Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR PRODUCING CARO S ACID
The invention is in the field of producing Caro's acid by reaction of
s hydr~gen peroxide and sulfuric acid in a controlled and effective manner.
Caro s acid which is peroxymonosulfuric acid is a strong oxidi~i, ,9
cor"pound which has been suy~ested for use in any applicdlions including
pulificalion of cyanide-containing effluents by conversion of their cyanides
into non-toxic derivatives. Caro's acid is usually produced by reacting
togetl,er cGr,cenl,dted sulfuric acid (85% to 98% by weight H2SO~) with
cGncentrated hydrogen peroxide (50% to 90% by weight H2O2) to produce an
equilibrium mixture of Caro's acid containing peroxy",onosulfuric acid
(H2SOs), sulfuric acid and hyd,ogen peroxide. I lov:ever, since the Caro s
acid is not stable for long periGJs it must be made and imme~ tely used on
site or quickly cooled and stored at refrigerated te",per~lures. In general, theCaro's acid is manufactured on site as needed and in just the amounts
required for the specified ap~l c~tion without the necessity of having to store
any excess amounts.
One procedure for producing Caro s acid is set forth in U.S. Patent No.
3 900 555 by using an apparatus descril,ed in U.S. Patent No. 3,939 072 for
mixing the sulfuric acid and hydlogen peroxide and cooling the mixture with a
water-cooled jacket to prevent overhearing of the reactants and premature
clecGI~position of the monoperoxysulfuric acid product. These ~,atents teach
the use of the monoperoxysulfuric acid product for treating waste aqueous
effluents from an electroplating plant containing cyanide ions while
simultaneously adding an alkali in amounts suitable for neutralizing the
added acid. This assures that the pH of the treated solution in maintained at
a specified alkaline value, nommally pH 9, by neutralizing any acidity resultingfrom the added acid.
Another procedure is set forth in U.S. Patent No. 4 915 849 wherein
the Caro s acid is used to treat cyanide-containing effluents from an ore-
p.ocessing plant. The Caro s acid is manufactured by reacting sulfuric acid
with hydrosJen peroxide in propo,liGns cor,espol-ding to between 0.01 and
0.5 moles of sulfuric acid per mole of hydrogel) peroxide. The resulting acid
is then added to the cyanide-containing effluent simultaneous with aqueous
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lime or sodium hydroxide mixtures in order to maintain the effluent at the
preferred pH of between 9.5 and 11.5.
Still another procedure is set forth in PCT Publication No. WO
92/07791, a published patent ~pplic~tion of Lane et al., which teaches
s production of peroxy.,.Gnosulfuric acid by introducing a hyd~ùgen peroxide
solution into a stream of sulfuric acid ~lu~:;, ~y through a reaction chamber, the
HzO2 intro~luction being made between the sulfuric acid inlet and the reaction
mixture outlet. Both the hyd~ùgen peroxide solution and sulfuric acid are
introduced under pressure into the closed tubular reaction cl ,aml,er of the
invention. In the reaction, chamber, the through-put per minute of the
reaction chamber is at least about 20 times its intemal volume measured
b~l~con the inlet for the hyd~oge" peroxide and the outlet.
In carrying out the production of Caro's acid in industrial ~pl)lic~lions,
two prùble."s have arisen in the scale-up of the Caro's acid generating unit to
commercial proportions. The first prùble." is the protection of a large amount
of hyd~os;en peroxide in storage tanks, used to feed the Caro's acid
producing generator, from possible conta"-i.,ation. The need to prevent
contamination of this large hydloyel1 peroxide source from either Caro's acid,
sulfuric acid, or other such impurities is critical to the safe containment and
use of the hy.l.ogen peroxide. The second problem is to control the Caro's
acid reaction so that the Caro's acid is folllled efficiently with maximum use
of the hydloyen peroxide reagent and without having the hot reaction mixtur
formed during the reaction go out of control and overflow or rupture the
reaction chamber.
