Note: Descriptions are shown in the official language in which they were submitted.
l~S6~;8
This invention relates to an improvement in the process for the
electrorefining of lead and, more particularly, relates to addition agents
in the electrorefining of lead in a hydrofluosilicic acid - lead fluosilicate
electrolyte.
In the process for electrorefining of lead using a hydrofluosilicic
acid - lead fluosilicate containing electrolyte, lead bullion electrodes are
placed in electrolytic cells through which electrolyte is continuously cir-
culated, an electrical current is applied and refined lead is cathodically
deposited, while impurities more noble than lead are anodically retained as a
layer of metallic slimes adhering to the surface of the undissolved electrodes.
After completion of the refining cycle, electrodes are removed from the cell,
the refined lead is recovered, the slimes are removed, washed and treated for
recovery of metal values and residual lead bullion is melted and recycled as
electrodes. In the conventional Betts Process, cast anodes of impure lead
bullion and pure lead cathode starting sheets are placed in alternate order
in the cells. Current is applied to give current densities which are usually
in the range of lO0 to 300 A/m2. After completion of the refining cycle,
which may vary from 3 to 14 days, the cathodes and anodes are removed from
the cells. The cathodes are washed, melted and cast into shapes for sale.
The undissolved portions of the anodes are separated from the wet slimes, re-
melted and re-cast into anodes. However, as is more fully described in our
co-pending application Serial No. 300,642, it is also possible to use a bipo-
lar system. In the bipolar system, lead bullion electrodes are placed in
cells and the first and last electrodes in each cell are connected to an
electrical power source. Electrical current is applied to give current den-
sities in the range of 100 to 500 A/m2. The first and last electrodes act as
a cathode and as an anode respectively, while electrodes between the cathode
and the anode act as bipolar electrodes from which lead dissolves from the an-
odic sides and on which lead deposits on the cathodic sides. After completion
of tho rofirling cycle, which usually varies from 3 to 7 days, electrodes are
S6~8
removed from the cells. The refined cathodic lead is separated from the resi-
dual electrodes, melted and cast into shapes for sale, slimes are removed and
residual electrodes melted and recast into electrodes. In both types of pro-
cesses, the refined lead is of high grade with a purity exceeding 99.99%.
A problem associated with electrolytic refining of lead is that of
establishing and maintaining a dense, smooth and level deposit of refined
lead. It is known that, without the aid of suitable addition agents, lead
tends to deposit in a rough irregular pattern with the formation of projec-
tions known as dendritesJ wires, trees or peanuts, which will eventually
lead to bridging between electrodes and short-circuiting, with a consequent
loss in efficiency and a likely deterioration in the quality of the refined
lead.
Associated with the use of addition agents are the interrelated
phenomena of grain refining, levelling and cathode polarization.
Grain refining is related to the production of a smooth cathodic
deposit with a small grain size. A smooth deposit is obtained by adding a
grain refining addition agent, or grain refiner, to the electrolyte.
Levelling is the phenomenon of evening out the distribution of the
deposit of refined lead on the surface and edges of the electrode. The level-
ling out occurs as a result of polarization effects at the cathodic surface
and is brought about by the use of levelling addition agents, or polarization
producing reagents. In distinction, it is noted that grain refiners, used
alone, do not create sufficient cathode polarization to cause levelling.
Cathode polarization is defined as the difference in voltage be-
bween the operating potential of the cathodic surface and the standard lead
reference potential of +0.126 volts versus the Standard Hydrogen Electrode
(SHE).
A large number of compounds that are suitable as addition agents in
tho electrodeposition of lead are disclosed in the prior art. In the prior
art there are disclosed grain refining agents, WhiCil are usually high molecu-
l~lS~io~
lar weight organic materials, such as lignin sulfonates ~by-products obtained
from wood-pulping operations) (United States Patent 1,544,726) used in com-
binations with organic levelling addition agents such as anthraquinone and
anthraquinone sulfonate ~United States Patent 2,415,169), Aloes extract
~United States Patent 2,664,393), Aloin ~United States Patent 2,773,819),
Chestnut extract ~United ~tates Patent 2,827,410), Western Red Cedar extract
~Canadian Patent 584,176), Mimosa extract ~Canadian Patent 681,943), water
soluble block co-polymers of polypropylene oxide and ethylene oxide ~United
States Patent 3,554,884) or alkoxylated fatty acid alkylolamide surfactant
(Canada Patent 972,704). Other levelling addition agents disclosed in the
literature are hydroquinone, l-amino-4-hydroxy-anthraquinone; 6-amino-1-
naphthol-3-sulfonic acid, eugenol, 1,4-naphthoquinone, ligninsulfonic acid
and l-naphthol-4-sulfonic acid ~Chemical Abstracts, 60, 15431 f).
