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(NH3)2Cl2 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 xe2x80x9czinc oxidexe2x80x9d crystals. Burrows indicates that these crystals range in size up to xe2x85x9c 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 must be added to remove all of the lead, a poor practice for economic reasons. Further, if it is desired to separate the lead and some cadmium from the zinc so that all of these metals can be sold or reused, the higher the zinc concentration in the metals, the larger the mass to be processed per unit mass of zinc.
U.S. Pat. No. 4,071,357 to Peters discloses a method for recovering metal values which includes a steam distillation step and a calcining step to precipitate zinc carbonate and to convert the zinc carbonate to zinc oxide, respectively. Peters further discloses the use of a solution containing approximately equal amounts of ammonia and carbon to leach the flue dust at room temperature, resulting in the extraction of only about half of the zinc in the dust, almost 7% of the iron, less than 5% of the lead, and less than half of the cadmium.
Steam distillation is directly contrary to temperature lowering; steam distillation precipitates zinc carbonate, other carbonates and iron impurities, whereas temperature lowering advantageously precipitates a number of crystalline zinc compounds. Steam distillation also disadvantageously results in an increase in temperature which drives off ammonia and carbon dioxide, resulting in the precipitation of iron impurities and then zinc carbonate and other dissolved metals. The purity of the zinc carbonate obtained depends on the rate of steam distillation and the efficiency of solids separation as a function of time. Calcining at temperatures between 200xc2x0 C. and 1100xc2x0 C. converts the zinc carbonate to zinc oxide, whereas washing and drying at temperatures between 100xc2x0 C. and 200xc2x0 C. converts the zinc compounds to zinc oxide. In addition to the advantages of temperature lowering, the present process also employs a 23% NH4Cl solution at temperatures ranging from 90-110xc2x0 C., and has several distinct advantages over the Peters process:
1. The solubility of zinc and zinc oxide is relatively high in NH4C1 solution which is important to the efficiency of the present process in terms of the rate of the leaching, the mass of dust that can be processed, and the ability to recycle the solution. The rate of the leaching (which is a dissolution process) is a function of the difference between the zinc concentration in solution and the saturation concentration; the higher the saturation concentration the more rapid the leaching. The present process leaches for only 1 hour, while the Peters process leaches for at least several hours. In addition, the ammonium chloride solution has the added property that the solubility of zinc and zinc oxide in the solution declines rapidly with temperature, which is the basis for the crystallization-based separation which is used later in the present process.
2. Lead and lead oxide, as well as cadmium and cadmium oxide, are soluble in the ammonium chloride solution while iron oxide is virtually insoluble. During the leaching process of the present invention, 95-100% of the zinc present as zinc oxide is extracted, compared to about 55% in Peters; 50-70% of the lead present is removed, compared to less than 5% in Peters; and 50-70% of the cadmium is removed, compared to less than half in Peters. In effect, Peters does not remove a significant amount of the impurities so as to leave an acceptably clean effluent. Peters indicates that his residue, which is high in lead and is a hazardous waste, is discarded. By leaching out a significant portion of the lead and cadmium, the present process produces a material which can be used by the steel producer as they use scrap metal.
3. Peters adds powdered zinc to the solution, which has a tendency to clump reducing the surface area available for the dissolution of the zinc and the plating of the lead and cadmium. The present process teaches a method to minimize this effect through the use of an organic dispersant.
In the present process the filtrate from the cementation step is already hot (90-110xc2x0 C.) and contains a large amount of dissolved zinc with small amounts of trace impurities. Upon controlled cooling of the solution, crystals of zinc salts begin to appear. Control of the cooling rate and temperature versus time profile is important in controlling the size distribution of the crystals and in reducing or eliminating many of the impurities which might occur. This is especially true of included solution; control of the crystallization can reduce this to virtually zero. In addition, since crystallization is based on differential solubility, and none of the impurities is present in a concentration which can crystallize, the zinc salts are virtually free of any metal impurities.
The final purification step in Peters is a calcining of the zinc carbonate at 600xc2x0 C. to zinc oxide. In the present process, the mixture of zinc oxide hydrates and diamino zinc dichloride are suspended in hot (90-100xc2x0 C.) water. The zinc oxide is not soluble; however, the diamino zinc dichloride is very soluble and completely dissolves. The remaining solid which is zinc oxide hydrates is then filtered and dried at 100-350xc2x0 C. to remove the water of hydration. The result is a very pure zinc oxide powder of controlled particle size.
