Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A PROCESS OF CONTROLLING FOAM WHICH
LIMITS ACID ~IST DURING ELECTROWINNING
The present invention relates to foam used in limiting acid
mist during electrowinning. More particularly, it relates to
controlling the level of surfactant used in producing foam bubbles.
BACKGROUND OF THE ART AND PROBLEM
During the copper electrowinning process from sulfate
electrolyte, oxygen bubbles are generated at the anodes. These
bubbles have the capability of bursting at the electrolyte air
interface ~surface) and producing acid mist in the electrowinning
tankhouse. A surfactant is added to the electrolyte to effect the
bubbles to form a stabilized protective foam layer on the electrolyte
surface. This foam layer prevents bubbles from bursting acld
directly into the atmosphere. If the protective layer of foam
becomes too thick, the foam deposits on bus bars which supply energy
to the anodes and to the cathodes. The foam evaporates on the bus
bars leaving a deposit which interferes with the conductivity at
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the bus bar connections to the anodes and cathodes.
There are several factors which affect the height of the
foam ]ayer. The major factors are: addition rate of fresh
surfactant5 addition rate of recycled surfactant, plating rate,
electrolyte flow rate through the cell, anode surface condition and
temperature. The increase of fresh and recycled surfactant increases
the amount of ~oam produced. The amount of foam produced is also
increased by increasing the metal deposition rate, which increases
the rate of 2 production. Increasing the flow rate through the
cell, increases the surfactant in the cell, which increases the foam
produced. A rough clean anode surface produces an increased number
of small bubbles having a greater surface area than a decreased
number of large bubbles. The resulting small bubbles remain intact
longer, increasing the level of the foam. An increase in temperature
has also been found to increase the life and thickness of the foam.
During commercial electrowinning the above factors may
constantly change. For this reason the rate or amount of fresh
surfactant added to the electrolyte must often be ad~usted. This
ad~ustment is difficult to perform by visual inspection of the
electrolytic cells, because it may take several hours for the
increased or decreased amount of surfactant to reach a steady state
when electrowinning on a large scale.
An improved method of controlling foam levels during
electrowinning is desired to eliminate open foam-free electrolyte
surface which allows acid mist to flow directly into the atmosphere
and over production of foam which adversely affects the bus bar
contacts.
SUM~IARY OF THE INVENTION
The present invention contemplates a process for measuring
foaming capacity of surfactant to effect foaming used in controlling
acid mist production during electrowinning. An electrolyte
containing surfactant is provided for use in an electrolytic cell. A
portion of the electrolyte containing surfactant is sent ~o a test
chamber. The electrolyte in the test chamber is gasified to produce
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bubbles which rise and accumulate above the electrolyte in the test
chamber as a foam layer. The foam layer has a height proportional to
the foamlng capacity of the electrolyte. The height of the foam in
the test chamber is measured to determine the amount of surfactant to
add to the electrolyte for effective foaming in the electrolytic
cell.
Preferably, a probe in the chamber produces a signal which
indicates the height of the foam. The signal controls an automatic
adjustable pump which controls addition of the surfactant to the
electrolyte. The pump preferably increases the flow of surfactant
when the signal indicates a drop in foam below a predetermined level.
The pump decreases the flow of surfactant when the signal indicates a
raise in foam level above the predetermined level.
BRIEF DESCRIPTION OF THE DRAWING
Figure 1 is a schematic view of a test chamber which
controls the flow of surfactant into a constant head tank for
supplying a stream of electrolyte for electrowirming in an
electrolytic cell.
PARTICULAR DESCRIPTION OF THE INVENTION
Referring to Figure 1, constant head tank 10 supplies a
stream of electrolyte such as, aqueous sulfuric acid electrolyte 12
containing surfactant through a gravity feed pipe 14 to an
electrolytic cell 15 for electrowinning. Bypass inlet pipe 16
transports electrolyte at an essentia]ly constant rate into a
rectang~larly shaped test chamber 20. Electrolyte 12 flows from test
chamber 20 out overflow outlet 17. A skimmer 18 extends across the
test chamber 20 to prevent foam 30 from exiting through outlet 17.
The overflow outlet 17 may transport electrolyte 12 to electrolyte
cell 15 or back to the constant had tank lO. Baffles 19 and 21
extend across the test chamber 20 to bufer the velocity of
electrolyte 12 from inlet pipe 16. The flow of electrolyte 12
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through the test chamber 20 ia buffered to prevent movement of the
foam layer 30 from currents of electrolyte 12 which would di~rupt the
level of -the foam, decrea~ing the accuracy of the test chamber 20.
Test chamber 20 can be maintained at the temperature of electrolytic
cell 15 by thermostat maans not illustrated.