Generally, levelling addition agents must satisfy a number of re-
quirements in order to be useful in electrodeposition processes for lead.
Levelling agents must have strong polarizing effects, have good levelling
properties, be soluble in and compatible with electrolyte, be stable and not
be subject to decomposition and the like phenomena which reduce their effect-
iveness under the conditions obtained in the electrolytic cell. The agents
should also be compatible with other addition agents, such as the grain re-
fining agents, and provide reproducible results. For control of the cathode
polarization in the electrolytic process, the polarization caused by the
levelling agents should be reducible by an agent such as thiosulfate (cf.
Canadian Patent 988,879).
Other problems related to the levelling agents are the complex
nature of agents derived from plants, the varying compositicn of plant ex-
tracts, the economics of usage, i.e. unit-price and amount required per ton
of lead deposited, and availability. These problems appear to be at least
in part associated with a lack of understanding of the basic structural re-
quirements of the chemical compounds that produce polarization and levelling
-- 3 --
.
in the electrolytic process. As can be seen from the above listed levellingagents, the variety of agents is great, but not all of the mentioned organic
compounds produce satisfactory levelling or reproducible and consistent re-
sults.
We have now found that, if certain chemical compounds are to satisfy
the requirements for levelling addition agents, the structural formula of the
chemical compounds should contain at least one hydroxyl group in close proxi-
mity to an aromatic ketonic group and at least one group which provides solu-
bility of the compounds in electrolyte. Satisfaction of the first require-
ment has been shown to provide strong polarizing effects and satisfaction ofthe second requirement has been shown to improve the levelling properties of
the addition agents. Thus, we have found that these structural requirements
are satisfied in some cases in chemical compounds contained in certain level-
ling addition agents of the prior art, but not in others. In those wherein
the structural requirements are satisfied, the agents have good polarizing
and levelling properties, while in those wherein the requirements are not sa-
tisfied, the agents have properties which are generally unsatisfactory for an
efficient and economic electrodeposition process.
We have now found that a group of naturally occurring flavonoids
which includes flavones, flavanones, their isomers, and chalcones, have excel-
lent cathode polarizing and levelling properties, in conjunction with lignin
sulfonate as grain refining addition agent, when added to electrolyte of lead
electrodeposition processes, in amounts effective to cause formation of dense,
smooth and level deposits of lead.
Thus this invention provides a process for the electrorefining of
lead in a cell containing a plurality of electrodes including at least one
anode and at least one cathode, wherein the electrolyte comprises an aqueous
solution of lead fluosilicate and fluosilicic acid to which is added as grain
refining and levelling agents an effective amount of a lignin sulphonate and
an effective amount of at least one electrolyte soluble flavone, isoflavone,
- 4 -
~Sf.~8
flavanone, isoflavanone or chalcone having at least one hydroxyl group in
close proximity to the aromatic ketone group.
Flavonoids are of a group of naturally occurring compounds
which comprise compounds of the chemical groups known as flavones, flavanones,
their isomers, and chalcones, which derive from flavanones. The compounds
flavone and flavanone, of which the flavanoids are derivatives, have the
structural formulae:
O O
Flavone Flavanone
Isomeric flavone and flavanone compounds are also known in which
the substituent C6 ring is attached in the 3-position rather than in the 2-
position as in the flavones and flavanones. Flavanones ~but not isoflava-
nones) may convert to their corresponding chalcones in alkaline solutions by
rin~ opening at the heterocyclic oxygen atom in the l-position and chalcones
may revert to the corresponding flavanones in acidic solutions. The reversi-
ble conversion reaction of flavanone to chalcone may be illustrated, for
example, for the flavanone known as hesperidin, by the following reaction
~u~t iOIl:
OH
0~1
~rocH3 ~ l
011 0
hesperidin chalcone
hesperidin (R = O-glycoside)
(R = O-glycoside)
.3~.