Another process offered by Engitec Impianti SpA, of Milan, Italy purports to recover zinc metal and lead cement directly from EAF flue dust using an electrowinning technology. Electrowinning is the technique of extracting a metal from its soluble salt by an electrolytic cell. Typically, it is used in the recovery of zinc by subjecting the zinc salt in solution to electrolysis and electrodepositing the elemental metal on a zinc cathode in the electrolytic cell. In the Engitec process, the EAF flue dust is leached with a spent electrolyte, such as ammonium chloride, which dissolves the zinc, lead, copper and cadmium in the EAF dust into solution while leaving the iron in solid form. The solution containing the dissolved zinc is placed in an electrolytic cell which draws the zinc from the solution onto a cathode plate, while the other heavy metals are filtered out in solid form into cement cakes. Engitec claims to obtain a zinc yield that is 99.5% pure and a lead cake consisting of a minimum of 70% lead. In effect, the Engitec process takes the product solution from the Burrows process and subjects it to electrowinning. A neutral solution of ammonium and sodium chlorides is heated to between 70xc2x0 C. and 80xc2x0 C. The EAF dust is mixed into the chlorides solution in which the zinc and heavy metals are dissolved. The iron, calcium, magnesium and aluminum oxides are insoluble in the chlorides solution. After leaching and residue filtration, the solution is purified by cementation using zinc granules or powder. After removal of the cement, consisting of lead, copper, silver and cadmium, the purified solution is fed to the electrolysis cell.
Apparently, the electrolysis of the zinc amino complex in the purified solution occurs in a conventional open cell using a titanium permanent blank cathode and a proprietary graphite anode. In the electrolysis cell, the zinc plates on the titanium cathode. However, the deposition time for the zinc is 24 to 48 hours, depending on the current density. In addition to the electrowinning of zinc, the electrolysis cell consumes ammonia and evolves nitrogen. Because of this, in order to maintain the pH of the electrolyte in the desired range of 6 to 6.5, additional ammonium must be added to the cell in the range of 180 kg per ton of product zinc.
Although the Engitec process appears to be theoretically possible, the use of an electrolysis cell adds additional costs to the process due to the energy consumption of an electrolysis cell, the consumption of ammonia in the electrolysis cell, additional costs in handling nitrogen evolved in the electrolysis cell, and the cost of maintaining the components of the electrolysis cell itself. For example, the titanium cathode can be costly, while the apparently proprietary graphite anode also may be costly. The Engitec process also results in the formation of metallic zinc. Although metallic zinc has a certain value, zinc oxide has more value. The residue removed from the Engitec process is composed primarily of iron oxide and zinc ferrite. Iron oxide can be used in the steel making process. The presence of zinc ferrite likely is not a significant detriment to the use of the residue from the Engitec process, but it does inject an additional impurity into any future process. It would be more advantageous to obtain a residue comprising primarily iron oxide with no zinc ferrite or other impurities, or only an insignificant amount of such other impurities.
The electrowinning of metals from chloride solutions is known in the art. U.S. Pat. No. 4,155,821 to Grontoft discloses and claims a method for recovering chlorine using electrolytic separation. Chlorine and metal are produced from a chlorine containing electrolyte by an electrolytic process having an anode surrounded by a membrane connected to a hood. The process is maintained at a partial vacuum so that any chlorine gas generated by the anode together with some of the electrolyte is drawn away from the anode. The vacuum also is devised to control redifussion of chlorine containing electrolyte back through the membrane into the surrounding electrolyte. The process is for use with nickel recovery where the nickel chloride containing electrolyte is introduced at such a rate that the pH is maintained below a certain level. The process also may be used for cobalt recovery.
