Note: Descriptions are shown in the official language in which they were submitted.
PC~ /CAN
986~7~
The invention relates to a process for separating
the elements cadmium, bismuth, rhenium, lead, molybdenum,
tin, antimony, zinc, and arsenic as volatile gases from the
elements copper, silver, cobalt, iron, nickel, gold, and
precious metals contained in a feed material. Selective
reaction and volatilization of the first group of elements
is accomplished because of the presence of specific
quantities of sulfur dioxide in an oxygen-rich combusted
gas containing controlled additions of chlorine.
Smelting and refining operations ~enerate large
quantities of dust and hydrometallurgical wastes which are
collected and reverted to the smelter. Recycling of such
materials, which are prone to contain elements potentially
harmful to the final product, can lead to eventual buildup
o the levels of these undesired elements in the feed and
contamination of the final product. Thus, a need e~ists
for a more effective means for separatin~ contaminating
constituents such as cadmium, bismuth, lead, etc., from
such waste materials in order that they may be treated for
recovery o~ the desired metallic values.
Chlorination is a well known process for separating
various elements from their ores. Elements such as copper
and lead form chlorides when contacted with chlorine at
elevated temperatures. Such chlorides are highly volatile
and can be separated readily in gaseous form from solid feed
material. However, commercially used chlorination methods
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generally require that the ore or feed material be in an
oxidized state, requiring costly intermediate roasting
operation, prior to being brought into contact with chlorine
gas.
The foremost use of chlorination is in the separa-
tion by volatilization of the elements copper, zinc, lead,
and arsenic from ores containing nickel or iron. Such
processes are conducted at temperatures of about 800C to
1300C. There are variations on the basic process which
are aimed at the selective volatilization of individual
elements and subsequent recovery as well as a process in
which all of the chlorinatible metallics are volatilized,
collected, and fractionally distilled. One of the disadvan-
tages of such processes is that the order in which the
metallic elements are volatilized may not be entirely
desirable for further processing and recovery of these
elements.
Othar methods, substantially diferent from those
aforedescribed, do not involve volatilization. For example,
in one technique a chloridized cinder is formed at a somewhat
lower temperature, i.e., 200C to S00C, from which the
desired elements are recovered by leaching. The major
problem inherent in such methods is the need for aqueous
leaching which, with current requirements for purity of
water effluent, entails considerable purification expense.
The order in which the elements are volatilized
is o prime importance in all of the prior art processes.
A published order of volatilization for metallic chlorides
at 1000C, starting with the most volatile element and
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ending with the least volatile element, follows: Ag, Hg,
Pb, Cd, Cu, Mn, Ni, Zn, Sn, Fe, Mg, Cr, Ti, Al, Si. Based
on this information, numerous processes have been devised
for separating the various fractions and subsequently
concentrating elements in the volatilized gas or in the
residue. In many instances, it would be advantageous if
the order of volatilization differed from the restrictions
imposed by the natural order of volatilization.
It has now been discovered that the restrictive
order of volatilization of the metallic elements known in
conventional chlorination processes can be circumvented.
Indeed, it has been found that the volatilization sequence
can be advantageously rearranged by introduction of sulfur
dioxide to the chlorinating gas stream. Sulfur dioxide
may be formed in situ through reaction of excess oxygen
with sulfur in the feed material or by intentional addition
of sulfur dioxide to the chlorinating gas stream. In the
presence of sulfur dioxide, the order of volatili2ation is
chan~ed from that known and aforedescribed to the following
sequence which includes those elements of importance to a
currently used smelting and refining operation on a particular
feed material: Cd, Bi, Re, Pb, Mo, Sn, Sb, Zn, As, Cu, Ag,
Co, Fe, Ni, Au, and precious metals. Through control of
processing conditions, elements forming chlorides having
grea-ter volatility than co~per are removed from the feed
material. The elements remaining behind as solids in the
feed material are subsequently refined by conventional
methods.