2s With respect to the first problem, it has been the custom in the industry
to isolate the storage tank of peroxide from the reactor where Caro's acid is
pro~!lce-l by means of an intermediate tank (sG"-eli",es called a "break"
tank) to interrupt the stored hydroyel, peroxide source from the line deliveringhydrogen peroxide to the Caro's acid generator. The peroxide from the
storage tank is passed by pump means or by gravity into the top of an
intemnediate tank to a given level in the intermediate tank without requiring a
direct liquid connection b~t~Joon the perùxide in the intermediate tank and
the line flowing from the storage tank. This assures that any possible
contamination which may be sucked back from the Caro's acid generator into
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the i"te""eJiate tank will not be able to flow into the hy.J,uyel) peroxide
storage tank.
The secG"d problem arises bec~l~se the reaction of sulfuric acid and
hyJu~gen peroxide is an exothemmic reaction and some h~,d~oyell peroxide
JecGI"poses to fomm large amounts of gas which may cause pressure build
up capable of rupturing the reactor or causing the reagents to overflow. This
may cause the hot reaction mixture to go out of control with the waste of both
sulfuric acid and hydrogen peroxide and further, if no break tank is used, may
become a possiblE source of conta",i"alion of the hyJ,ugen peroxide storage
10 tank if it backs up into the hyclrogen peroxide line conl-e~;ti,-3 the hydroyell
peroxide storage tank to the Caro's acid reactor.
In copending U.S. Application Serial No. 08/351 987 filed Decer,lber 8
1994, (now Patent No. 5 470,564) in the names of James L. Manganaro et
al. a process is described that overcomes the fore~oi. ~y problems. This
process for producing Caro's acid is carried out by reacting sulfuric acid
having an cGncel llraliGn of at least about 85% by weight and hydrogen
peroxide having a cGncenl~dlion of at least about 50% by weight wherein the
hydrogen peroxide is introduced through a first feed line and the sulfuric acid
is introduced through a second feed line into a funneling zone open to the
atmosphere, the first feed line and secGn d feed line having air gaps between
their ends and the funneling zone, passing said hyd-ogen peroxide and
sulfuric acid by gravity flow from said funneling zone into one end of a
reaction zone that has been sized to permit a pressure drop therein which is
at least 8 times the theoretical pressure drop for liquids flowing through such
reaction zone and removing a mixture containing Caro's acid from an exit
end of the reaction zone.
While the above process has been very suGcessful in eliminating the
need for break tanks and avoiding pressure buildups in the reactor, the
Caro s acid that issues from the end of the reactor open to the atmosphere is
quite hot bec~use of the exothemmic reaction between the sulfuric acid and
h~,J,ogen peroxide and tends to form an undesiled Caro s acid mist. The
Caro's acid mist is an irritant to the mucus ",eull,ranes of workers who are in
the vicinity of the outlet of the Caro s acid reactor. The mist is especially
objectionable when the reactor is inside a building where the mist can collect
and build up in conce"lf~lion to a point where protective gear and/or special
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exhaust provisions are required. Even outdoors the mist is notice~hlQ and
most objectionable to any workmen who may be downwind from the Caro's
acid reactor. Further, when the Caro's acid is used to treat tailing slurries toreduce their cyanide conce"l,dlions, the mixing of the Caro's acid with the
tailing slurries is not instanlaneous due to the syrupy nature of the
COI Ice, Itrdl6cl Caro's acid that issues from the Caro's acid generator and thehigh density of the tailing slurries (35% to 40% solids). This is undesired
bec~l ~se it allows time for the Caro's acid to decompose before reacting with
the cyanide in the tailing slurry.
It has now been found that the Caro's acid mist, that fomms when
Caro's acid is produced by reaction of sulfuric acid and hyd~ogen peroxide in
a reaction zone and the hot Caro's acid mixture is released from the end of
the reaction zone, can be reduced or eliminated by quenchi. ,9 the hot Caro's
acid mixture with water to both cool and dilute the Caro's acid.