The conversion can be made irreversible by reacting a chalcone in alkaline
solutions with compounds that cause, for example, a methyl or other alkyl
group or a solubilizing group, to become attached to the former heterocyclic
oxygen after which reversion to the corresponding flavanone is no longer
possible.
The naturally occurring flav~noids, as well as the chalcones de-
rived from them, generally contain one or more hydroxyl groups attached to
the ring-structures. For example, hyd~oxyl groups may occur in the 3, 5, 6 J
7, 8, 2', 3', 4', 5' and 6' positions. Isomeric flavones and flavonones may
contain hydroxyl groups in the 2, 5, 6, 8, 2', 3', 4', 5' and 6' positions.
When one or more hydroxyl groups occur in close proximity to the aromatic
ketonic group, the compounds exhibit strong polarizing effects. When flavones
and flavanones have a hydroxyl group attached in the 3 and/or 5 position,
excellent polarizing effects are obtained, whiie those with a hydroxyl group
attached in the 5 position are particularly effective. Chalcones and isomeric
flavones and flavanones with a hydroxyl group attached in the 5 positions are
similarly effective in causing polarizing effects.
Particularly effective as polarization causing agents are the
flavones:
~O quercetin (OH at 3, 5, 7, 4', 5');
fisetin (O~l at 3, 7, 3', 4');
chrysin (O}l at 5, 7);
morin (Oll at 3, 5, 7, 2', 4');
rutin (Ol-l at 5, 7, 4', 5'; O-glycoside at 3); and
apigetrin (OH at 5, 4'; O-glycoside at 7);
the flavanones:
dihydroquercetin (Oll at 3, 5, 7, 4', 5');
naringeniTI ~OI-I at 5, 7, 4');
hespcr~tirl (Oll at 5, 7, 3'; methoxy group at 4');
3~ hesperidirl (Oll at 5, 3'; methoxy group at 4'; O-glycoside at 7);
l~! lS~
neohesperidin (OH at 5, 3'; methoxy group at 4'; Q-glycoside at 7);
naringin (OH at 5, 4'; O-glycoside at 7);
the isomeric compounds of flavones and flavanones such as
biochanin A tO~I at 5,7; methoxy group at ~'); and
the methyl chalcone of hesperidin.
Dihydroquercetin is a principal constituent of conifer barks such as
Douglas Fir. O~uercetin, the flavone analog, is readily prepared by heating
dihydroquercetin in a solution of sodium sulfite. Rutin is obtained from buck-
wheat. Hesperidin, neohesperidin, hesperetin and naringin are derived from
citrus fruit peels.
Most of the compounds derived from natural sources are obtainable
in impure and purified forms. It has been shown that the use of relatively
in~pure compounds in electrodeposition processes is satisfactory to obtain
excellent results and is of major economic advantage.
A number of flavones, flavanones and their isomeric compounds are
soluble in acidic electrolyte, while others are less soluble or insoluble.
It was noticed that the less soluble compowlds also exhibited less satisfact-
ory levelling characteristics than the soluble compounds. The solubility
was found to reside, in some of the compounds, in the presence of a glycoside
group attached to the carbon atom in the 3 or 7 position via an oxygen or
carbon atom, i.e. an O-glycoside or C-glycoside, respectively. Glycosides
are cyclic ~ugars containing one or more rings with 5 or 6 carbon atoms, such
as, ~or cxample, glucose, fructose and rhamnose. Compounds which contain an
O-glycoside or C-glycoside group in the 7 position are stable in the acidic
electrolyte of the electrodeposition processes. These compounds, which in-
clude apigetrin, hesperidin, neohesperidin and naringin, exhibit excellent
polarization as well as levelling properties. Compounds having an O-glyco-
side group attached ir, the 3 position, such as for example rutin, are sub-
ject to hydrolysis in the acid electrolyte and, although these compounds have
good polarizing and levelling properties upon addition to the electrolyte,
1~ 15~5~
their suitability as addition agents decreases rapidly with time, and such
compounds are only useful for short term depositions such as of 4 to 8 hours
duration, after which the electrolyte has to be renewed.
