Gold, silver, and platinum group metals are extensively utilized in digital and electrical elements to offer long-term reliability, Development of army gear consumes the most important proportion of the valuable metals used in the digital and electrical business. Resulting from obsolescence and injury, army electronics are presently being scrapped on the fee of about 15,000 tons per yr. This scrap averages approximately 100 troy ounces of silver, 5 troy ounces of gold, 1 troy ounce of palladium, and lesser amounts of different valuable metals per ton.
The high degree of improvement and use of army digital hardware that quickly becomes out of date assures an expanding provide of scrap electronic elements from army sources.
Because of its highly variable and sophisticated nature, army digital scrap is virtually inconceivable to sample and analyze for metallic content material. This reality coupled with the inflexibility of presently used scrap-processing strategies, limits disposal to a relatively small half of the scrap that’s now generated and has been accrued by army reclamation operations. The Bureau of Mines is at present investigating various strategies for efficient, low-cost restoration of the valuable and base metals in numerous mixtures of army digital scrap.
Leaching methods have been tried initially for processing the electronic scrap. Nitric acid dissolved copper and silver but left a residue from which the gold could not be separated. Cyanide leaching proved ineffective for dissolving heavy silver and gold plate on copper. The massive quantities of soluble copper and aluminum in typical scrap made leaching difficult and dear.
After preliminary studies revealed that straightforward leaching methods were not applicable, the investigation was shifted to devising- a smelting technique capable of recovering the valuable metals and copper. The next is a quick account of the progress made towards solving the issue. Some of the remaining, unsolved problems are additionally discussed.
Nature of Military Electronic Scrap
The character and composition of army scrap is very variable. While scrap from out of date B-47 bombers includes wired-vacuum tube-type elements with relatively low valuable metallic content, the electronic elements of later gear are composed of printed and miniaturized circuits of more complicated nature and excessive valuable metallic content material. In the extra trendy digital elements, gold and platinum group metals have replaced part of the silver used in older sort electronics. With miniaturization, use of plastics has elevated, and the amount of aluminum and iron encasing the elements has decreased considerably.
Valuable metallic assays of some typical electronic elements are proven in table 1.
Table 2 exhibits a comparison between the composition of B-47 bomber electronic elements and the composition of a 400-pound lot of extra just lately generated scrap.
Typical army digital scrap may be economically processed together with copper sulfide focus, in a standard copper smelter to recuperate the copper and valuable metals which represent the principal values. Sadly, this easy answer to the issue isn’t practical, as a result of such scrap can’t be sampled and assayed to define its composition, and therefore to find out the smelter fees and funds. The one apparent various is to plan methods for batchwise electric furnace smelting of small tonnages of complicated scrap.
Military electronic scrap incorporates comparatively giant quantities of iron and aluminum which require oxidation and slagging to supply high-grade metallic bullion containing the copper and the valuable metals. Because slagging a large half of the furnace feed was sure to create difficulties, the feasibility of rejecting clean iron and aluminum previous to smelting was examined. Magnetic, hand-sorting, and sink-float methods have been tried on samples of digital scrap shredded in hammer mills. None of these strategies was efficient for significantly decreasing the iron and aluminum content of the scrap without high loss of valuable metals and copper.
Smelting High Aluminum Electronic Scrap
A number of small and subsequently one larger scale smelting check have been made on samples of typical excessive aluminum and iron content scrap. In the bigger check, 458 kilos of scrap have been smelted at 1,500° C in an electric arc furnace. A small amount of silica was added simply previous to j pouring the melt and the dross layer skimmed out of the furnace. The results are summarized in desk three.
As proven by the info in desk 3, the extraordinarily viscous dross which assayed 23.0 % aluminum and 20.four % iron retained 59 % of the copper and over 50 % of the valuable metals, mainly as small particles of metallic. The metallic bullion produced was contaminated with considerable iron and aluminum that was not oxidized. Though an simply handled clean dross and top quality bullion were not produced, the 2 merchandise might be precisely sampled and presumably marketed to a customized copper smelter.
Smelting with additions of adequate silica to flux and slag all the aluminum and iron additionally was unsuccessful. The slag shaped was extraordinarily viscous even at 1,500° C, and its relatively giant volume retained a big part of the copper and valuable metallic values.
Smelting Low Aluminum Scrap
Low aluminum scrap that incorporates 33 % iron was efficiently smelted’utilizing the identical circumstances as tried on excessive aluminum scrap. The results are summarized in desk four.