In carrying out the present invention, the Caro's acid is produced by
reacting sulfuric acid and hydrogen peroxide together, preferably in a
continuous manner and in accordance with the process set forth in our
copending U.S. Ar, '.c~tion SN 08/351,987filed Dece"lber8, 1994 (now
Patent No. 5,470,564) in the names of James L. Manganaro et. al. While
other processes may also be employed for producing the Caro's acid, this is
the preferred method bec~l~se the Caro's acid is for",eJ efficial,lly with
maximum use of hyclrogel) peroxide reagent and without having the hot
reaction mixture fommed during the reaction go out of control and overflow or
rupture the reaction chamber. In this process the reactants are added into a
funneling zone, or other type of feeding zone, open to the alrnosphere and
then the hydrogen peroxide and sulfuric acid flow from the funneling zone by
gravity into one end of a reaction zone where the reaction takes place. The
reaction zone which is loc~ted downstream from the funneling zone is
preferably a pipe-like or tube reactor, whose diameter may be variable or
constant, which may be either vertically oriented or h~ri~o,ltally oriented or
any skew angle intermediate these two extremes and is fed by gravity from
the funneling zone. Further, it is preferred that the reaction zone have a size
which pemmits a pressure drop in the reaction zone which is at least eight
times the theoretical pressure drop for liquids passing through such reaction
zone. Such reaction zones are nommally static reactors containing several
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mixing elements which ensure a complete mixing and reaction of the two
reagents.
The sulfuric acid employed in the reaction can be of any c~r,ce, ll-dliO"
from about 85% by weight to about 98% by weight H2SO4 with about 93% by
s weight sulfuric acid being preferred bec~use of its ready availability and
workability. The hydr~gen peroxide employed can be of any conce"l,dlio
from about 50% by weight H202 to about 90% by weight H202 with 70% by
weight hyd~oyel, peroxide being ~,refer,tzd bec~use of safety considerations
and availability. The mole ratios of sulfuric acid to hydr~yel, peroxide
lo (H2SO4/H202) can range from about 1/1 to about 4/1 with about 2/1 to about
3/1 being prefe~.ed. The reaction results in Caro's acid being formed in a
solution which is an equilibrium mixture of hydrogen peroxide, sulfuric acid,
Caro's acid and water. The equation for this reaction is set forth below:
H2S~4 + H2~2 ~ H2SOs + H20
A typical cGl"~,osilion prepared from a 2.5/1 mole ratio of 93 weight
percel ,l sulfuric acid and 70 weight percent hydrogel) peroxide is as follows:
Caro's Acid (peroxymonosulfuric acid) 25 weight percent; sulfuric acid 57
weight percent; hydrogel) peroxide 3.5 weight percent; and water 14.5 weight
percel,t. A cG"si~lerable amount of heat is rcloascd as a result of mixing the
hydrogen peroxide with sulfuric acid. The amount of heat rel ~ced depends
on the concenlrdliGI) of the starting reagents and the mole ratios of sulfuric
acid to hydrogen peroxide. Using the preferred cGncenlrdliol)s of 70 weight
percent H202 and 93 weight percent H2SO4 and a preferred mole ratio of
H2SO4/H202 of 2.5/1, an increase in te.n~.eralure of 58QC will occur. If, for
example, the raw materials are at 34~C, the Caro's acid will reach a
temperature of 92~C.
Caro's acid mist is generated as the hot Caro's acid mixture exits the
reactor unit. In the case of treating precious metal tailings slurry to reduce
their cyanide values (CN ), Caro's acid is added directly from the reactor to
30 the tailings slurry. The Caro's acid mist is released as the Caro's acid
mixture free falls between the end of the Caro's acid reactor and the surface
of the tailings slurry. Further, mist is released as the Caro's acid spreads outonto the surface of the slurry before it mixes with the tailings slurry. The
higher the Caro's acid tel"perdl~re, the more acid mist is generated. Mixing
3s of the Caro's acid with the tailing slurry is not instantaneous due to the syrupy
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nature of the cGncent.dted Caro s acid and the high density of the tailings
slurry (35% - 40% solids). The temperature of the tailing slurry also can
affect the amount of acid mist generdted. Warm or hot slurry will conl,ibute
to more mist than slurry at ambient te".pe,dl.Jres. The Caro s acid mist thus
s generated can be particularly objsvtiol)able if the Caro s acid is employed
indoors, where acid mist can build up. At some gold mines the tailing sump
a CGIlllllOn i"je_tio,) point for Caro s acid to control the cyanide
CGnCel llraliGnS of the tailings slurry is locAte-J i. .JoGr~. The acid mist has a
sharp odor which can cause irritation of the mucus membranes. The acid
10 mist is also cG"~,sive to equipment and instruments used in precious metal
recovery sites. Where the injection point for Caro s acid to a tailings pond is
locAtecl outdoors, the Caro's acid mist is exl-~l"ely objectionable to anyone
downwind from the site where the mist is generated.