It is an object of this invention to pro~ide a
process for reducing most substantially the presence of at
least one of the elements: cadmium, bismuth, rhenium, lead,
1~86~74
molybdenum, tin, antimony, zinc, and arsenic from a feed
material as volatile chlorides thereby leaving behind a
material enriched in at least one of the elements copper,
silver, cobalt, iron, nickel, gold, and precious metals better
suited to conventional smelting and refining operations.
Generally speaking, the present invention is
directed to a process for treating a finely-divided residue
containing at least about 1.5%, by weight, of sulfur and
at least one metal value from the group consisting of copper, -
silver, cobalt, iron, nickel, gold, and precious metals to
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remove therefrom at least one impurity from the group
consisting of Cd, Bi, Re, Pb, Mo, Sn, Sb, Zn, and As which
comprises: pelletizing said residue; and heating said
pelletized residue at a temperature of about 800C to about
1150C in an atmosphere containin~ chlorine in an amount of
about 0.1 percent to about 5 percent, by weight of charge
weight, of said residue and about 8% to about 20% by volume
of sulfur dioxide to volatilize said impurities and provide
a treated residue containing at least about 1 weight percent
sulfur enhanced in said metal value content. The sulfur in
the finely-divided residue and the treated residue is sulfide
~ sulfur and/or elemental sulfur.
; Advantageously, the gaseous stream contains from
about 2% to about 20% oxygen by volume of gas. Upon reaction
~ with the sulfur, this results in about 8% to about 20% sulfur
,~ dioxide, by volume, in the gas stream. Below about 8% sulfur
;j, dioxide in the gas stream present in the reaction zone, the
preferred volatilization order is not achieved while there
~ is little advantageous increase in volatilization with sulfur
', 30 dioxide contents above about 20%
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In carrying the invention into practice, it has
been found that it is particularly effective when used to
separate at least one of the elements cadmium, bismuth,
rhenium, lead, molybdenum, tin, antimony, zinc, and arsenic
as volatile chlorides from a feed material such as a finely-
divided residue containing electrostatic precipitator dusts
and reverts from hydrometallurgical operations. It is pre-
ferred that such a feed material contain from about 2~ to
about 20% total sulfur, by weight, with at least about 2%
of the sulfur being in the form of sulfide or elemental
sulfur, which quantity is sufficient to react with oxygen
in the gas stream and thus introduce the highly desirable
sulfur dioxide level of 8~ to 20%, by volume, into the gas
stream. In the event that insufficient sulfur is present
in a particular feed material, supplemental quantities of
sulfur dioxide may be ~ntroduced within the gas stream.
Sulfates can serve as an additional source of
sulfur dioxide by reaction with copper sulfide, and the
following reduction reaction is believed to occur:
a 3CuSO4 t- CuS ~ 4CuO t- 4SO2.
; For efficient operation, it is essential that sufficient
sulfur ~as sulfide sulfur and/or elemental sulfur) be present
in the feed material so that the amount of sulfate (SO4), in
wei~ht percent, is less than about
sulfur 4
0.3
Advantageously, the process is carried out in a
continuous fashion. A vertical shaft furnace was used for
the processing described herein and is considered useful for
this purpose; however, a rotary kiln, fluid bed roaster, or
other suitable apparatus could also be used for this purpose.
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The oxygen content within the reaction bed is of
considerable importance to the principal reactions. Also,
it has been found that in a highly-oxidizing, low-moisture
. atmosphere, accelerated volatilization of copper and arsenic
occurs; however, in turn, the volatilization of bismuth and
zinc is depressed. It is preferred that a sulfur dioxide
to oxygen ratio of at least 1:1, and preferably at least
4:1, or even 10:1 or more, be maintained for o~timum
volatilization. Further in this regard, the moisture
content of the combustion gas should be limited since
increasing the water content decreases the volatilization
of arsenic, zinc, and copper. It is preferred that the
moisture content of the gas stream be maintained between
5% and 20% by volume.
