USOO5208004A United States Patent (19) 11 Patent Number: 5,208,004 Myerson (45) Date of Patent: May 4, 1993 (54) METHOD FOR THE RECOVERY OF ZINC Attorney, Agent, or Firm-Todd Deveau; Laurence P. OXDE Colton; Arthur A. Gardner 75) Inventor: Allan S. Myerson, Brooklyn, N.Y. 57) ABSTRACT 73 Assignee: Metals Recycling Technologies Corp., A method for the recovery of zinc oxide from industrial Atlanta, Ga. waste materials of various components, including zinc, (21) Appl. No.: 820,987 lead, iron and cadmium, comprising the steps of treating the waste material with an ammonium chloride solution (22) Filed: Jan. 15, 1992 maintained at an elevated temperature, separating the 51) int.C. .............................................. C22B 19/24 undissolved components from the solution, treating the 52 U.S. C. ...................................... 423/622; 75/724; solution with zinc metal to displace undesired metal 423/623 ions from the solution, cooling the solution to precipi 58 Field of Search ................... 423/622,623; 75/724 tate therefrom zinc compounds, washing the precipi 56) References Cited tated zinc compounds to remove unwanted compounds such as diamino zinc dichloride, and drying the remain U.S. PATENT DOCUMENTS ing zinc compound which is essentially hydrated zinc 2,503,479 4/1950 Griffith ................................. 75/724 oxide, resulting in essentially pure zinc oxide. 4,071,357 1/1978 Peters .................................. 423/622 Primary Examiner-Peter D. Rosenberg 22 Claims, 6 Drawing Sheets U.S. Patent May 4, 1993 Sheet 1 of 6 5,208,004 r e d o O d C ed O d d O d on o n Wo ar eas y N - a d d O C d o d C o U.S. Patent May 4, 1993 Sheet 2 of 6 5,208,004 U.S. Patent May 4, 1993 Sheet 3 of 6 5,208,004 s U.S. Patent May 4, 1993 Sheet 4 of 6 5,208,004 s s s 5,208,004 1 2 ride solutions which have been used to treat the waste METHOD FOR THE RECOVERY OF ZINCOXIDE metal dust. In the Burrows method, powdered zinc dust is added to the ammonium chloride solutions and an BACKGROUND OF THE INVENTION electrochemical reaction results in which lead in ele 1. Field of the Invention mental form deposits on the surface of the powdered The present invention relates generally to a method zinc dust. For this reaction to proceed, a large surface for the recovery of essentially pure zinc oxide and spe area of zinc initially must be present because as the lead cifically to a method for the recovery of essentially pure covers the zinc dust particle, the particle becomes no zinc oxide in a recycling operation from metal dust longer available for the electrochemical reaction. For containing zinc compounds. O this reason, very fine powder is used. However, in the 2. Prior Art Burrows method as disclosed, there is a major disadvan Zinc oxide typically is a coarse white or greyish pow tage in that the powdered zinc dust, when added to the der which has a variety of uses including as an accelera solutions, immediately aggregates to form large clumps tor activator, as a pigment, as a dietary supplement and which sink to the bottom of the vessel. Rapid agitation in the semiconductor field. Zinc oxide is found in com 15 does not prevent this from happening. Because of the merical by-products including waste material streams aggregation of zinc, a large amount of zinc must be such as fly ash and flue dust. Methods for recovering added to remove all of the lead, a poor practice for zinc oxides are known in the art, including recovering economic reasons. Further, if it is desired to separate zinc oxide from industrial waste materials. Such previ the lead and some cadmium from the zinc so that all of ous methods have included leaching with mineral acid, 20 these metals can be sold or reused, the higher the zinc caustic soda, ammonium hydroxide, and ammonium concentration in the metals, the larger the mass to be carbonate solutions. However, these methods have low processed per unit mass of zinc. yields of zinc oxide and typically do not recovery pure Thus, there exists a need for a method which will zinc oxide, the recovered zinc oxide being contami allow the recovery of elemental lead, cadmium, and nated with other metal salts. Therefore, in order to 25 obtain pure zinc oxide, subsequent roasting and evapo other metals from industrial waste streams which allows ration processes were necessary. the powdered zinc dust to remain dispersed in the solu Burrows, U.S. Pat. No. 3,849,121, assigned to a prin tion so as to minimize the amount of zinc dust needed to cipal of the assignee of the present invention, discloses a remove lead, cadmium and other metals. Minimizing method for the selective recovery of zinc oxide from 30 the amount of zinc required increases the economy of industrial