In accordance with the present invention the Caro s acid mist can be
lS subst~ntially reduced or even eliminated by quenching the hot Caro s acid
mixture with water to both cool and dilute the mixture as it exits the Caro's
acid reactor. The above quenching of the above hot Caro s acid mixture can
take place in a number of ways. One way is to direct water sprays at the
base of the pipe where the Caro s acid mixture exits. The water sprays
20 simultaneously emit water streams directly into the Caro s acid mixture and
also form a curtain of water droplets that surround the end of the pipe from
which the Caro s acid mixture exits. A second ",ell,od for carrying out the
above quenching of the Caro s acid mixture is to introduce dilution water into
an annular tube surrounding the outside of the pipe from which the Caro's
25 acid mixture exits resulting in a ring of water flowing co-currently with theCaro s acid contained within the water ring. The Caro's acid and water
i"li".alely mix as they fall from the exit pipe of the Caro's acid reactor and the
ring of water prevents any release of the Caro s acid mist before it is
scrubbed and reacts with the water to quench and dilute the Caro s acid.
Another system for quenching the Caro's acid mixture is to attach a
water aspirator to the pipe from which the Caro s acid mixture exits. As the
Caro s acid mixture exits from the pipe water will be aspi, dted into the
mixture, mixed with the Caro's acid and both cool and dilute the Caro s acid
before it exits the aspirator pipe. Another allel"ate technique for quenching
35 the hot Caro's acid mixture is to attach a static mixer to the exit pipe of the
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Caro's acid reactor wherein water and the Caro's acid mixture are passed
simultaneously into the static mixture in order to quench the Caro's acid. The
static mixer is a desirable ,nell,od of carrying out the quenching operation
bec~use it allows i"li",ate contact of the Caro's acid and water
5 simultaneously and with varying amounts of water as desired. The instant
mixing of the water and Caro's acid assures immediate dilution and cooling of
the Caro's acid before it exits from the static mixer into the tailings sump.
Finally, the simplest and most economic way of carrying out the quenching of
the Caro's acid mixture is to place the exit tube of the Caro's acid reactor into
10 a container in which a continuous water stream is directed with the exit tubebeing below the level of the water that is present in the container. In other
words the exit tube is placed into a container full of water, below the water
level, and into which a continuous water stream is directed and overflows
the container. In this way there is instantaneous quenchi"g of the Caro's
15 acid mixture as it exits the tube into the container of water and the constant
supply of water into the container results in overflowing a diluted Caro's acid
mixture from the container into the tailings pond. Since the exit pipe from the
Caro's acid reactor is below the water level in the container no mist or Caro's
acid solution can esc~pe from the tube without first contacting and being
20 quenched and cooled by water which is in the container. Thus, both
quenchi, ,9, that is cooling of the Caro's acid mixture, is obtained along with
simultaneous dilution.
The amount of water employed for water dilution and quenching is not
critical and can vary widely. For example we have found that from 1/1 to
25 200/1 weight dilution of water to Caro's acid is effective. Obviously, largeramounts of water can be used without del~terious affects. The dilution water
can be at any temperature which will cool the Caro's acid mixture.
Temperatures from about 5Q to about 45~C are useful with ambient
te"".6rdlures being preferred bec~use they are the most economical. Since
30 the temperature of the Caro's acid mixture exiting from the Caro's acid
reactor is nommally from about 52~C to 98QC, any temperature below 52~C will
usually be effective. Obviously the lower the temperature of the dilution
water the more effective will be the results since it will minimize
cJecGn,posilion and hydrolysis of the Caro's acid.
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At an eA~eri",e"tal test site where the invention was ~,"Jer~o..lg
testing an unusual event u"ex~ecte-lly allowed the evaluation of the
invention in a visual mode. The testing was carried out with a tailing slurry
recovered from a gold extraction using sodium cyanide as the extracting
5 agent. In this case the Caro's acid is employed to reduce the cyanide levels
in the tailings slurry. In the first instance the Caro's acid was added to the
tailing slurry without any water to quench the Caro's acid mixture. The
Caro's acid mixture exited from the outlet pipe of a static Caro's acid reactor
several inches above the tailing slurry pond and dropped into the tailing
10 slurry. The tailing slurry in contact with undiluted Caro's acid tumed green. It
is believed that the green color was due to some co""~onent of the ore slurry
such as a heavy metal changing color under oxkl;;liol) and/or acid conditions.