Although theoretically only the stoichiometric
- quantity of chlorine required to form volatile chlorides of
the impurity elements need be added, it has been Eound that
-~; a considerably greater quantity is required in practice.
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For feed materials of the type described herein, the presence
` 20 of 3% chlorine, by weight of charge weight, in the gas
stream is sufficient to lower all impurities to acceptable
levels. It is preferred that the chlorine addition be
maintained between about 1.2~ and about 3.5%, and still
more preferably between about 1.5% and 3.0~ by weight of
charge weight. It is preferred that chlorine be added as
a gas althou~h other sources of chlorine might be used.
However, as found with HCl addition, other sources may not
be as effective as gaseous chlorine.
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: Optimum volatilization conditions were achieved
with a throughput time of 1-1/2 to 2 hours. The temperature
, of the reaction bed should be controlled so that temperatures
in excess of 1150C are avoided since melting of the feed
material may occur. The rate of reaction at temperatures
below 800C is too low to be of commercial interest.
Generally, reaction bed temperatures between 925C and
1050C are preferred. The off-gases from the reactor are
treated in a scrubbing system to remove the volatilized
elemental chlorides, sulfur dioxide, and other contained
gases. Impurities retained in the scrubbers can be recovered
by conventional means.
For the purpose or giving those skilled in the art
a better understanding of the invention, the following
illustrative examples are given:
EXAMPLE I
. A mixture of electrostatic precipitator dusts and
reverts from hydrometallurgical operations were pelletized
readily by the addition of about 20% by weight of water as
a fine spray. The pellets were then dried at 110C to a
5~ moisture conten-t and temporarily stored as feed stock.
A pellet size of about minus 6 to about plus 30 mesh was
used in the vertical shaft furnace. The pellets were fed
at prescribed intervals into the top of a vertical shaft
furnace which was constructed Erom a 5 centimeter diameter
"Vycor" glass tube capped at both ends with fire-clay
crucibles and fitted with inlet and outlet ports to handle
solid and gas throughput. The shaft was heated in a
vertically-positioned furnace that was equipped with
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automatic heat control. Combustion gases, oxygen, and
chlorine were fed in countercurrent flow to the solids.
Off-gases from -this reactor were treated in a two-stage
scrubbing system consisting of a water scrubber and a
scrubber containing 4N sodium hydroxide solution. Neutral-
ized gas left the scrubbers through a regulator and vacuum
pump.
The 24-hour continuous operation was started by
heating a false charge of dead roasted pellets in the shaft.
After the desired temperature was reached and conditions ~-
had stabilized, the flow of chlorinating gases was started
and the scrubber system energized. Temperature was controlled
at about 1000C for the first 12 hours of the 24-hour run
and subsequently reduced to about 925C. The minus 6 to
plus 35 mesh pellets were added at the rate of 60 milliliters
every 15 minutes, which is equivalent to an average feed
rate of 3.9 grams per minute with an average retention time
of about 2 hours. The throughput of the gas mixture varied
between 1900 and 2500 cubic centimeters/minute with excess
oxygen content ranging from 2 to 20~ by volume. Moisture
and carbon dioxide were kept between about 6% and 10% by
volume with the balance of the gas mixture consisting of
nitrogen for all tests to simulate the effect of oil
; combustion. Typically, in addition to chlorine, the feed
~as consisted of, in volume percent, 74.6% nitrogen, 7.3%
carbon dioxide, 8.8% oxygen, and 9.3% water. In order to
f" obtain the desired oxygen content, it was necessary to add
liquid oxygen at the rate of 6% by volume of air which is
equivalent to 4% by weight to the weight of the feed.
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No sulfur dioxide was added to the gas s-tream. The charge
contained 7.45% sulfide sulfur with 4.15~ as sulfate prior
to treatment.
