and their corresponding compounds) the relevant analytical details of these
two metals have been combined and presented in a single volume.
In compiling this review, the aim has been to include the latest information on chemical
and instrumental methods of analysis, involving, for example, solvent extraction, polaro­
graphy, atomic-absorption. X-ray fluorescence, and radiochemical analysis, as weh as
"classical" chemical determinations. Detailed methods for determining molybdenum and
tungsten in typical samples, and the analysis of these metals and their alloys by both classical
and modern techniques, have also been included.
The authors have endeavoured to adopt a uniform, succinct style, especially in presenting
the more detailed analytical procedures, and in doing so have taken every reasonable care
to avoid any departure from the practical recommendations of the original authors.
They have left the choice of method for any particular problem very largely to the reader's
discretion; the objective has been to present the relevant information in such a way as to
enable a rehable assessment to be made of the requirements, uses, and hmitations of a wide
range of procedures.
In making a compilation from such a wide area of knowledge, the authors have drawn
freely on the published work of others. The value of this information is gratefully acknow­
ledged, and any omission to give full credit in the references is a genuine oversight.
vii
VIH PREFACE
It is hoped that this book will provide a useful source of reliable information not only to
analysts who have had experience with these metals, but to those for whom the analytical
chemistry of molybdenum or tungsten opens up new fields.
Research & Development Department^
Imperial Metal Industries Ltd,,
Kynoch Works,
Witton,
Birmingham, B6 7BA
9th September 1970
INTRODUCTION
IN PRESENTING the information contained in this publication it is assumed that the reader
is fully conversant with the Safety Precautions pertaining to the handling of reagents,
apparatus, etc.
REAGENTS—ĽĹŃÉÍÉąĎÍ8
To avoid repeating the quality, strength and preparative details of certain reagents that
are frequently mentioned throughout the book, the undermentioned reagents and, where
applicable, their preparation, are defined as follows (analytical-grade reagents of the highest
quality must be used):
Ammonia solution Ammonia solution (sp.gr. 0.91)
Concentrated hydrochloric acid Hydrochloric acid (sp.gr. 1.16 to 1.18)
Concentrated nitric acid Nitric acid (sp.gr. 1.42)
Concentrated sulphuric acid Sulphuric acid (sp.gr. 1.84)
Concentrated phosphoric acid Orthophosphoric acid (sp.gr. 1.75)
Hydroñuoric acid Hydroñuoric acid (40 per cent, w/w)
Concentrated perchloric acid Perchloric acid (sp.gr. 1.54)
EDTA The disodium (dihydrate) salt of ethylenediamine-
tetra-acetic acid. This acid is also known by the
synonym diaminoethanetetra-acetic acid
Dithiol Toluene-3,4-dithiol. Store the reagent in a refriger
ator
Fluoroboric acid To 280 ml of hydroñuoric acid (maintained at lO^'C)
add, in small amounts, 130 g of boric acid. Store
in a polyethylene bottle
Sulphurous acid Water saturated with sulphur dioxide
GENERAL INFORMATION
Dilute solutions of liquid reagents are made by appropriate dilution of the concentrated
reagent with water, e.g. sulphuric acid (1 + 4) is made by adding 1 volume of concentrated
sulphuric acid to 4 volumes of cold water; the water is kept cool and well agitated until all
the acid has been added; any contraction in volume brought about by the dilution is
ignored.
The use of distilled or demineralized water is intended at all times and, unless the diluent
is specified, "dilute" implies **dilute with cold water and mix the solution well". It is equally
ix
χ INTRODUCTION
important to mix solutions thoroughly after adding a reagent. Where the addition of a solid
reagent is speciñed, it is intended that the reagent should be dissolved before proceeding with
the next stage.
If water is not used in the preparation of a particular reagent solution, the solvent to be
used is implied in the text, e.g. "an ethanolic solution of. . .
Details for preparing what is essentially the same reagent solution are not necessarily
the same in all procedures; this has been unavoidable because the information has been
taken from the pubhshed work of many independent authors.
Unless otherwise stated, solutions of ah solid reagents are made by dissolving the required
weight (g) of the reagent in the appropriate volume (ml) of water, the percentage composition
(w/v) being based on the assumption that the reagent does not contain any water of
crystallization; otherwise the weight of the reagent used, as opposed to that recommended,
must be increased to take into account the additional water. On those occasions when the
hydrated reagent is normally available, the degree of hydration is stated in the text to make
it quite clear that calculations or instructions are based on the use of that particular hydrated
reagent.
