Note: Descriptions are shown in the official language in which they were submitted.
2B~7~
LOW-L~AD ZINC ALLOY POWD~RS FOR ZERO-M~RCURY
ALKALIN~ BATTERIXS
This invention relates to low-lead zinc alloy
powders for zero-mercury alkaline batteries displaying low
and iron resistant after discharge gassing, hereinafter
simply called gassing.
sackground of the Invention
The role of mercury in suppressing gassing (due
to hydrogen evolution) of zinc powders in alkaline battery
electrolytes is well known. However, mercury is toxic and
it has become highly desirable to provide mercury-free
alkaline batteries.
Addition of alloying elements to zinc, namely
bismuth, indium, gallium, aluminum, and other elements is
known to reduce corrosion of mercury-free alkaline
batteries, such as disclosed in U.S. patent No. 5,082,622.
However, as disclosed in U.S. patent No. 5,108,494,
gassing of mercury-free alkaline batteries made of zinc
alloy powders is strongly dependent upon the iron content
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of the zinc power. According to the above patent, the
iron content of the zinc powder used for making mercury-
free alkaline batteries must be kept below 1 ppm.
It is the object of the invention to provide
zinc alloy powders for mercury-free alkaline batteries
which display low gassing even in the presence of higher
amounts of iron.
Summary of the Invention
The present invention provides mercury-free
(also called non-amalgamated) zinc alloy powders which are
characterized by a low gassing rate in the presence of
iron up to 30 ppm. The zinc alloy powder in accordance
with the present invention consists of 0.001 to 0.1 wt %
lead, 0.01 to 0.1 wt % bismuth, 0.01 to 0.1 wt % indium
and 0.01 to 0.1 wt % Al, the balance being zinc and
unavoidable impurities.
The bismuth or indium content of the zinc alloy
powder is preferably from 0.05 to 0.1 wt % so as to
produce a zinc alloy powder exhibiting negligible
sensitivity to iron concentration up to 30 ppm.
Short Description of the Drawing
The invention will now be disclosed, by way of
example, with reference to specific examples and to the
accompanying drawing which illustrates the iron effect on
gassing for various zinc alloy powders.
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Detailed Description of the Invention
Preliminary tests have shown that the addition
of bismuth, or indium to a zinc alloy containing 500 ppm
of lead and less than 5 ppm iron increases gassing, while
the combination of bismuth and indium is not efficient in
reducing gassing. Only aluminum was found to reduce
gassing. The above is shown in the following table.
TAB~ I
I~ . =
Nominal composition (ppm) Gassing
Bi In Al microl/g-day
0~- __ 0 44~-
0 0 300 ~5
I
0 500 0 264
0 500 300 60
. 11
500 0 0 144 1
.. _ 11
500 0 300 59
500 500 0 208
500 500 300 18
___ .. _
It also has been found that a reduction in
impurity content such as antimony, is not sufficient to
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suppress gassing of regular lead-zinc alloys (lead at
500 ppm), unless iron is also reduced to the lowest level
(1 ppm). Indeed, a linear relationship was found between
gassing of regular lead-zinc alloys and the corresponding
iron content. The sensitivity was found to be 20
microl/g-day per ppm.
Applicant has surprisingly found, in accordance
with the present invention, that the problem of
sensitivity of zinc powder towards iron contamination can
be solved by adding to a low-lead zinc powder specific
combinations of Bi, In and Al.
The following describes the embodiments of the
present invention. The requisite alloys were prepared by
addition of the respective alloying elements in their
metallic form to molten zinc. The molten alloys were
converted to powder using low-pressure, dry-air atomizing.
The obtained product was not sieved.
Examples 1 to 4
Zinc containing 250 ppm lead was alloyed with
bismuth, indium and aluminium. The starting material was
zinc ingot having an iron content around 2 ppm. Iron was
successively added by dissolving steel in molten zinc.
As shown in Table 2, Bi-In-Al zinc alloys
containing 250 ppm lead and more than 200 ppm of indium
or/and bismuth display a strong resistance to iron
contamination. In comparison, a lead (250 ppm) - zinc
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powder containing less than 5 ppm iron exhibits a gassing
rate of 130 microl/g-day.
TABL~ 2
_ _ .. __
Added elements Gassing
. _
Lead Bismuth Indium Aluminium Iron
.___
(ppm) (ppm) (ppm) (ppm) (ppm)microl/g-day
250 200 200 600 3 46
250 500 200 300 5 33
I
250 200 500 300 2 34 l
__. 11
250 500 500 600 4 33 1
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Exam~les 5 to ~
Zinc containing less than 25 ppm of lead was
alloyed with bismuth, indium and aluminium. The starting
material was zinc ingot having an iron content lower than
2 ppm. Iron was successively added by dissolving steel in
molten zinc.
As shown in Table 3, Bi-In-Al zinc alloys
containing less than 25 ppm lead and more than 200 ppm of
Indium or/and Bismuth display a strong resistance to iron
contamination. In comparison, the gassing of zinc powder
containing no alloying element and less than 5 ppm of iron
is around 400 microl/g-day.
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TABLE: 3
_ _
Added elemen~s Gassing
L0ad Bismuth Indium Aluminium Iron
(ppm) (ppm) (ppm) (ppm) (ppm)microl/g-day
5 ~ 25 200 200 300 5 42
500 200 600 6 64
200 500 600 3 72
.
500 500 300 4 63
I
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A graph showing the effect of iron composition
in zinc alloys powder on the gassing rate is shown in the
accompanying drawing. All the alloys described in example
1 to 8 above are plotted. The dependence of regular lead
zinc powder on iron contamination is also shown by a
dashed line.