Chlorine gas was injected into the gas mixture
at flow rates of 18 or 36 cubic centimeters/minute, which
is equivalent to 1.5% and 3% chlorine by weight of feed.
The solid discharge from the shaft bottom was combined into
hourly composite samples. The samples were analyzed and,
together with operational data, evaluated for the efficiency
of the removal of impurities. The high concentration of
sulfur dioxide required for optimized volatilization is
spontaneously generated from the sulfide and sulfur content
of the charge by controlled oxidation and formation of
sulfur dioxide. The degree of sulfide and sulfur oxidation
has a controlling effect on the volatilization efficiency
of the elements. This effect is more obvious at the lower
chlorine addition level of, by weight, 1.5~ than at the
3% level. It was found that losses of copper, silver, and
precious metals increased with excessive oxidation, and
further increase was observed with increased chlorine
addition.
Analysis of the exit gas showed, in volume percent,
1~.4% carbon dioxide, 1.9~ oxygen, and 10~ sulfur dioxide.
Calculation based on the amount of chloride recove,red from
the exit gas showed that the gas stream contained an
average of 2.6~, by weight of feed, of chlorine equivalent
chlorides.
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Table I contains the initial composi-tion of the
5,570 grams of pellets prepared from a blend of electrostatic
precipitator dusts and hydrometallurgical wastes. It also
shows the assay of the resultant 4,362 gram composite sample
from 24-hour continuous operation, the composition of the
32.8 grams of scrubbed solids (insoluble in water and
removed by filtering), and the composition of the 15 liter
water and the 12.5 liter, 4N, NaOH scrubbing soIutions.
Control of the processing conditions in the
aforedescribed manner provided a treated residue containing,
by weight, 3.17~ sulfur as sulfide and 0.63~ sulfur as
sulfate. The water scrubber solution contained 0.49 g/l
and ~N sodium hydroxide scrubber solution contained
34.1 g/l of sulfur dioxide.
Operation of the continuous process showed that
the volatilization of copper, nickel, cobalt, iron, silver,
and precious metals was depressed and that these elements
were retained in the solids. This action was due to the
presence of sufficient sulfide sulfur in the residue at
the start of treatment and, of as great importance, to the
control of the process conditions so that the treated
residue contained more than about 1 weight percent sulfide
sulfur after treatment.
The loss of moisture and sulfur from the charge
was reElected by increased percentages oE the retained
elements in the processed pellets. Preferential volatili-
zation and separation was also demonstrated in the results
of analyses for the solids recovered from the scrubbers.
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TABLE I
Material Balance for 24-Hour Opera-tion
: : Composi-tion of
: : Scrubber : Total
: Composition in Weight, % : Solutions, g/1 Volatil-
: Feed :Processed:Scrubbed: ized,
Element : Pellets: Pellets : Solids : Water :4N NaOH :Weight %
Cu : 7.3 : 9.26 : 6.06 : 0.0053 : 0.0012 : 0.51
Ni : 5.66 : 7.22 : 0.50 : 0.0033 : 0.004 : 0.22
Co : 0.205 : 0.26 : 0.03 : 0.0005 : 0.0005 : 3.24
Fe : 26.20 :33.40 : 2.63 : 0.2327 : 0.001 : 0.30
Pb : 0.37 : 0.044 : 26.62 : 0.7740 : 0.001 : 98.7
As : 0.097 : 0.045 : 3.89 : 0.140 : 0.0006 : 62.6
Zn : 0.17 : 0.040 : 6.61 : 0.370 : 0.014 : 83.4
Bi : 0.038 : 0.0062 : 2.93 : 0.0585 : 0.0007 : 87.3
Mo : 0.003 : <.001 : 0.001 : 0.008 : <.001 : 79.5
Cd : 0.027 : 0.0011 : 0.93 : 0.0832 : 0.002 : 99.9
Sulfide*: 7.45 : 3.17 : 19.78 : - : - :
S2 : ~ : ~ : - : 0.0490 :34.10
Sulfate*: 4.15 : 0.63 : 0.41 : 0.510 : 0.716
Total S : 11.60 : 3.80 : 20.1~ : 1.00 :34.82
Cl- : 0.16 : 0.030 : 3.&5 : 9.60 : 0.060
Composition in Parts per Million ;
Ag : 7S.00 :94.64 : 31.79 : nil : nil : 0.25
Au : 1.54 : 2.39 : trace : nil : nil : nil
~t : 5.04 : 6.68 : trace : nil : nil : nil
Pd : 5.86 : 7.93 : trace : nil : nil : nil
Rh,Ru,Ir: 1.07 : 1.46 : trace : nil : nil : nil
Re : 0.84 : 0.07 :134.64 : nil : nil : 93.9
* Sulfide and Sulfate reported as weight % sulfur.