The use of the hydrated salts is invariably intended when the following reagents are
referred to:
Stannous chloride SnCl2 · 2H2O
Ferric sulphate Fe2(S04)3 · 9H2O
Sodium tungstate Na2WO4 · 2H2O
Sodium molybdate Na2Mo04 · 2H2O
Ammonium molybdate (ΝΗ4)6Μθ7θ24·4Η2θ
In most instances it wih be obvious when weighings and volumetric measurements are to
be made accurately, but where this may not be so apparent, particularly with volumetric
additions, the volume is speciñed, e.g. as "10.0 ml", and accurate additions (to within
± 0.05 ml in this example) must be made.
In the colorimetric procedures, absorbances are measured at 20° ± 1°C. As a general
guide, conditions (e.g. ceh size and ñnal volume of solution) should be adjusted, if necessary,
so that the absorbance of the test solution is within the range 0.15 to 0.75; in preparing
the calibration graphs this absorbance range should be slightly extended beyond these
lower and upper levels. Where a reagent blank is to be determined, this must be made
concurrently with the analysis, and a suitable correction or compensation applied to the
final absorbance.
Many books dealing with gravimetric analysis contain information on the care and
attention necessary during the drying, ignition, and calcination of precipitates, and this
information is not repeated in the present text. It is emphasized, however, that filter papers
and filter-paper pads, including the material to be calcined, must be thoroughly dried and
the paper well charred at a conveniently low temperature (less than 500°C) before the final
ignition. The paper must not be ahowed to inflame and cause mechanical loss of the residue;
it is often an advantage to place a loose-fitting lid on the crucible or dish.
REVIEWS
Methods for the separation and determination of molybdenum have been reviewed by
Gmelin,^^^ Jensen and Weaver,^^^ and, more recently, by Busev.^^^ Reviews of the analytical
chemistry of tungsten have been pubhshed by Chernikhov and Goryushina,^*^ Bagshawe,^^^
INTRODUCTION χί
and Busev et al}^^ Chalmers^"^^ has reviewed methods for the gravimetric and titrimetric
determination of these metals, and reviews on their analysis and determination have
appeared biennially over the past ten years or so in Analytical Chemistry, Chemical methods
for determining molybdenum and tungsten in rocks have been reported by Jeffery/^^
REFERENCES
1. GMELIN, Handbuch der anorganischen Chemie, 8th edn., Verlag Chemie G.m.B.H., Berlin, 1935, Vol. 53,
p. 8.
2. JENSEN, K. J., and WEAVER, B., Analytical Chemistry of the Manhattan Project, McGraw-Hill, New York,
1950, p. 445.
3. BUSEV, A. I., Analytical Chemistry of Elements Series: Analytical Chemistry of Molybdenum, Daniel
Davey, New York, 1964.
4. CHERNIKHOV, YU. Α., and GORYUSHINA, V. G., Zav. Lab., 1946,12, 517.
5. BAGSHAWE, B., Chemical Age, 1954, 70, 267 and 309.
6. BUSEV, A. I., TIPTSOVA, V. G., and KHLYSTOVA, A. D., The Present State of the Analytical Chemistry of
Tungsten, Zav. Lab., 1962, 28, 1414-24.
7. CHALMERS, R. Α., in Comprehensive Analytical Chemistry (ed. by Wilson, C. L., and Wilson, D. W.),
Elsevier, Amsterdam, Vol. Ic, 1962.
8. JEFFERY, P. G., Chemical Methods of Rock Analysis, Pergamon Press, Oxford, 1970, pp. 325-333.
CHAPTER 1
HISTORY, OCCURRENCE, AND
APPLICATIONS OF MOLYBDENUM
AND TUNGSTEN
MOLYBDENUM
From about 350 B.C. to the early part of the eighteenth century, considerable confusion
prevailed in the use of the names molybdoena, galena, plumbago, and graphite. During the
seventeenth century, all of these terms were used to mean graphite. Later, galena was
identified with the mineral which now bears this name, and it was soon recognized that
molybdoena and graphite did not contain lead. The confusion which continued to exist
between the composition of molybdoena (or molybdenite) and graphite was resolved by
Scheele in 1778 when he discovered that the molybdoena used in experiments by Qvist was
an acidic substance, metallic by classification, and combined with sulphur, whereas the
molybdoena described by Von Cronstedt was a mineral composed of carbon and a small
proportion of pyrites. Accordingly, molybdoena was renamed molybdenite, and in 1782
Hjelm^^^ separated the metal from molybdenite and called it molybdenum.