Once the Caro's acid mixed with the slurry the green color ~is~ppe~red.
Thus, the green color provided a unique visual opportunity to observe mixing
15 pattems of Caro's acid with the slurry and to observe any unreacted Caro's
acid which was green in color and which floated on top of the tailing slurry. Inthe absence of water quenching and dilution, a mist formed around the outlet
pipe and scattered pools of Caro's acid could be observed, green in color,
floating on top of the wamm slurry (containing 37 weight percent solids) and
20 could be seen bubbling (dec~""~osi"g) and el"illi"y an acid mist prior to
disappearing into the slurry. Also, green streaks were also seen at the
surface of the slurry at a slurry outlet pipe loc~ted downstream about 12 feet
from the Caro's acid injection point. After water quenching and dilution of the
Caro's acid mixture exiting from the outlet pipe, the pools of Caro's acid
25 floating on the slurry were much smaller, disappearing very quickly, and the
bubbling previously seen in the aL,sence of water dilution was absent. In
addition no green streaks were cviJel ,t at the slurry pipe outlet 12 feet
dGw"~l,eam from the Caro's acid ~ddition point. The mixing of the Caro's
acid into the slurry was due to the thinning effect of the water on the Caro's
30 acid. The water quel)cl ,ing and dilution of the Caro's acid eliminated the acid
mist and improved mixing of the Caro's acid due to the cooling and thinning
- effect of the added water.
While an important use of the invention is the ~pplic~liGIl of Caro's
acid for detoxification of cyanides (CN-) and WAD (Weekly Acid Dissociable)
35 CN in gold mine tailings slurries, the invention can also be used in any other
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application where Caro's acid is prepared on site and injected into a
sul,str~te to destroy, deodorize, decolor, or alter a chemical comrosition.
Among such applications is the delignificaliGn of wood pulp, phenol
destruction and de~olo,iLali~,) and cleodo,i~dlioll of a wide variety of waste
s water solutions and slurries.
By carrying out the ~.rese!nt process of quenching the hot Caro's acid
mixture before using the Caro's acid to treat tailing slurry for reduction of their
cyanide concenl~alions, it has been found that the efficiency of cyanide
destruction is materially i"creased without increasing the amount of Caro's
10 acid used in the process. This additional benefit of using the instant
quenching operation results bec~use of two separate actions that take place
when the Caro's acid is quencl ,ed. Initially the Caro's acid is rapidly cooled
by the quenching step and this prevents rapid decGmpositiGI ~ of the hot
Caro's acid with the loss of oxidizing potential. In addition the water dilutes
15 the Caro's acid considerably and makes it a much thinner solution than the
viscous mixture that exits from the Caro's acid reactor. This is important
bec~llse when this Caro's acid mixture is added to the viscous tailing slurry
the diluted Caro's acid which has much less viscosity can readily mix with the
tailing slurry as compared with the more syrupy viscous Caro's acid which is
undiluted. The faster mixing diluted Caro's acid can thus react quickly with
the cyanide values in the tailing slurry and thereby reduce decG",position of
Caro's acid from unmixed pools of Caro's acid floating on top of the tailing
slurry. By CGIItldSt, when the Caro's acid is not diluted, the syrupy Caro's
acid mixture exiting from the Caro's acid reactor fomms many large pools on
the surface of the tailing slurry and these pools can be seen to be bubbling
indicating that the hot Caro's acid is decGI"posing. This is wasteful of
oxidizing potential of the Caro's acid and of course, dil"inishes the
effectiveness of the Caro's acid in reducing the cyanide concel,l.dlion in the
tailing slurry. By quenching the Caro's acid mixture as it issues from the
30 Caro's acid reactor, decomposition of the hot Caro's acid is eliminated by
cooli"y it and the now thinned Caro's acid can more readily react and mix
- with the tailing slurry without first being subjected to decor,~positiGn before it
mixes and reacts with the cyanide in the tailing slurry. This was completely
unexpected and is an additional bonus of the pr~cess along with elimination
35 or reduction of the Caro's acid mist.
