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
06569311
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention provides a continuous 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, concentration of zinc was low (approximately 30 grams per liter) and a low current density of 1200-1500 amperes/sq. meter 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 at a low current density (1000-3000 A/m
2
). Use of a divided cell and low current density makes this process significantly more capital intensive. In their study, 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, the 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.
One advantage of the present invention is that by providing a continuous process for the electrochemical reduction of zinc oxide (or any other zinc compound that reacts with an aqueous base to produce zinc oxide) to zinc powder, it provides for a very high volume efficiency, as solid zinc oxide (or the other zinc compound) is added continuously during the electrochemical process to maintain the concentration of zinc based species (such as Zn
2+
ions). Furthermore, the use of high current density coupled with high volume efficiency makes the presently claimed process industrially attractive.
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 a current density of about 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 electrowinning 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 this process uses a relatively more complex cell, and a lower current density.
SUMMARY OF THE INVENTION
The present invention provides a continuous electrochemical process for preparing zinc powder, which comprises the steps of:
a) providing to an electrochemical cell a solution or suspension in an aqueous 1.25 Molar to 10.0 Molar base solution of zinc oxide or any other zinc compound that reacts with an aqueous base to produce zinc oxide, the solution or suspension containing at least 2 millimoles of solubilized zinc based species per 100 grams of electrolyte; and
b) passing current to the cell at a current density of about 500 to 40,000 A/m
2
, for a time period sufficient to electrochemically reduce the zinc based species to zinc powder, while continuously or intermittently adding a sufficient amount of the zinc oxide or the other zinc compound to the cell to maintain the concentration of the zinc based species at a level of at least 2 millimoles per 100 grams of electrolyte and continuously or intermittently removing at least a portion of the zinc powder formed;
wherein in steps a) and b), the electrolyte consists of the aqueous base solution and the zinc oxide or the other zinc compound, and the solubilized zinc species are derived from the zinc oxide or the other zinc compound.
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 solution or suspension in an aqueous 1.25 Molar to 10.0 Molar base solution of zinc oxide or any other zinc compound that reacts with an aqueous base to produce zinc oxide, the solution or suspension containing at least 2 millimoles of solubilized zinc based species per 100 grams of electrolyte.
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 sim
Bandlish Baldev K.
Martin Vincent Wise
Banerjee Krishna
Clariant Finance (BVI) Limited
Nguyen Nam
Parsons Thomas H.
Silverman Richard P.
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