Molybdenum does not occur in nature in the elemental form, and its compounds, though
widely distributed throughout the world, are among the rarer constituents of the earth's
crust; the average proportion of molybdenum in igneous rocks is about 1.5 X 10"^ per
cent.^^^ Various studies on the occurrence and distribution of molybdenum have been
puWished.(^-^>
The most important mineral, molybdenite (molybdenum disulphide), is found chieñy in
granites. Deposits occur in the United States (Colorado and Arizona), Canada (Queensland),
Chile, Norway, Mexico, Korea, and China. It is a bluish-grey substance with a metallic
lustre, has a specific gravity of about 4.7, and occurs as soft hexagonal ñakes, resembling
graphite. Molybdenite ores usually contain more than 0.5 per cent, of molybdenum
disulphide.
The metal is obtained by roasting the ore to oxide, followed by reduction either with
hydrogen, carbon, or aluminium, or electrolytically. The basic commercial process for the
production of high-purity molybdenum is by reduction of ammonium molybdate, or
molybdenum trioxide, with hydrogen. The metal obtained in this way is in the form of
powder, which is subsequently converted to fabricated forms by sintering processes or,
less frequently, by arc-melting techniques.
1
2 ANALYnCAL CHEMISTRY OF MOLYBDENUM AND TUNGSTEN
Another mineral of practical importance is wulfenite (lead molybdate); deposits of the
mineral have been reported in New Mexico, Morocco, Angola, Somaliland, and in the west
of the United States. Wulfenite is a secondary mineral produced by oxidation of associated
lead and molybdenum sulphides. Molybdenum also occurs as molybdate, associated with
magnesium, iron, calcium, and cobalt, in the minerals belonosite (MgMo04), molybdite
[Fe2(Mo04)3], powellite (CaMoOJ, and pateraite (C0M0O4).
The residues of copper smelting in Utah and New Mexico provide another source of
molybdenum. Molybdenum also occurs in the crusts of the fumaroles in the craters of
Vesuvius, and in volcanic lava in Hawaii. Traces of molybdenum have been found in the
ashes of many plants, e.g. Scotch fir, silver fir, vine, oak, poplar, and hornbeam.
To date, the United States has dominated the world in the production and consumption of
molybdenum; the estimated production of concentrates in 1955 was in the region of
70,000,000 lb, when there were known reserves of molybdenum in the United States for
about 100 years, based on the highest recorded rate of consumption at that time. The world
production of molybdenum in 1967 was estimated at about 65,000 metric tons, of which
North and Central America contributed about 77 per cent., Europe about 11 per cent,
and the Near East and Asia about 3 per cent.
Molybdenum is used extensively as an alloying constituent of steels, for electrodes in
resistor furnaces (particularly in the glass industry), for making anodes, grids and supports
for filaments in lamps and electronic tubes, and for electrical contacts generally.
High melting-point and excellent strength-to-density ratio account for the use of molyb­
denum in rockets, missiles, and space vehicles. High-temperature applications, however,
are limited by the fact that the metal oxidizes in air at elevated temperatures, although
recent work has shown that this can be overcome, to some extent, by the provision of a
protective refractory coating that is impervious to oxygen.
Molybdenum sulphide is used in lubricants.
TUNGSTEN
The history of tungsten is closely associated with a mineral which was thought to contain
tin, and the mineral was named lapides stanniferi spathacei by Wallerius.^^^ Various other
names linking the mineral with tin were also used. In Sweden the mineral was called tungsten,
from the Swedish tung (heavy) and sten (stone). The name wolframite was given to the
mineral by Breithaupt.According to Agricola, the term wolfram comes from the German
words wolf and ram (froth), hence his use of the term lupispuma for wolfram. The miners of
Saxony and Bohemia called it wolfart, i.e. wolfish or devouring ore, because of the low
yield of tin obtained from tin ores associated with this mineral. The metal is called wolfram
in Germany and tungsten in England and elsewhere. The first published account of the
isolation of the metal from the ore was in 1783 by the brothers de Elhuyar;^^^ they
obtained the metal by reducing the oxide with carbon.
