Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Group iib metal
Reexamination Certificate
2003-08-27
2004-08-31
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Group iib metal
C423S109000, C423S622000, C075S724000, C075S725000, C075S961000
Reexamination Certificate
active
06783744
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
Many of the uses of zinc oxide require that the zinc oxide have certain particular size, shape and purity characteristics. Therefore, many grades of zinc oxide having different purity and particle characteristics have been developed to meet the diverse industry requirements. Today, most zinc oxide is made by the so called French Process which involves controlled burning of zinc metal vapor in air to obtain zinc oxide having exceptional chemical purity. The present invention provides a zinc oxide purification process which involves precipitating zinc oxide in such a manner that the desired purity and particle characteristics can be obtained. One method to control particle size is through the control of the conditions of the washing step. Additionally, although the zinc oxide purification process preferably utilizes a sodium hydroxide solution as the intermediate, the purification process of the present invention also provides for preparation of zinc oxide having particular purity and particle characteristics by utilizing intermediates such as ammonium chloride liquor, ammonium sulfate, ammonium phosphate, potassium hydroxide, ammonia/ammonium oxalate and ammonia/ammonium carbonate solutions. Once the zinc oxide has been dissolved in the solution, controlled dilution results in the precipitation of zinc oxide having predetermined purity and particle characteristics.
The present invention relates generally to a process for the recovery of zinc products including essentially pure zinc oxide and, optionally, an iron-carbon residual from industrial waste streams comprising zinc compounds and iron compounds. The present invention relates more specifically to a process subjecting a waste materials stream comprising zinc compounds and iron compounds, such as electric arc furnace (EAF) dust, to a combination of leaching and reducing steps, for the recovery of essentially pure zinc oxide in a recycling operation which recycles process solutions for reuse, and produces a cake product from undissolved iron and carbon compounds which can be used as a feedstock for steel mills. Once the essentially pure zinc oxide has been recovered, the zinc oxide is further purified by a process which is preferably based on the solubility of zinc oxide in a concentrated sodium hydroxide solution. This final purification process can be controlled in such a manner that the particle size and surface area of the zinc oxide produced can be controlled. Additionally, zinc compounds can be quickly, easily, and economically, synthesized from the aqueous zinc oxide slurry resulting from this process.
2. Prior Art
Zinc oxide typically is a coarse white or grayish powder which has a variety of uses including as an accelerator activator, as a pigment, as a dietary supplement and in the semiconductor field. Zinc oxide is found in commercial by-products including waste material streams such as fly ash and flue dust. Methods for recovering zinc oxides are known in the art, including recovering zinc oxide from industrial waste materials. Such previous methods have included leaching with mineral acid, caustic soda, ammonium hydroxide, and ammonium carbonate solutions. However, these methods have low yields of zinc oxide and typically do not recover pure zinc oxide, the recovered zinc oxide being contaminated with other metal salts. Therefore, in order to obtain pure zinc oxide, subsequent roasting and evaporation processes were necessary.
U.S. Pat. No. 3,849,121 to Burrows, now expired but which was assigned to a principal of the assignee of the present invention, discloses a method for the selective recovery of zinc oxide from industrial waste. The Burrows method comprises leaching a waste material with an ammonium chloride solution at elevated temperatures, separating iron from solution, treating the solution with zinc metal and cooling the solution to precipitate zinc oxide. The Burrows patent discloses a method to take EAF dust which is mainly a mixture of iron and zinc oxides and, in a series of steps, to separate out the iron oxides and waste metals. However, the material obtained in the last step is a mixture of a small amount of zinc oxide, hydrated zinc phases which can include hydrates of zinc oxide and zinc hydroxide, as well as other phases and a large amount of diamino zinc dichloride Zn(NH
3
)
2
Cl
2
or other similar compounds containing zinc and chlorine ions. Currently, the Burrows method is not economically viable because of Environmental Protection Agency guidelines established subsequent to the issuance of the Burrows patent. Additionally, the Burrows method is not a continuous method and, therefore, is not economical as a continuous process.
The first step in the Burrows patent is the treating of the EAF dust with an ammonium chloride solution. The action of the treatment is the leaching of zinc oxide, lead oxide and cadmium oxide in the solution without any leaching of the iron oxides present. Twenty to fifty percent of the zinc present in the Burrows dust is in the form of an iron-zinc complex (known as a spinel) which cannot be leached by the ammonium chloride solution. The Burrows process therefore cannot leach and recover a significant portion of zinc present in the EAF dust.
The second step in the Burrows process is cementation in which the solution obtained from the initial leach is filtered to remove any remaining solids and then zinc dust is added. The zinc dust causes an electrochemical reaction which causes the lead and cadmium to deposit on the zinc particles. Burrows does not teach the need to remove the lead and cadmium in this step efficiently without using a large amount of zinc. If the process requires too much zinc in this step, it will not be economically viable. The zinc powder when added tends to clump together reducing the available surface area and requiring the addition of more zinc.
The third step in the Burrows patent then takes the filtrate from the cementation process and cools the filtrate and obtains what are called “zinc oxide” crystals. Burrows indicates that these crystals range in size up to ⅜ of an inch. Burrows does not produce zinc oxide of any degree of purity; x-ray diffraction figures clearly show that upon crystallization there is a mixture of many phases. Washing the crystals is not sufficient to purify the material to zinc oxide since zinc hydroxide and hydrates are also present, so that a drying step is necessary. In addition, the control of the size of the zinc oxide along with the purity is crucial. Commercial zinc oxide normally has a requirement that 99% of the particles fit through 325 mesh (44 microns). Burrows indicates no method of cooling or controlling either purity or size, and the particles produced do not meet commercial requirements. Further, a significant portion of the ammonium chloride is lost in the crystal washing step when the diamino zinc dichloride decomposes.
Waste metal process dust typically has varying amounts of lead, cadmium and other metals contained in the dust. For various reasons, it is desirable to remove such metals from the waste metal dust, for example to recycle the lead and cadmium and/or to prevent introduction of the lead and cadmium into the atmosphere. The Burrows patent includes a method for removing dissolved lead and cadmium from the ammonium chloride solutions which have been used to treat the waste metal dust by the addition of powdered zinc dust to the ammonium chloride solutions. The resulting electrochemical reaction forms elemental lead deposits on the surface of the powdered zinc dust. For this reaction to proceed, a large surface area of zinc initially must be present because as the lead covers the zinc dust particle, the particle becomes no longer available for the electrochemical reaction. For this reason, very fine powder is used which, unfortunately, immediately aggregates to form large clumps which sink to the bottom of the vessel. Rapid agitation does not prevent this from happening. Because of the aggregation of zinc, a large amount of zinc
Myerson Allan S
Robinson Peter
Tabah Stephan
Bos Steven
Technoprop Colton LLC
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