Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing single metal
Reexamination Certificate
2001-02-02
2003-05-27
Nguyen, Nam (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing single metal
C205S603000
Reexamination Certificate
active
06569310
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention provides an electrochemical process for the preparation of zinc powder from zinc oxide.
2. Description of the Related Art
Zinc powder is widely used in the chemical industry in various industries. Zinc oxide containing other zinc salts, metal impurities, etc. is produced as a byproduct. Recycling of the zinc oxide to produce pure zinc powder is highly desirable from a cost as well as an environmental point of view.
The electrodeposition of zinc metal is a well-known reaction in electrochemical technology (See, for example, D. Pletcher and F. C. Walsh,
Industrial Electrochemistry
, Blackie Academic, 1993). The electrogalvanizing of steel is a process carried out on a very large scale and aqueous acid is the normal medium. High speed, reel to reel galvanizing of steel is carried out in sulfuric acid with dimensionally stable anodes and uniform deposition is achieved at high current density by inducing very efficient mass transport by rapid movement of the steel surface. The deposition of zinc metal is also the critical electrode reaction in the electrowinning and electrorefining of zinc. In addition, there are a number of technologies, which have been demonstrated for the removal of Zn(II) from effluents. However, in these technologies, concentration of Zn(II) is low, commonly less than 100 ppm. Finally, the deposition of zinc has been widely investigated as the cathodic reaction in candidate secondary batteries. In all these applications, however, the objective is to select the conditions so as to give an adhesive and smooth zinc coating.
Zinc powder can be produced by electrolysis either in strong alkaline or neutral zinc containing solutions. The first patents obtained on the alkaline electrolysis process date back to the early thirties (German Patents, 581013, 506590, 653557). In these methods, a low current density of 1200-1500 amperes/sq. meter (A/m
2
) was used. Volume efficiency and current density of these batch type processes are too low to be industrially attractive. I. Orszagh and B. Vass (Hung. J. Ind. Chem., 13,(1985) 287) used these methods to recycle zinc oxide byproduct from zinc dithionite production. They, however, used a divided cell, and lower current densities, which is significantly more capital intensive than using an undivided cell. In their study, at lower current densities no significant difference was observed at different sodium hydroxide concentrations.
For the recycling of zinc oxide containing waste by an alkaline electrolysis process to be industrially attractive, alkaline electrolysis process needs to be improved to lower capital as well as operational expenses. Capital expenses can be significantly reduced by increasing the current density and by providing a process that is capable of being carried out in an undivided cell. Furthermore, electrolysis conditions need to be improved to achieve high volume efficiency and minimum corrosion of the electrodes. The present invention unexpectedly fulfills these and other needs.
J. St-Pierre, D. L. Piron (Electrowinning of zinc from alkaline solutions at high current densities;
J. Appl. Electrochem
(1990), 20(1), 163-5), discloses experimental results conducted at high current density (2000 to 8000 A/m
2
) to obtain cell voltage and current efficiency data necessary for specific energy computations.
U.S. Pat. No. 5,958,210 discloses a method for electrowinning metallic zinc from zinc ion in aqueous solution, said method comprising performing electrolysis on a mixture of solid conductive particles and aqueous alkali solution, said solution ranging in concentration from about 3N to about 20N alkali and containing dissolved zinc ion at an initial concentration ranging from about 50 to about 500 grams of zinc ion per liter of said solution, in an electrolytic cell containing first and second vertically arranged, parallel flat plates defined as a current feeder and a counter electrode, respectively, said counter electrode coated with a substance that is catalytic for oxygen evolution, said cell further containing an ion-permeable diaphragm parallel to each of said plates and interposed therebetween to define a gap between said current feeder and said diaphragm, by passing said mixture of particles and solution through said gap such that said particles contact said current feeder and passing a current across said gap, thereby depositing metallic zinc from said solution onto said particles. The elelctrowinning process is disclosed to yield high current efficiency and low energy consumption. The process, however, is not industrially attractive for a large-scale production of zinc powder because it uses a relatively more complex cell, and a lower current density.
SUMMARY OF THE INVENTION
The present invention provides an electrochemical process for preparing zinc powder which comprises the steps of:
a) providing to an electrochemical cell a basic solution of zinc oxide or any other zinc compound that reacts with an aqueous base to produce zinc oxide, the basic solution prepared by dissolving the zinc oxide or the other zinc compound in an aqueous 2.5 to 10.0 Molar (M) base solution; and
b) passing current to the cell at a current density of about 10,000 to about 40,000 Amperes/meter
2
(A/m
2
) for a time period sufficient to electrochemically reduce the zinc oxide to zinc powder, wherein the electrochemical process has a current efficiency of at least 70% and is substantially free from electrode corrosion.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The first step a) of the presently claimed electrochemical process for preparing zinc powder involves: providing to an electrochemical cell a basic solution of zinc oxide or any other zinc compound that reacts with an aqueous base to produce zinc oxide, the basic solution prepared by dissolving zinc oxide or the other zinc compound in an aqueous 2.5 to 10.0 M base solution.
As used herein, the phrase “zinc powder” encompasses zinc metal particles of various particle sizes known to one of ordinary skill in the art and is not limited to fine particles.
The electrolytic cell employed in the present invention may be an undivided or divided cell, with the undivided cell being preferred. Use of an undivided cell requires lower capital. Furthermore, operational costs are also lower when an undivided cell is used. Therefore, it is important that the process be capable of being carried out in an undivided cell, if desired.
Since zinc powder formed at the cathode by the reduction reaction can react with the oxygen generated at the anode, cathodic and anodic chemistries are generally separated by some kind of a porous diaphragm which allows the current to pass, but suppresses mixing of anolyte and catholyte. Cells of this kind are called divided cells.
The design of the undivided cell is simpler and the cell voltage required to achieve the desired current density is lower because of the lower ohmic resistance. This means that the electrical cost is generally lower where an undivided cell is used. Furthermore, capital cost required with the undivided cell is significantly lower than the divided cell.
The anode may be made from any conventional suitable material such as platinum, or iridium, either of which may be coated over an inert support such as niobium or titanium. The anode may also be made of nickel, or from conventional materials having good alkali corrosion resistance, e.g., lead or stainless steel. The cathode may be made from any conventional suitable materials having good alkali corrosion resistance, such as magnesium, magnesium alloy, nickel, lead and stainless steel. Preferably, the anode in the present invention is formed of stainless steel, and the cathode is formed of stainless steel, magnesium, or magnesium alloy.
The basic solution of zinc oxide is prepared by dissolving zinc oxide, or the other zinc compound (such as zinc sulfate) in an aqueous 2.5 to 10.0 M base solution, preferably 2.5 to 8.0 M base solution, and in one embodiment 2.5 M to 5.0 M, and in one embodiment 5.0 to 10.0 M
Bandlish Baldev K.
Martin Vincent Wise
Banerjee Krishna
Clariant Finance (BVI) Limited
Silverman Richard P.
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