The electrodeposition of zinc from chloride solutions also is known in the art. U.S. Pat. No. 4,292,147 to Fray discloses and claims a method for the electrodeposition of cadmium or zinc from chloride solutions derived from chlorine leaching of materials. An aqueous solution having 15 to 30% by weight of zinc or cadmium chloride is electrolyzed at a pH of 2 to 3.5 at a temperature of below 35xc2x0 C. with gas agitation at a current density above 100 A/m2 to form coherent zinc or cadmium at the cathode. A typical zinc containing material such as flue dust is leached with a saturated chlorine solution, preferably in the presence of chlorine hydrate. The zinc chloride solution preferably contains 20 to 30% by weight zinc or cadmium chloride and up to 20% by weight alkaline metal or ammonium chloride. The electrolysis preferably is carried out at 0xc2x0 C. to 9xc2x0 C. and above 2500 A/m2 with intermittent current reversal. Chlorine hydrate liberated at the anode may be recycled to affect leaching.
In addition to the uses of zinc oxide as a pigment, there are several important zinc compounds which are synthesized from zinc oxide. Methods to make various zinc compounds from zinc oxide are well known in the art. The methods usually involve the reaction of zinc oxide with the appropriate acid to form the zinc compound. Generally, commercial zinc oxide is first made by the combustion of zinc vapor in air, resulting in dry powdered zinc oxide. This dry powder must first be suspended or dissolved in an aqueous solution prior to treatment with the appropriate acid to produce the desired compound. In many cases the zinc oxide is dissolved in an aqueous solution of an acid. The zinc compounds may be present in solution or alternatively may be precipitated as a solid.
Thus, there exists a need for a method which will recover high purity zinc oxide products from industrial waste streams. The method disclosed below relates to the preparation of essentially pure zinc oxide. In addition, since zinc oxide is the desired product and diamino zinc dichloride is undesired, the method disclosed herein demonstrates how to increase the formation of desired zinc products and decrease the formation of diamino zinc dichloride. Furthermore, once the essentially pure zinc oxide has been obtained, the zinc oxide is further purified by a process which is based on the solubility of zinc oxide in a concentrated sodium hydroxide solution. This purification process can be controlled to produce zinc oxide having a desired size and surface area.
There also exists a need for a method which will allow the recovery of iron oxide from industrial waste streams which can be used with little or no additional treatment as the feedstock for other processes. Producing an iron oxide with a minimum amount of impurities, such as zinc ferrite, is advantageous because the iron oxide can be used as the feedstock for steel production processes. A method which results in the recovery of iron oxide would have additional value in that the iron oxide could be sold for use in other processes.
There also exists a need for a quick, easy, and economical method for the synthesis of various zinc compounds from recovered zinc oxide.
Additionally, with specific regard to this disclosure, there exists a need for methods of purifying zinc oxide which allows a highly purified zinc oxide to be obtained which has predeterminable and controllable purity, and particle size and shape characteristics.
The present invention satisfies these needs in a method which recovers essentially pure zinc oxide ultimately from waste material containing zinc or zinc oxide. Along with the essential pure zinc oxide, zinc metal can be recovered, along with values of other metallic elements contained in the waste material such as lead, silver, and cadmium. The solutions used in the process are recycled such that the process does not have any liquid wastes. The solids recovered from the process, namely, the zinc oxide, zinc, metal values, and other residues all can be used in other processes. One such residue, an iron oxide cake, is of such a quality that it can be used directly as the feedstock for the typical steel production process. The zinc oxide recovered in this preliminary process is further purified by the preferred process which is based on the solubility of zinc oxide in a concentrated sodium hydroxide solution. This purification process can be controlled to produce zinc oxide having a desired size and surface area.
One method for purifying zinc oxide to obtain zinc oxide crystals having predetermined purity and particle characteristics comprises the steps of dissolving a zinc oxide containing product in an intermediate, filtering out any undissolved materials, precipitating zinc oxide crystals out of the intermediate in a controlled manner such that the zinc oxide crystals have predetermined purity and particle characteristics, filtering out the zinc oxide crystals, washing the zinc oxide crystals, and then drying the zinc oxide crystals.
The intermediate is preferably selected from the group consisting of sodium hydroxide, ammonium sulfate, ammonium chloride liquor, ammonium phosphate, potassium hydroxide, ammonia/ammonium oxalate, and ammonia/ammonium carbonate solutions. Most preferably, the intermediate is a concentrated 50%-70% sodium hydroxide solution. The precipitation step is accomplished by diluting the solution at a predetermined rate by a factor ranging from 3 to 30 at a temperature ranging from about 25xc2x0 C. to 100xc2x0 C. at atmospheric pressure, and even over 100xc2x0 C. at pressures greater than atmospheric pressure, to precipitate the zinc oxide crystals.