The poorly acid soluble or insoluble flavones~ flavanones, their
isomeric compounds and chalcones, that is, those compounds that do not con-
tain a solubilizing glycoside group, can be modified by reacting with reagents
that introduce a group, or groups, to confer solubility. Such reagents are
preferably non-aromatic compounds which contain a hydrophilic group or groups
such as for example, carboxy-alkoxy groups containing from 2 to 10 carbon
atoms, sulfo groups, aliphatic-amine groups and aliphatic-oxy groups such as
hydroxyalkyl or polyether alcohol groups. We have found that suitable poly-
ether alcohol groups are easily attached to the poorly soluble and insoluble
compounds of the invention by reacting these compounds with ethylene oxide,
propylene oxide, or mixtures thereof in a closed reaction vessel under agita-
tion of the reaction mixture while maintaining temperatures below ambient
temperature. For example, poorly soluble quercetin was dissolved in 2N KOH
and ethylene oxide was added to the solution at a rate of 2 l/min in a closed
reaction vessel for one hour. The vessel contents were continusouly agitated
and maintained at a temperature of 10 C. The ethoxylated quercetin, which
contains hydrophilic polyether alcohol groups, believed to have the structure
-O~C2H4O~nC2H4OH, had an improved solubility and exhibited improved levelling
characteristics in the electrodeposition of lead as compared with quercetin
as shown in Table 1. Similarly, ethoxylation of fisetin, morin, dihydroquerce-
tin, hesperetin, and naringenin improved their levelling characteristics.
TABLE
Amount of quercetin Amount of ethylene Amount of ethoxylated Levelling
in g in 2N KOH oxide added in g quercetin in g/l of deposit
required for correct
polarization
0 0.02 poor
0.06 0.02 poor
0.81 0.05 good
1.60 0.10 very good
3.92 0.14 excellent
4.80 0.14 excellent
-- 8 --
l~ ~S6~5~3
In the procedure for modifying poorly soluble or insoluble com-
pounds, care must be ta~en to preserve the structural groups that are active
in causing polarization. During the reaction to incorporate solubilizing
groups, the polarizing activity per unit weight of the starting compound de-
creases approximately linearly with time while the levelling activity per
unit weight increases with time, but after a time the levelling activity be-
comes constant. The modifying reaction should be terminated at the point
where the levelling activity first becomes satisfactory.
The determination of the polarizing and levelling properties of the
compounds of the present invention was carried out by using the following pro-
cedures. The polarizing properties were determined using a cathode polariza-
tion technique. Compounds having satisfactory polarizing properties were then
tested for levelling properties in laboratory electrolytic cells wherein lead
bullion electrodes were electrorefined. A number of compounds were ethoxylat-
ed or propoxylated, according to the method described hereinbefore, prior to
being subjected to the procedure for determining their levelling properties.
The polarizing properties of addition agent compounds were deter-
mined using a small cell with removable electrode holders having an area
for the exposed portion of the electrodes of 6.45 cm2. Refined lead was
used for the cathode and lead bullion was used for the anode. A reference
electrode of refined lead placed in a Luggin probe was positioned in the cell
in such a manner that the tip of the probe was in touch with the surface of
the cathode exposed to electrolyte. The electrodes were connected to a vari-
able direct current power source and a X-Y recorder. The cell and the probe
were filled with electrolyte containing 70 g/l lead as lead fluosilicate and
90 g/l fluosilicic acid. For each test, 4 g/l Trastan (Trademar~), a lignin
sulfonate, was added to fresh electrolyte and increasing amounts of polariza-
tion and levelling causing addition agent were added to this electrolyte.
~le electrolyte was agitated and maintained at a constant temperature of ~0C.