waste. The Burrows method comprises leach the process first by reducing the quantity of zinc ing the a waste material with an ammonium chloride needed, second by reducing the mass of material to be solution at elevated temperatures, separating iron from processed, and third by allowing the removal of a pro solution, treating the solution with zinc metal and cool portionally greater quantity of lead and cadmium. ing the solution to precipitate zinc oxide. The Burrows 35 BRIEF SUMMARY OF THE INVENTION patent discloses a method to take metal dust which is mainly a mixture of iron and zinc oxides and, in a series The present invention satisfies these needs in a of steps, to separate out the iron oxides and waste met method which recovers essentially pure zinc oxide from als. However, the material obtained in the last step is a waste material containing zinc or zinc oxide. The waste mixture of a small amount of zinc oxide, hydrated zinc 40 material is added to an ammonium chloride solution at a phases which can include hydrates of zinc oxide and temperature of about 90° C. or above. The zinc and/or zinc hydroxide, as well as other phases and a large zinc oxide dissolves in the ammonium chloride solution amount of diamino zinc dichloride Zn(NH3)2Cl2 or along with other metal oxides contained in the waste other similar compounds containing zinc and chlorine material, such as lead oxide and cadmium oxide. The ions. Currently, the Burrows method is not economi 45 resultant solution is filtered to remove the undissolved cally viable because of Environmental Protection materials, such as iron oxides and inert materials such as Agency guidelines established subsequent of the issu silicates, which will not dissolve in the ammonium chlo ance of the Burrows patent. Additionally, the Burrows ride solution. Finely powdered zinc metal then is added method is not a continuous method and, therefore, is not to the resultant solution at a temperature of about 90 C. economical as a continuous process. or above. A dispersant may be added at this point to Thus, there exists a need for a method which will prevent the finely powdered zinc metal from flocculat recover zinc oxide from industrial waste which results ing and becoming less effective. Through an electro in a product the majority of which is zinc oxide, and not chemical reaction, lead metal and some cadmium plates mixtures of zinc oxide and other zinc phases. The out on the surface of the zinc metal particles. The addi method disclosed below relates to the preparation of 55 tion of sufficient powdered zinc metal results in the pure zinc oxide. In addition, since zinc oxide is the removal of virtually all of the lead from the resultant desired product and diamino zinc dichloride is unde solution. The resultant solution is filtered to remove the sired, the method disclosed herein demonstrates how to solid lead, zinc and cadmium. These initial steps, with increase the formation of zinc oxide and decrease the the exception of adding the dispersant, have been gener formation of diamino zinc dichloride. 60 ally disclosed in the prior art, yet have not resulted in Waste metal process dust typically has varying the production of essentially pure zinc oxide. amounts of lead, cadmium and other metals contained in The filtrate then is cooled to a temperature of be the dust. For various reasons, it is desirable to remove tween about 20 C. and 60' C. resulting in the crystalli such metals from the waste metal dust, for example to zation of a mixture of zinc compounds. This mixture recycle the lead and cadmium and/or to prevent intro 65 contains a significant amount of diamino Zinc dichlo duction of the lead and cadmium into the atmosphere. ride, or other complex compounds which involve zinc The Burrows patent includes a method for removing amino complexes, as well as hydrated zinc oxide and dissolved lead and cadmium from the ammonium chlo hydroxide species. The solid precipitate is filtered from 5,208,004 3 4. the solution, the solution recycled, and the solid precipi FIG.2C is an X-ray diffraction of the precipitate after tate washed with water at a temperature between about washing and drying ZnO. 25 C. and 100 C. The diamino zinc dichloride dis solves in the wash water leaving the majority of the DETALED DESCRIPTION OF A PREFERRED hydrated zinc oxide species as the precipitated solid. EMBODIMENT The precipitated solid then is filtered from the solution, The method for recovering zinc oxide disclosed the resulting solution being recycled, and the solid pre herein is carried out in its best mode in recovering zinc cipitate placed in a drying oven at a temperature of oxide from the waste streams of industrial or other between about 100 C.