Chemistry of inorganic compounds – With additive
Reexamination Certificate
1999-04-08
2001-02-06
Langel, Wayne (Department: 1754)
Chemistry of inorganic compounds
With additive
C423S029000, C423S275000, C423S371000, C423S379000, C423S658500
Reexamination Certificate
active
06183710
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a sodium cyanide composition, to a process for producing the composition, and to a process for using the composition.
BACKGROUND OF THE INVENTION
Sodium cyanide (NaCN) has a variety of uses. For example, it is used in electroplating, treating metal surfaces, extracting and recovering precious metals from ores, and a number of other chemical applications. The use of NaCN in the leaching of ores that contain precious metals, such as gold and silver, is especially well known in the art.
The production of NaCN for the above use is accomplished by the so-called “wet” process, wherein hydrogen cyanide (HCN) is neutralized with aqueous sodium hydroxide (NaOH) solution, followed by evaporative crystallization to produce a slurry of NaCN crystals. The NaCN crystals are separated from the slurry, dried, and usually formed by dry compression methods into briquettes. The resulting briquettes are about 99 weight percent anhydrous NaCN.
The briquettes are shipped to consumers generally in containers designed to exclude exposure to atmospheric air since the anhydrous NaCN is very hygroscopic and can absorb substantial quantities of water when exposed to atmospheric air. If exposed to atmospheric air, serious difficulties in shipping and storage can result due to caking. Also, there is the added cost of excluding atmospheric air.
The majority of consumers generally convert the NaCN briquettes into an aqueous solution. They dissolve the NaCN, sometimes after breaking the briquettes into smaller particles, to produce a solution containing about 20 to 25 weight percent NaCN. To avoid hydrolysis with the resulting evolution of hazardous hydrogen cyanide vapors, an alkali such as NaOH is added to the dilution water. Sufficient alkali should be added to raise the resulting solution pH to about 12 or higher.
To avoid difficulties and cost associated with storage of the anhydrous product and industrial hygiene hazards due to generation of respirable dust when handling anhydrous NaCN briquettes or breaking the briquettes into smaller particles, some of the larger consumers have changed to direct solution storage. This has been accomplished by dissolution of the briquettes in the shipping container, usually tank trucks or railway cars, or in a storage tank, and unloading the resulting solution into storage tanks.
The manufacture of briquettes has the disadvantages of added investment and operating cost associated with concentrating, separating, drying and compacting anhydrous NaCN only to dissolve and dilute it prior to use. It would thus appear that direct shipment of NaCN solution, particularly where the NaCN production facilities are located in reasonable proximity of the consumer, would be highly desirable. However, the shipment of NaCN solution presents high shipping costs and a high environmental risk of spills in the event of an accident during transportation.
Anhydrous crystals of NaCN can be prepared by a number of processes known in the art. For example, McMinn, U.S. Pat. No. 2,708,151 and Oliver, U.S. Pat. No. 2,726,139, disclose processes for reacting substantially pure HCN with substantially pure NaOH to form solutions containing NaCN. Mann, et al., U.S. Pat. No. 3,619,132; Cain, U.S. Pat. No. 2,616,782; Mittasch, et al., U.S. Pat. No. 1,531,123; and Rogers, et al., U.S. Pat. No. 4,847,062 disclose processes for preparing NaCN employing impure starting materials. Isolating dry crystals of NaCN from reaction solution presents the hazards of exposing to harmful dust of anhydrous NaCN and additional manufacturing costs.
Accordingly, there is a need for a NaCN composition that can be inexpensively produced and packaged in sufficiently high NaCN weight percentage concentrations, with low risk to the environment when shipped over distances, and efficiently dissolved into solution upon delivery to the consumer. There is also a need to develop a process for producing a NaCN that eliminates the need to separate, dry and compact the NaCN prior to shipment by providing a composition with a paste-like consistency which is highly desirable from the standpoint of safe transport of product to consumers.
An advantage of the present invention is that the NaCN concentration in the paste composition is sufficiently high that the cost of shipping water does not become an overriding concern. Another advantage of the present invention is that the paste composition of this invention eliminates for consumers the hazards of exposure to harmful dusts associated with handling the anhydrous NaCN briquettes, and the need to add caustic to avoid the generation of hazardous HCN vapors due to hydrolysis of the NaCN. Also an advantage of the paste compositions of this invention is that the paste incorporates sufficient base and, therefore, consumers may not need to provide additional base to avoid hydrolysis, when used in an application. Other advantages will become more apparent as the invention is more filly disclosed hereinbelow.
SUMMARY OF THE INVENTION
According to a first embodiment of the present invention, a composition is provided which comprises NaCN, a base, water, and optionally a rheology modifier.
According to a second embodiment of the present invention, a process for preparing a NaCN paste composition is provided which comprises: (a) contacting HCN with an aqueous medium comprising a base and optionally a rheology modifier at an elevated temperature; and (b) cooling the medium to provide a NaCN paste composition.
According to a third embodiment of the present invention, a process is provided which comprises contacting an ore with a composition which is the same as the composition disclosed in the first embodiment of the invention and can be produced by the process disclosed in the second embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a NaCN paste composition that facilitates applications requiring NaCN in solution, such as electroplating, treating metal surfaces, extracting and recovering precious metals from ores, and a number of other chemical applications.
For purposes of this invention, a paste is defined as a suspension of solid particles in a liquid phase wherein the viscosity of the suspension is generally about 1 to about 500 Pa.s at 25° C. in the shear rate range of about 1s
−1
to about 10 s
−1
. The shear viscosity, or simply viscosity, is the proportionality constant between shear stress and shear rate for a material, and is a common measurement which can be used to characterize the fluidity of the paste compositions of this invention. As described in “An Introduction to Rheology”, by H. A Barnes, J. F. Hutton and K. Walters, Elsevier, 1989, pp. 26-31, viscosity measurements may be performed in any of several types of rotational instruments employing different probe configurations: parallel plate (or disk), cone and plate, or concentric cylinder. The measurements may further be made in either of two common modes: commanded rate under steady or constant shear conditions, or commanded force under steady stress conditions. The paste compositions of this invention, characterized using a parallel disk instrument under steady shear conditions, exhibit approximate power-law shear-thinning behavior on a log-log plot of viscosity versus shear rate over the shear rate range of about 0.1 to 100 s
−1
.
Such NaCN paste compositions can be inexpensively produced and packaged, and can be transported more safely than NaCN solution. The paste composition of the invention also provides the consumer with a sufficiently high NaCN weight percentage to minimize cost to the manufacturer of shipping material other than NaCN, i.e., shipping water.
According to the first embodiment of the invention, a composition is provided which comprises sodium cyanide, a base, water, and optionally a rheology modifier.
Sodium cyanide produced by any methods known to one skilled in the art can be used in the composition of the present invention. Sodium cyanide can be present in the composition at least about 45, pref
Rogers Janet Marie
Tufano Thomas Peter
E. I Du Pont de Nemours and Company
Langel Wayne
Shay Lucas K.
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