~he cathodic polarization volta~e was measured against the reference electrode
~lS6~3
voltage and recorded on the Y-axis of the recorder while the current supplied
to the cell was measured on the X-axis. During each test, the current to the
cell was increased at a linear rate equivalent to 15 A/mZ/sec to a value equi-
valent to 400 A/m2. The polarization curve traced on the recorder was identi-
fied and the slope of the curve observed. Only those additives which had suit-
able polarizing characteristics, as defined by the optimum amounts of the
agent that gave an approximately linear polarization curve having a slope at
80 to 90 mV in the range of 0.3 to 0.4 mV/A/m2, were then tested for their
levelling properties. The levelling properties were determined using an
electrorefining cell containing a lead cathode starting sheet and two cast
lead-bullion anodes. Electrolyte containing 70 g/l lead as lead fluosilicate,
90 g/l fluosilicic acid, 4 g/l Trastan ~Trademark), and the optimum amounts of
levelling agent, was continuously circulated through the cell at a rate of
30 ml/min. Electrolyte, Trastan (Trademark) and levelling addition agent addi-
tions were made to the cell during the seven day refining cycle to maintain
volume, lead content and addition agent concentrations. All of these tests
were run at a current density of 215 A/m2 and at 40~C. The levelling of the
cathodically deposited refined lead was visually compared with the levelling
obtained by using Aloes extract which has been shown to possess excellent
levelling properties in a large commercial lead refinery. The results of the
testing of the polarization and levelling producing agents are given by means
of the following non-limitative examples as shown in Table II. In order for
an addltion agent to be acceptable in a commercial lead deposition process,
the agent must have the designation of very good or excellent for its polari-
zing and levelling properties. We have found that amounts of grain refining
agent and polarization and levelling producing agents, effective to produce
dense, smooth and level deposits of lead, are from 2 to 4 g/l of lignin sul-
fonate and from 0.02 to 10 g/l of a flavone, isoflavone, flavanone, isoflava-
none, chalcone or solubilised flavone, isoflavone, flavanone, or isoflavanone.
--10-
t)~
T A B L E II
Name of agent Optimum amounts Polari~ing Levelling
in g/l properties properties
quercetin 0.05 - 0.10 good good
rutin 0.3 - 0.5 excellent** good
apigetrin 0.08 - 0.10 excellent excellent
fisetin 0.03 - 0.05 good N . D .
morin 0.02 - 0.03 good good
chrysin 0.02 - 0.03 good good
ethoxylated quercetin 0.1 - 0.2 excellent excellent
ethoxylated fisetin 0.05 - 0.07 excellent excellent
ethoxylated morin 0.20 - 0.25 very good very good
dihydro-quercetin 0.07 - 0.10 good good
hesperidin 0.4 - 0.6 excellent excellent
neo-hesperidin 0.25 - 0.35 excellent excellent
hesperetin 0.02 - 0.03 good N. D .
naringenin 0.07 - 0.10 good N.D.
naringin 0.3 - 0.5 excellent excellent
ethoxylated
dihydro-quercetin 5 - 6 excellent excellent
ethoxylated naringenin 0.07 - 0.10 very good very good
ethoxylatecl hesperetin 0.03 - 0.05 very good very good
flavanone* 0.4 poor N.D.
l~iochallin ~ 0.08 - 0.15 good N.D.
hespericlin
~.ethyl-chalcone 0.4 - 0.6 excellent excellent
p-ropoxylated quercetin 0.1 - 0.2 excellent excellent
propoxylated dihydro-
quercetin 0.06 - 0.08 very good very good
Notes * does not contain hydroxyl groups and could not be ethoxylated
_ _ _
** in:i.ti<llly excellent, poorer with tiDIe (glycosicle in 3 position~
hydroly~ed)
N.D. not deter~ ed
--11--
s~
From the tabulated results it can be seen that the flavone apige-
trin, and the flavanones hesperidin, neo-hesperidin and naringin rated excel-
lent. Ethoxylation of the flavones quercetin, fisetin and morin, which, as
such, only rated good, improved their rating to very good or excellent al-
though requiring larger optimum amounts. Similarly, the hesperedin methyl-
chalcone had excellent properties, while ethoxylation of dihydro-quercetin,
naringenin and hesperetin, and propoxylation of quercetin and dihydro-querce-
tin improved their rating. Ethoxylation of biochanin A shouldJ similarly,
improve its rating.
The above examples show that flavones and flavanones, wherein at
least one hydroxyl group is positioned in the structural formula in proximity
to the ketonic group and wherein a glycoside, preferably in the 7 position,
is present, have excellent polarizing and levelling properties as addition
agents in an electrodeposition process for lead. Insertion of a solubilizing
group, for example by propoxylation or ethoxylation of flavones and flavanones
which do not themselves contain a glycoside group or other solubilizing group,
improves the polarizing and levelling properties.
.~