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EXAMPLE I I
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The vertical shaft furnace and equipment described
in Example I were used for batch tests on pellets, prepared
in the previously described manner, having the composition
shown in Table II. The tests were of 6-hour duration with
pellets being fed into the equipment at the rate of 60 grams
every 15 minutes. The temperature within the shaft furnace
was maintained between 850-950C. About 500 grams of pellet
were maintained in the reaction zone at all times. The
degree of oxidation of the residue was controlled by the
amount of oxygen ln the gas stream which was between about
5 to 15% by volume. The gas stream also contained, by
volume, 7.5~ water, 7.5% carbon dioxide, 5% sulfur dioxide,
and balance nitrogen. Either 1.5 or 3.0 percent, by weight,
chlorine was added to the gas stream as shown in Table II.
The gas flow rate was 1900 to 2500 ml/min/500 grams of
furnace charge.
These tests show the importance of maintaining
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~;;' the sulfide level of the treated residue to be at least
' 20 about 1~ so that desired elements (Cu, Ag) are retained
and others (Pb, As, Zn, Bi, Cd) are volatilized. The
values shown in Table II for volatilization of the various
,- elements were calculated from the change in ratio of nickel
plus iron to other elements.
- EXAMPLE III
~- Essentially the same conditions as described for
Example II were used in this example except that chlorine
was injected into the gas stream in controlled amounts of
0, 0.7, 1.5, and 3.0% by weight of charge weight. The
30 results of these tests are contained in Table III and show
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TABLE II
EFFECT OF RESIDUAL SULFIDE CONTENT ON VOLATILIZATION
Composition of Pellets After Treatment
With Indicated Amount of Chlorine
(In Weight Percent)
; Feed
Ingredient Pellets 3 1.5 1.5 3 3
;
Cu 7.86 5.47.9 8.59.9 9.9
Ni 6.20 7.97.3 7.78.0 7.8
E'e 27.4 34.533.4 33.034.6 33.7
Pb 0.40 0.076 0.196 0.1450.014 0.014
As 0.097 0.037 0.066 0.0530.0076 0.013
Zn 0.17 0.180.204 0.1960.0440.026
Bi 0.037 0.021 0.029 0.0180.003 0.0027
Cd 0.028 0.058 0.008 0.00560.002 0.002
Ag 0.008 0.0003 0.002 0.0030.011 0.011
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Sulfate* 4.1 1.20.9 0.80.7 0.2
Sulfide* 8.1 0.00.2 0.61.1 4.6
Total Sulfur 12.2 1.21.1 1.41.8 4.8
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~ Percent Volatilization (calculated)
r Cu 45.616.614.7 0.6 Nil
Pb 84.959.570.1 96.8 97.1
As 69.943.855.0 93.8 89.3
'``Zn 17.00.94.479.6 87.6
~i 55.135.259.9 93.6 94.6
Cd 83.175.584.6 94.5 94.0
Ag 99.776.369.3 Nil Nil
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* SulEide and Sulfate reported as weight % sulfur.