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The following are exal"pl~s which illustrate the use of the present
process.
Example 1
Generation and Use of Caro s Acid Without Water ~uenching-Prior Art
sA Caro s acid generator co"sisli"g of a funneling zone attached to a
static tubular reactor inst~l'ecl in a vertical position beneath the funneling
zone and containing four (4) mixing elen,e"ls was placed over a sump filled
with tailing slurry that contained residual cyanide values. The funneling zone
was open to the atmosphere as was the outlet of the static reactor. Sulfuric
10 acid in the amount of 0.85 gallons per minute of 93% by weight H2SO4 and
0.20 gallons per minute of 70 weight ~,ercent of H2O2 was added to the
funneling zone and produced from the bottom of the reactor a mixture
containing 27% Caro s acid on a continuous basis. The Caro's acid mixture
was passed directly into the sump to treat a stream of 840 gallons per minute
15 of the tailing slurry containing 1.3 Ibs. per minute of cyanide (CN ). This
equated to a Caro s acid/cyanide mole ratio of 0.71/1 and res~lted in a 43.3%
destruction of WAD cyanide (CN ). The hot Caro s acid exiting from the
Caro s acid generator was not treated in any way before it fell into the sump
containing the tailing slurry. Caro s acid mist was prevalent in the vicinity of20 the Caro's acid addition point.
Generation and Use of Caro s Acid with Water Quenching-lnventive Process
The same eqL;,~r"ent and reagents were used as in the example
above except that the 840 gallons per minute of tailing slurry contained 1.5
Ibs. per minute of cyanide (CN-) and this was t,eat~d with the Caro s acid
2s prepared from 1.0 gallon per minute of 93% sulfuric acid and 0.25 gallons perminute of 70% hydrogen peroxide (all weight percent). This equated to a
cyanuric acid/cyanide mole ratio of 0.64/1 and resulted in a 68% destruction
of WAD cyanide (CN ). During this run a spray of water was directed towards
the end of Caro s acid outlet pipe so that it impinged directly on the hot
30 Caro's acid mixture exiting from the Caro's acid reactor. The water was at
ambient temperature and the amount of water used was estimated to be in
excess of a weight ratio of water to Caro s acid of 10:1. There was no
detectable Caro s acid mist pres6nl in the vicinity of the Caro s acid outlet
pipe. For a brief time during the trial the water that was being sprayed on the
3s outlet pipe was shut off and shortly thereafter the acid mist retumed
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11
i"""ediately. When the water was tumed on again, allowing the water spray
to impinge upon the outlet of the Caro's acid pipe, the acid mist quickly
disappeared.
Example 2
Another run was made using the same Caro's acid generator and
reagents as in Example 1, except that the 840 gallons per minute of tailing
slurry contained 1.8 Ibs/minute of cyanide values (CN-) and was l~ated with
Caro's acid prepared from 1.0 gallons per minute of 93% by weight H2SO4
and 0.25 gallons per minute of 70% by weight H2O2. This equated to a
Caro's acid/cyanide mole ratio of 0.64/1 and resulted in a 50% destruction of
WAD cyanide (CN-). During this run a large plastic bucket was positioned
under the Caro's acid outlet pipe and a stream of water was continually
added to the bucket. The level of the Caro's acid outlet pipe was below the
water level in the bucket and the Caro's acid and water mixture in the bucket
overflowed into the tailings slurry contained below it. No Caro's acid mist
was present and no bubbling of any pockets of Caro's acid pools floating on
top of the slurry were observable in the vicinity of the Caro's acid injection
point. Also no green streaks were evident on the surface of the tailings slurry
in the slurry pipe outlet 12 feet downstream from the Caro's acid injection
point. By contrast, when the Caro's acid was pellllitled to flow directly into
the tailing slurry without being quenched with water, pools of Caro's acid
which were green colored floated on top of the slurry and were seen to be
bubbling indicating Caro's acid decG"" osi"y. Further green sl,eaks were
evident at the slurry pipe outlet 12 feet downstream from the Caro's acid
injection point and there was subst~ntial Caro's acid mist surrounding the
area where the outlet pipe from the Caro's acid generator fed the hot Caro's
acid mixture to the tailing slurry.
