CA1115658A - Addition agents in lead electrodeposition - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Process for the electrorefining of lead in which a grain refining and levelling agent system comprising a lignin sulphonate together with a flavone, isoflavone, flavanone, isoflavanone or chalcone is used.

Description

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 .

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, ~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 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 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.

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 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.

.~

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the electrorefining of lead in a cell con-taining 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 agent an effective amount of a lignin sulphonate and as levelling agent an effective amount of at least one electrolyte soluble flavone, flavanone, isoflavone, isoflavan-one or chalcone having at least one hydroxyl group in close proxi-mity to the aromatic ketone group.

2. Process according to claim 1, wherein the cell contains a plurality of anodes and an equal number of cathodes.

3. Process according to claim 1, wherein the cell contains a plurality of electrodes, comprising at least one anode, at least one cathode, and at least one bipolar electrode placed between the anode(s) and cathode(s).
4. Process according to claim 1, wherein the flavone or flavanone is of general formula Ia or Ib I a Ib wherein Rl and R2 together represent either a second carbon-carbon bond (flavone or isoflavone) or each represent hydrogen (flavanone or isoflavanone);
R3 represents hydrogen, a hydroxyl group, or a solubilizing group;
R4 represents hydrogen, a hydroxyl group or a solubilizing group;
R5 represents one, or more, substituents chosen from hydrogen, hy-droxyl or a solubilizing group; and R6 represents one or more substituents chosen from hydrogen, hydrox-yl, etherified hydroxyl, or a solubilizing group;
provided that at least one of R3 and R4 is a free hydroxyl group.

5. Process according to claim 1, wherein the chalcone is of general formula II

wherein R3 represents hydrogen, a hydroxyl group, or a solubilizing group;
R4 represents hydrogen, a hydroxyl group or a solubilizing group;
R5 represents one, or more, substituents chosen from hydrogen, hy-droxyl or a solubilizing group;
R6 represents one or more substituents chosen from hydrogen, hydrox-yl, etherified hydroxyl, or a solubilizing group; and X represents hydrogen, an alkyl group, or a solubilizing group;

provided that at least one of R3 and R4 is a free hydroxyl group.

6. Process according to claim 4 or 5, wherein a solubilizing group pre-sent as one, or more, of R3, R4, R5 or R6 is chosen from an O-glycoside group, a C-glycoside group, an ethoxylated hydroxyl group, a propoxylated hydroxyl group, a carboxy-alkoxy group of 2 to 10 carbon atoms, a sulfo group, or an alkylamino group.

7. Process according to claim 4 or 5, wherein an etherified hydroxyl group present as R6 is a methoxy group.

8. Process according to claim 5 wherein X represents a methyl group.

9. Process according to claim 1, 4 or 5, wherein the flavone, flava-none, or chalcone is chosen from apigetrin, hesperidin, neo-hesperidin, rutin, naringin, hesperidin methyl chalcone; ethoxylated derivatives of quercetin, isetin, morin, dihydro-quercetin, hesperetin and naringenin; and propoxy-lated derivatives of quercetin and dihydro-quercetin.

10. Process according to claim 4 or 5, wherein a solubilizing group pre-sent as R3, R4, R5 or R6 is an )-glycoside or C-glycoside group derived from glucose, fructose, or rhamnose.

11. Process according to claim 4 or 5, wherein a solubilizing group pre-sent as R5 is anO-glycoside group present in the 7 position.

12. Process according to clai.m 1, wherein the applied current density in the electro-refining process is in the range of 100 to 300 A/m2.

13. Process according to claim 1, wherein the applied current density in the electro-refining process is in the range of 100 to 500 A/m .

14. Process according to claim 1, wherein the electrolyte contains about 70 g/l lead as lead fluosilicate, and about 90 g/l fluosilicic acid.

15. Process according to claim 1, 4 or 5, wherein the electrolyte con-tains frorm about 2 g/l to about 4 g/l of lignin sulphonate.

l6. Process according to claim 1, 4 or 5, wherein the electrolyte contains from about 0.02 g/l to about 10 g/l of flavone, isoflavone, flava-none, isoflavanone, chalcone, or solubilized flavone, isoflavone, flavanone, or isoflavanone. 15 17. Process according to claim l, 2 or 3, wherein the cell electrolyte is circulated and an effective amount of grain refin-ing agent and an effective amount of levelling agent is maintained in the circulating electrolyte.

18. Process according to claim 1, 2 or 3 wherein the effec-tive amount of levelling agent is such that the slope of the polarization curve is in the range of about 0.3 to about 0.4 mV/

A/m2 .