This invention relates to the drying of copper concentrates in a gas suspension or fluid bed dryer. In one aspect, this invention relates to a process for the drying of copper concentrate that is both energy efficient and that reduces the possibility of concentrate combustion in the dryer.
Present copper production from sulfide ores almost universally involves mining porphyry deposits composed mostly of chalcopyrite (Cu.FeS2). Raw ore is crushed and ground suitable for floatation concentration in which different materials are separated by agitating a pulverized mixture of the materials with water, oil, and chemicals. Differential wetting of the particles suspended during flotation causes unwetted particles to be carried by air bubbles to the surface for collection to produce a beneficiated ore containing about 25% copper and approximately equal portions of copper, iron, and sulfur. The concentrate is dewatered mechanically and subsequently dried to less than about 0.5% moisture by weight and preferably less than about 0.2% moisture by weight, in a direct fired dryer. However, at temperatures above 260° C. and in the presence of oxygen the sulfide concentrates will combust. Combustion in the dryer is undesirable and results in unsafe conditions and downtime. In order to stay below the ignition temperature of sulfide ores, significant amounts of excess air must be introduced into the gas heating means such as a hot gas generator that is upstream from the dryer. Ironically, this excess air invariably serves to provide oxygen for sulfide concentrate combustion.
Dried concentrate and suitable fluxes are introduced to smelters/converters where the iron and sulfur is oxidized and the iron combines with the flux agents. The sulfur is released as sulfur dioxide gas. Traditionally, the concentrated ore is processed in a primary smelting reactor, such as an oxygen/flash smelter; to produce a copper sulfide-iron sulfide matte, up to 60 percent copper. The matte is oxidized in a converter to convert the iron sulfides to iron oxides, which separate out in a slag, and to reduce the copper sulfide to blister copper, which contains at least 98.5 percent copper. Current technology combines the converting step with the proceeding smelting step. Fire refining of blister copper then removes most of the oxygen and other impurities, leaving a product at least 99.5 percent pure, which is cast into anodes. Finally, most anode copper is electrolytically refined, usually to a purity of at least 99.95 percent.
In order to generate high sulfur dioxide gas concentrations and reduce off gas volumes, present practice is to use oxygen enrichment in the smelter/convert processes.
While there are many processes known and in use today in the production of copper, uniformly they all require the use of substantial amounts of oxygen requiring the utilization of oxygen producing plants. The oxygen plants required to support the oxygen enrichment generate substantial waste nitrogen. It would be both environmentally and economically efficient to utilize the waste nitrogen produced in the oxygen plant in the copper process.
According to this invention, waste nitrogen produced in an oxygen plant is preheated to temperatures suitable to dry copper concentrates in a gas suspension or fluid bed dryer.
In a first embodiment of the invention waste nitrogen from the oxygen plant is directed to a hot gas generator, in the presence of heated ambient air, thereby substantially reducing the oxygen content of the preheated ambient air in the hot gas generator. The oxygen depleted ambient air and the heated nitrogen gas are directed to a fluid bed or gas suspension device where they are mixed with and dry wet concentrate. The dried concentrate is carried out of the device with the spent nitrogen and ambient air and is collected for further treatment.
In another embodiment of the invention removing the oxygen source eliminates the possibility of concentrate combustion in the dryer. Waste nitrogen from the oxygen plant is directed to an air to gas heat exchanger. A hot gas generator fires the heat exchanger. However, the ambient air from the hot gas generator used as the heat transfer medium is directed to atmosphere and not into the drying device. Nitrogen preheated to significantly above the concentrate ignition temperature (260° C.) is directed to a fluid bed or gas suspension device where it is mixed with and dries incoming wet concentrate. The dried concentrate is carried out of the device with the spent nitrogen and collected for further treatment.
In a third embodiment of the invention there is a method for drying copper sulfide concentrates in gas suspension and fluid bed dryers utilizing controlled sulfide ore oxidation. In this embodiment the risk of concentrate combustion is reduced by controlling the amount of oxygen utilized in the dryer. Waste nitrogen and a controlled amount of oxygen from the oxygen plant is directed to a heat exchanger and preheated to temperatures sufficient to dry the copper concentrates. The preheated oxygen and nitrogen gases are directed to a fluid bed or gas suspension device where they are mixed with incoming wet concentrate. The reactor temperature is maintained above the concentrate combustion temperature and the sulfide is allowed to oxidize in a controlled fashion—thus providing the dehydration energy without supplemental fuel firing. The dried concentrate is carried out of the device with the spent drying gasses, passes through the economizer and collected for further treatment.
The present process has the benefit of significantly reducing the combustion risk when drying sulfide concentrates in a fluid bed or gas suspension dryer. The present process also benefits from lower equipment costs due to smaller gas handling requirements and significant power savings by using waste nitrogen.
Various embodiments of the invention are further described in the drawings in which like numerals are employed to designate like parts. Although items of equipment, such as valves, fittings, holding tanks, pipes, and the like, have been omitted so as to simplify the description, those skilled in the art will recognize that such conventional equipment can be employed as desired.
It should be understood at the outset that identical reference numbers on the various drawing sheets refer to identical elements of the invention. It should also be understood that the following description is intended to completely describe the invention and to explain the best mode of practicing the invention known to the inventors but is not intended to be limiting in interpretation of the scope of the claims. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating principles of the invention.
Any conventional process can prepare the copper concentrates used in the practice of this invention, most preferably a flotation process, and such concentrates typically contain between about 10 and 50 percent by weight copper. The concentrates contain other metals, e.g. iron, lead, bismuth, arsenic, molybdenum, one or more precious metals. etc., that are associated with the copper in the ore deposit, and these metals, as well as the copper, are present in the concentrate principally as sulfides. The concentrate is typically in particulate form, typically with an average particle size less than about 65 U.S. mesh.
Oxidative-type smelting furnaces are also of two basic designs, bath and flash, the oxygen for which is provided by oxygen plants that generate waste nitrogen gas.
The copper concentrate is fed to the smelting furnace in conventional fashion. If the furnace is a flash smelting furnace, then the concentrate is mixed with flux and optionally recycled converter slag and/or slag concentrate (all of appropriate size), and the mix is then dried and fed (e.g. blown) into the furnace with oxygen or oxygen-enriched air.
The use of nitrogen gas, plus the reduced use of ambient air from which much of the oxygen is depleted it reaction with the fuel to support combustion within hot gas generator 5, substantially reduces the oxygen content in the heated gas mixture that leaves the hot gas generator 5 via conduit 25 and are directed into fluid bed dryer 8 in which they are utilized to dry the copper concentrate. The dried concentrate product is carried out of the fluid bed device with the spent nitrogen and ambient air via conduit 26, directed through the gas solid separator 11 and the separated concentrate is collected for the smelter/converter step.
The fluid bed and gas suspension dryers suitable for use in the present invention are of the type well known in the art.
Although this invention has been described in detail by reference to the drawings, this detail is for illustration only, and it is not to be construed as a limitation upon the invention as described in the appended claims.