The preferred method for purifying zinc oxide to obtain zinc oxide crystals having predetermined purity and particle characteristics comprises controlling the method by which the intermediate, preferably sodium hydroxide, is added to the zinc oxide can be used to control the particle size of the resulting purified zinc oxide. Preferably, the sodium hydroxide is dispersed into droplets averaging in size from 100 to 300 microns, with 150-250 microns being preferred, and the best results being obtained at 180 microns. Generally, the smaller the droplet size the larger the surface area of the resulting zinc oxide particles.
Once the substantially pure zinc oxide is recovered from the preliminary process, the purification process takes place resulting in zinc oxide which is at least 99.8% pure. In the preferred embodiment, this purification process is based on the solubility of zinc oxide in a concentrated sodium hydroxide solution. In the preferred process, zinc oxide is dissolved in a concentrated 50%-70% sodium hydroxide solution. Most of the metal impurities contained in the zinc oxide will not dissolve, including manganese, iron and cadmium. Any lead, calcium or chloride contained in the zinc oxide will dissolve. The solution is then filtered to remove the undissolved solids, which are then recycled back to the metals recovery section of the plant and thereby returned to the recycling process of the present invention.
The solution is then diluted by a factor ranging from 3 to 30, and preferably 3 to 8. The dilution preferably is performed hot at temperatures at or above 70xc2x0 C., preferably at temperatures ranging between 90 to 100xc2x0 C., so that the dilution step favors the formation of zinc oxide as compared to zinc hydroxide. The zinc oxide crystals which form are then filtered out, sent to a wash tank where they are washed with water, and sent to a dryer where they are dried, preferably at a temperature of 160xc2x0 C.
The diluted sodium hydroxide solution then is sent to an evaporated condenser where the solution is concentrated back to 50%-70% sodium hydroxide so that it can be reused. When a steady state has been achieved, this step will result in the formation of sodium chloride crystals which will be filtered out of the solution and recovered. This is because sodium chloride formed by the chloride present in the zinc oxide is less soluble in a concentrated sodium hydroxide solution than in dilute sodium hydroxide. After the sodium chloride is filtered out, the concentrated solution can be reused in the purification process of the present invention.
Periodically, lead will be removed from the sodium hydroxide solution by cementation. This involves the addition of zinc dust which will displace the lead in solution. The lead will be filtered out and sent to the lead recovery portion of the plant.
By controlling the manner in which the zinc oxide precipitates out of the intermediate during the zinc oxide crystallization step, it is possible to control the particle size hence the surface area of the zinc oxide produced as well as the purity. Furthermore, the purification process of the present invention can be used to purify zinc oxide obtained from other sources.
Therefore, it is an object of the present invention to provide a method for recovering zinc oxide from waste materials, such as fly ash or flue dust, which contain other metals, such as iron oxide, lead oxide, cadmium and other materials.
Yet another object of the present invention is to provide a method for recovering zinc oxide in which all leaching and washing solutions are recycled for further use, and no leaching or washing solutions are disposed of into the sewers or the environment.
Still another object of the present invention is to provide a method for recovering zinc oxide which also results in the precipitation in elemental form of any lead and cadmium metals contained in the starting materials.
It is another object of the present invention to provide a method for recovering zinc oxide in which all of the zinc can be recycled so that all of the zinc eventually will be converted to zinc oxide.
Still another object of the present invention is to provide a method for recovering iron oxide from waste materials, such as fly ash or flue dust, which contain other metals, such as zinc, lead oxide, and cadmium.
A further object of the present invention is to provide a method for recovering iron oxide which can be used as is as a feedstock for steel production processes.
Another object of the present invention is to provide a method for recovering zinc metal, zinc oxide and/or iron oxide which is economical, quick and efficient.
A further object of the present invention is to provide methods of purifying zinc oxide which allows a highly purified zinc oxide to be obtained which has predeterminable and controllable purity, and particle size and shape characteristics.
A final object of the present invention is to provide a quick, easy, and economical method for the synthesis of various zinc compounds from recovered zinc oxide.
These objects and other objects, features and advantages of the present invention will become apparent to one skilled in the art when the following Detailed Description of a Preferred Embodiment is read in conjunction with the attached figures.