Chemistry: fertilizers – Processes and products – Inorganic material
Reexamination Certificate
2000-10-27
2002-09-24
Nazario-Gonzalez, Porfirio (Department: 1621)
Chemistry: fertilizers
Processes and products
Inorganic material
C071S054000, C210S724000, C423S043000, C423S351000, C423S409000, C504S122000, C148S023000, C148S026000
Reexamination Certificate
active
06454828
ABSTRACT:
BACKGROUND OF THE INVENTION
Preparation of Zinc Diammine Chloride
Zinc diammine chloride (Zn(NH
3
)
2
Cl
2
, CAS #14826-66-5, a.k.a. zinc ammonia complex, diamino zinc dichloride, zinc diammine dichloride) (ZDC) has been primarily known as an intermediate for production of zinc chloride. It is not commonly known as a commercial product.
Crystallization of zinc diammine chloride from aqueous solutions has been a known technology for many years. Crystallization has been used as a method of separating and concentrating zinc from other cations, especially iron and heavy metals. A number of patents have been issued using this technology to recover zinc values from various by-product materials, such as steel mill dusts and other zinc-containing materials. Many of the patents use a process in which the zinc fume is collected in a scrubbing system containing an ammonium chloride solution. The zinc diammine chloride is crystallized from the ammonium chloride solution by adjusting the pH of the solution to near neutral.
Zinc diammine chloride can be produced in crystalline form. Zinc diammine chloride crystallizes as an unhydrated compound and can be grown into long needle-like crystals. The surface moisture can be easily dried by use of any typical material drying apparatus or process. Patents have been issued concerning the production of anhydrous zinc chloride from zinc diammine chloride crystals. In these patents, the material is heated to decomposition to drive off ammonia.
Uses of Zinc Compounds
Agricultural
Zinc deficiency in soil is a common cause of poor plant growth. Zinc deficiencies can be remedied by applying a zinc salt to the soil directly or in an admixture with other plant nutrients.
Aqueous solutions of zinc ammine chloride and zinc ammine sulfate have been manufactured and marketed for years. These products are typically zinc tetraammine complexes and have been used as a source of zinc. There have been several patents covering the manufacture and use of zinc and other micronutrient ammine complexes as aqueous solutions. The patents cover aqueous zinc ammine complexes with both inorganic and organic anions, the most common being sulfates, chlorides, and nitrates. Some of the other notable use patents cover the manufacture of zinc ammine acetate or other organic anions and its use in anhydrous ammonia. The aqueous zinc ammine complexes are stable enough to prevent precipitation of zinc compounds when added to liquid fertilizers, such as urea/ammonium nitrate and polyphosphate fertilizers. The fertilizer market for such liquid zinc ammine complexes is mature and limited.
Zinc diammine chloride could also be used as a zinc raw material in liquid fertilizers. Zinc diammine chloride does not, however, dissolve as rapidly or mix as easily as the aqueous tetraammine complex. Dry zinc diammine chloride would be more economical to transport since the zinc content is higher. The zinc content of pure zinc diammine chloride is 38.4%.
The majority of zinc applied as fertilizer is used as a blend with other dry fertilizer materials. The most common sources of dry zinc for fertilizer are zinc sulfate (36% Zn) or zinc oxy-sulfate (20% Zn). These products are marketed as granules with the optimal size being matched with the particle size of the fertilizer material to prevent segregation. Some dry zinc fertilizer products, especially the zinc oxy-sulfates, contain high levels of iron and heavy metals. Environmental concerns are driving the market towards purified zinc sources with low heavy metal content. Zinc diammine chloride can solve this need by having very low heavy metal content and provide both zinc and nitrogen as sources of plant nutrients.
In order to produce particles of sufficient size to use the ZDC as a fertilizer, a need exists for a method to produce granules which are consistent in size, hard and nonfriable and which does not drive off an excess amount of ammonia resulting in zinc chloride. What constitutes a “sufficient size” is generally thought to be that which meets uniformity index (UI) and size guide numbers (SGN) for blending with other fertilizer components. For example, The Fertilizer Institute published a Bulk Blend Quality Control Manual in September 1987 which is designed to help producers and blenders in matching (which eliminates segregation) and compatibility of materials. Blended fertilizers are generally mixtures of granules of −6+16 mesh, but the most important consideration is matching the other components in the mixture. See e.g., Cecil Russell, Quality of a Blend Can be Determined, Farm Chemicals, March 1965, p. 86; George Hoffmeister, et al. Bulk Blending of Fertilizer Material: Effect of Size, Shape, and Density on Segregation, Agricultural and Food Chemistry, 1964, p. 64. Alternatively, crystallized zinc diammine chloride can be made into aqueous solution for fertilizer use at distant locations to lower the cost of transportation.
Adjustment of Galvanizing Preflux
There are many processes which produce zinc-containing by-products or waste products which can be utilized as a raw material for production of zinc complexes which have value. One such process is galvanizing. Galvanizing produces “sal ammoniac skimmings, sal skimmings, or wet skims”. Many galvanizers use a “top flux” composed of zinc chloride/ammonium chloride double salt in what is called a “wet kettle process”. The ratio of zinc chloride to ammonium chloride for a galvanizer's top flux can vary depending on the particular application. The most common ratio is a double salt containing 1 mole of zinc chloride to 3 moles of ammonium chloride. The metal to be galvanized is pickled in either a dilute hydrochloric acid or sulfuric acid solution to clean and prepare the surface, and then it is dipped into a vat of molten zinc through a layer of “top flux”. The flux is floated as a melt on top of a vat of molten zinc and the pieces to be galvanized are passed through this layer of flux to aid deposition of zinc. Over time, much of the ammonium chloride portion of the flux is sublimed and the flux becomes contaminated with zinc oxide, charred organics, and various other impurities. When the flux is no longer effective, it is skimmed off and sold as a by-product to various industries as a source of recyclable zinc, i.e., the “sal ammoniac skimmings”, “sal skimmings”, or “wet skims”.
Other galvanizers use a prefluxing process in which the zinc chloride/ammonium chloride double salt is dissolved in water and the pieces to be galvanized are dipped into the flux solution and then into a vat of molten zinc. The zinc chloride/ammonium chloride ratio in these preflux solutions varies depending upon the application. Most operations use ratios of either 1:3 or 1:4 zinc chloride/ammonium chloride. This type of galvanizing is known as “dry kettle”. Over time, oxidation of the molten zinc occurs and a layer of zinc, iron, and other metal oxides form on the surface of the molten zinc. This layer is also skimmed off to prevent problems in galvanizing and is commonly known as “dry zinc skims”. It is composed of the various metal oxides as well as metallic zinc. This material is more easily recycled than the “wet skims” and commands a higher price.
Dry galvanizing also goes through a pickling process to clean and prepare the metal surface. Unless the metal is well rinsed, carryover from the acid-pickling tank gradually contaminates the preflux solution and lowers the pH to a point where it no longer performs properly. At this point the flux solution must either be changed or adjusted chemically. Properly maintained preflux solution has a pH of 4.0-4.5. One option for adjusting the solution pH is to add some of the “dry skims” from the zinc kettle. The zinc oxide reacts with the excess acid to form zinc chloride. This option changes the optimum zinc chloride/ammonium chloride ratio and requires the addition of ammonium chloride to maintain the balance. Another option is to add ammonia either as anhydrous or aqueous solution. This also changes the optimum zinc chloride/ammonium chloride rat
Lohry Dirk W.
Meeker Doyle E.
McKee Voorhees & Sease, P.L.C.
Nazario-Gonzalez Porfirio
Nulex, Inc.
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