Tungsten does not occur in nature in the elemental form. Its compounds, though widely
distributed, are among the rarer constituents of the earth's crust; the average proportion of
tungsten in igneous rocks is estimated to be about 5 χ 10"^ per cent.^^^ Studies on the
occurrence and distribution of tungsten have been published by Jeffery^^^^ and Vinogradov
et al}^'
Its occurrence is confined almost exclusively to the siliceous rocks, and it is found chiefly
as tungstate associated with iron, manganese, calcium, copper, and lead in the minerals
HISTORY, OCCURRENCE, AND APPLICATIONS 3
ferberite (FeW04), hübnerite (MnW04), wolframite [Fe,Mn(W04)], scheelite (CaW04),
cuproscheelite [Ca,Cu(W04)], and stolzite (PbW04). Tungsten is also found as the disul­
phide [tungstenite] and in small quantities with niobates and tantalates. Tungsten minerals
are conveniently divided into the wolframite and scheelite groups.
Tungsten ores are found in many parts of the world; deposits of commerical importance
occur in Burma, China, the United States, Africa, Australia, Portugal, Korea, and Bolivia.
Most ores are of low-grade quality, and rarely contain more than 2 per cent, of tungsten
trioxide. They occur irregularly, and the lodes are more or less sporadic. The primary ores
are those which occur in lodes, veins, or dykes, and are usually associated with granites;
the secondary ores are those found in alluvial and residual deposits.
The wolframite group (wolframite, ferberite, and hübnerite) range in colour from black
to dark reddish brown, and have a specific gravity of about 7.3 to 7.5. Scheelite ranges in
colour from brown, through yellow to white, and its specific gravity varies from 5.4 to 6.1.
An important property used in prospecting for scheehte is the fluorescence of the mineral
under ultraviolet light; uncontaminated scheehte gives a blue fluorescence, but when about
1 per cent, of powellite (CaMo04) is present, the fluorescence is white: higher concentrations
of molybdate produce a gradation of colours from yellow to flesh pink.
Ores of minor commercial importance include tungstite (H2WO4) and cuprotungstite.
In the production of tungsten metal, wolframite ore is first converted into an alkaline
tungstate, tungstic acid, or insoluble ammonium paratungstate. Any of these forms is
calcined to tungsten trioxide, which is then reduced with carbon or hydrogen; hydrogen is
used to obtain the metal in a state of high purity. Scheelite is usually treated with hydro­
chloric acid to yield tungstic acid, which is subsequently reduced to the metal. The metal
powder obtained by these processes is compacted into ingots, and these are heated in a tube
furnace at about 1000°C, in a reducing atmosphere, to impart adequate strength to the
product for subsequent handling. The ingots are then sintered electrically in a hydrogen
atmosphere at about 3200°C.
The world production of tungsten in 1967 was about 28,000 metric tons; about 50 per
cent, of this was produced in the Near East and Asia, about 27 per cent, in Europe, and about
14 per cent, in North and Central America.
Tungsten is particularly useful for high-temperature applications because of its very high
melting point and exceptional strength at high temperatures.
The metal forms an oxide which is not significantly volatile up to 1000°C, and in this
respect it has an advantage over molybdenum. Because of this characteristic, tungsten
would be used in place of molybdenum for many metallurgical applications if it were less
difficult to fabricate and more abundant.
It is used as a filament in incandescent lamps and as an alloying constituent (up to 20
per cent.) in high-speed cutting tools. Tungsten steels are also used in springs, valves,
magnetos, contact points, spark plugs, and numerous other products where strength,
hardness, resistance to corrosion, and a high melting point are essential.
Targets in X-ray tubes and electrodes used in inert-gas or hydrogen-arc welding are also
made of tungsten.
Tungsten carbide (Carbaloy), which is extremely hard, is used extensively for lathe tools,
cutters, drills, and dies for wire drawing. Kennametal (WTÍC2) and Stellite (Co,Cr,W) are
used for similar purposes.
Sodium tungstate is used as a mordant, and for the fireproofing of fabrics.