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TABLE III
`. EFFECT OF CHLORINE ON VOLATILIZATION
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. Composition of Pellets After Treat-
,i ment with Gas Stream Containing
Indicated Amount of Chlorine
. (In Weight Percent)
s Feed
In~redient Pellets 0 0.7 1.5 3.0
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" Cu 7.869.7 9.7 9.8 9.9
Ni 6.207.4 7.5 7.7 7.8
.:, Fe 27.433.4 32.8 32.233.0
.. Pb 0.400.32 0.10 0.0710.014
.~' As 0.0970.075 0.045 0.035 0.003
Zn 0.170.21 0.11 0.0740.025
.;. Bi 0.0370.04 0.010 0.0086 0.002
Cd 0.0280.02 0.007 0.0033 0.002
", Ag 0.0080.0097 0.010 0.010 0.011
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~ Sulfate* 4.1 0.5 0.6 0.4 0.7
; Sulfide* 8.1 2.2 2.4 2.1 1.1
. Total Sulfur 12.2 2.7 3.0 2.5 1.8
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~ Percent Volatilization (calculated)
.;. Cu ' <1.O <1.0 <1.O<1.0
Pb 34.7 79.2 85.197.3
~-` As .36.4 61.3 69.793.8
.~ Zn Nil 44.3 63.787.9
Bi 10.9 77.5 80.593.6
`~` Cd 41.0 79.1 90.094.4
~- Ag <1.0 <1.0 <1.0Nil
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* Sulfide and Sulfate reported as weight ~ sulfur.
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effective volatilization of undesirable elements at the
- 1.5 and 3.0% chlorine levels. Although in theory only
about 0.48% by weight of chlorine should be required for
volatilization of Pb, As, Zn, Bi, and Cd in pellets of the
compositions shown, efficient levels of volatilization are
not obtained until a minimum addition level of about 1.5
wei~ht percent chlorine is attained.
Observation of the reaction zone during continuous
operation has shown the formation of two distinct zones
within the reaction vessel. The first zone develops close
to the oxygen and chlorine gas impact area and is relatively
oxidizing. The temperature rises about 50C to 100C higher
(although not exceeding about 1150C) in this zone than in
- other parts of the reaction bed and copper loss is high.
The following reactions, written for copper but also repre-
sentative of the other volatile elements, are believed to
occur:
(1) CuS ~ 1/2 2 ~~ CuO + S02
. (2) 2CuO + C12 > 2 + 2 CuCl (volatile)
A reducin~ zone develops above the oxidizing zone due to
the diminishing oxygen content of the process gas which has
just reacted with sulfur. As a consequence, the ratio of
SO2/O2 in the gas phase approaches the range where conditions
Eavor the reduction of the volatile co~per chloride converting
it to oxide or to metallic form. The reactions believed to
occur in this reduction zone follows:
(3) 4CuCl + 2H2O + 2 ~ 4HCl + 4CuO
(4) 2CuCl + SO2 + H2O ~ 2HCl + SO3 + 2Cu
(5) 2CuCl + CuS + H2O + 2 - 3 2HCl + SO2 + Cu2O + Cu
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During initial start up oE the process (i.e.,
about the first 5 hours), it has been observed that the
:- copper content of the processed pellets is considerably
lower than the copper content of later processed pellets
even though the process gas does not show the presence of
large quantities of volatilized copper. Although the exact
mechanism is not precisely understood and without being
bound to any particular theory, it is believed that the
~, process is dependent upon the concentration of copper in
the upper reducing zone, which in turn is controlled by
, the presence of sulfide sulfur in an amount of at least
about 1.5%, by weight. Under controlled roasting condi-
tions, this is believed to cause the observed change in the
order of volatilization.
Although the present invention has been described
in conjunction with preferred embodiments, it is to be
understood that modifications and variations may be resorted
, to without departing from the spirit and scope of the inven-
tion as those skilled in the art will readily understand.
Such modi~ications and variations are considered to be
within the purview and scope of the invention and appended
claims.
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