A pump sends a ~as such a3 air, nitrogen or oxygen at an
e~sentially constant rate through a flowmeter. The flowmeter
iq used to determine whether or not the air i~ flowing at a constant
rate. The air flows at a con3tant rate throu~h a heat exchanger 25.
The heat exchanger 25 provides a 3upply of air to porous glass 26 at
a temperature nea~ the temperature of the electrolyte 12. Heat
exchanger 25 is preferably constructed out of stainles~ steel or of
other corrosive re~istant materials having good thermal conductivity
properties. After the air is pumped through heat exchanger 25, the
air is forced through porou~ glas~ 26 to create air bubbles 28.
The air bubbles 28 rise to the surface to form a foam
layer 30. The height of the foam layer 30 reache~ a steady state
when the rate of ga~ entering the foam layer equals the rate of gas
entering the atmo phere. The level of the foam 30 i~ an effective
measure of the foaming capacity of the electrolyte 12. Changes in
the foaming capacity of the electrolyte predict increases and
decrease3 in the height of the foam 31 in the electrolytic cell 15.
A decrease of the foam height in the test chamber 20 indicates a
future decrease in the foam height in the electrolytic cell 15.
Similarly, an increase of foam height in the test chamber 20
indicates a future increase of foam 31 in the electrolytic cell 15.
The rate oP adding surfactant is adjusted according to the
predictions of the test chamber 20 to avoid substantial decreases and
increase~ in the layer of foam 31.
~ surfactant 3uch as alkyl diphenyl oxide disulfonate is
supplied from a drum through pipe 34, adju~table valve 36, and a
rotometer to a tee connection 40. Water is supplied from pipe 42
through valve 44, through a second rotometer to tee connection 40.
If the surfactant i~ alkyl diphenyl oxide di3ulfonate, the surfactant
is corrosive and should be mixed with about 10 part~ water per one part
surfactant to reduce corrosion. The rotometers and are used to
monitor the 10 to 1 ratio and the valve~ 36 and 44 are used to adjust
the flow rates of the surfactant and the water to achieve the desired
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10 to 1 ratio. The water and ~urfactant ~olution travel3 through
pipe 48 to an adjuatable pump. The adjustable pump, ~uch as worm
gear type pump, ~upplies surfactant and water at a con~i~tent
controllable rate through pipe 52 to the electrolyte constant head
tank 10. The flow rate of the ~urfactant into the con~tant head
tank, is adjusted with a rheostat which controls the revolution rate
of the pump. In the con~tant head tank 10, the surfactant is
dissolved into the electrolyte. The recycle pipe 54 al~o ~upplies
recycled ~urfactant to the constant head tank 10. This recycled
surfactant i9 supplied from the cell outlet pipe 56 and from the
washing of cathodes 58. Skimmer S9 prevents foam 31 from flowing
directly out cell outlet pipe 56. At the anode~ 60, oxygen bubble~
61 are produced. Oxygen bubbles 61 and the stabilizing surfactant
form the foam layer 31.
Preferably a foam levsl probe send~ a ~ignal indicating
the height of the foam to a controller. The probe may consist of a
capacitance type probe or any other probe ~uitable for measuring the
foam height. The controller is linked to and electrically controls
the adjustable pump. The controller increases adju table pump when
the level of the foam 30 ha~ fallen below a predetermined value. The
predetermined value i9 a specific Ievel of foam in the test chamber
which corre~ponds to an acceptable level of foam in the electrolytic
cell. Similarly, the controller decrea~es adjustable pump when the
level of foam 30 ha~ risen above the predetermined value.
Alternatively, the controller may be constructed to operate an
adjustable valve on pipe 52 to increase and decrease surfactant flow.
A less de3irable alternative would be to ~imply place a
foam level probe into an électrolytic cell and connect the probe to a
controller. This alternative would operate~ but it would not have
the effectiveness and accuracy of the invention, due to the extended
lag time required for the foam level to reach a 3teady state.
Typical lag times required for the foam level to reach steady state
are often between 3 and 4 hours. The invention by comparison
measures almost immediate changes in foaming potential in the
electrolyte, providing an effective and accurate proce~3 for
controlling foam level in an electrolytic cell.
The proce~ of this invention ha~ proven to be an effective
means of controlling foam height in electrolytic cells. The amount
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of surfactant added is ad~usted to the several factors which affect
foam height with sufficient speed as to prevent the uncovered
electrolyte problems w~ich results from insufficient surfactant and
to limit the conductivity problems occurring with the bus bars in the
S electrolytic cells associated with over supply of surfactant.
While in accordance with the provisions o~ the statute,
there is illustrated and described herein specific embodiments of the
invention, those skilled in the art will understand that changes may
be made in form of the invention covered by the claims and the
certain features of the invention may sometimes be used to advantage
without a corresponding use of the o~her features.