The relatively small quantity of dross was straightforward to skim from the furnace. Nevertheless, this product still contained an excessive amount of copper and valuable metals to be discarded, and the metallic bullion produced is contaminated with considerable iron. Both products may be sampled, accurately assayed, and bought to a custom main copper smelter.
Oxidation Smelting and Electrorefining of Low-Aluminum Scrap
One other attainable choice for processing digital scrap is to smelt suitable low-aluminum scrap to supply a copper bullion low in iron for subsequent electrorefining to recuperate high-purity copper and a saleable valuable metallic wealthy sludge.
In follow, the preliminary reduction smelting of scrap could be followed instantly by oxidation smelting and slagging of the iron and aluminum to supply a high quality copper bullion. Nevertheless, within the laboratory these operations have been finished separately to enable evaluating the benefit of finishing only the preliminary discount smelting.
The crude bullion produced from low aluminum scrap and described in table four was used for the oxidation smelting check. After remelting the bullion in a gas-fired pot furnace, the temperature was raised to about 1,500° C, and air was blown via the charge for five hours. Approximately 0.3 pound of silica per pound of bullion was added to the soften in the course of the blowing operation. Slag was skimmed off, the melt was poled, and the metallic was poured into cast iron molds. Over 95 % of the copper and valuable metals have been recovered in the bullion, and over 99.5 % of the iron and aluminum have been slagged off. Table 5 exhibits the assay of the purified bullion.
Aluminum and iron rejection was relatively good nevertheless, the iron focus in the copper is considerably larger, and the nickel and valuable metallic content material are much greater than in normal blister copper. The concentrations of other metals are low. If bought to a standard copper smelter, such bullion can be so diluted with regular blister copper as to impress no issues in anode refining or electrorefining.
The copper bullion made by oxidation smelting is considerably greater in nickel, iron, gold, and silver than the anode copper produced by regular copper smelter apply. Some problem was anticipated in getting ready copper meeting the specifications for electrolytic grade. To examine this drawback, electrolytic exams have been made simulating normal copper refinery apply. The copper sulfate electrolyte contained 45 grams copper, 200 grams free sulfuric acid, four milligrams-glue, and 40 milligrams sodium chloride per liter. Electrolyte temperature was 50° C and the current density was 15 amp per sq. foot of cathode area. Desk 6 exhibits assays of the anode used and the cathode and anode slimes produced in a typical check.
The impurities in the cathode copper are approximately 10 occasions higher than usually encountered in industrial follow. Because of the excessive nickel content in the anode copper, an auxiliary nickel restoration unit can be required. This operation is a normal auxiliary process in lots of refineries and shouldn’t present any critical problems. A second electrolysis to further refine the copper and recuperate the remaining valuable metals may be advisable. Research is constant to aim to lower the impurity content material in the first cathode.
No experimental work has been completed on the remedy of the anode slimes for the restoration of valuable metals. The slimes are just like these produced:in commonplace electrolytic copper refinery operations and.should respond to regular remedy procedures for the restoration of the varied valuable metals.
Plans For Future Work
The research aim to plan methods sufficiently versatile for environment friendly processing of every type of complicated, high-value army electronic scrap has not, as but, been achieved. The principal unsolved drawback is find out how to selectively take away and preferably get well the comparatively large amount of aluminum current in a lot of the scrap. Smelting aluminum-rich scrap is troublesome and dear; moreover, smelting loses the aluminum in worthless dross and slag. Incineration of plastics, both prior to or concurrently with smelting, creates a air pollution drawback and in addition might symbolize a attainable loss of valuable metals by volatilization. Studies are needed to guage numerous methods for controlling pollution, as well as recovering values which can be carried in the fumes.
A number of strategies are being studied and evaluated for recovering aluminum from scrap prior to smelting. These embrace (1) dismantling the scrap to remove the aluminum packing containers and shields that encase the digital elements, (2) caustic leaching to selectively dissolve the aluminum, (3) sweating aluminum from the electronic scrap, and (4) separating aluminum from shredded and incinerated scrap, using molten salt to float the lower density aluminum from the other metals.
Because complicated army electronic scrap cannot be precisely sampled and assayed, such scrap just isn’t marketable for simple and economic processing by copper smelters. Attempts to separate and recuperate the precious valuable and base metals by leach methods weren’t profitable. Smelting checks indicated that low aluminum scrap could be processed to supply slag and crude bullion saleable to a copper smelter. Alternatively, the copper bullion could be electrorefined to recuperate the majority of the copper and valuable metals. Excessive aluminum scrap could be smelted only with problem to supply a comparatively large amount of dross and a crude bullion. These merchandise may be sampled, assayed, and presumably additional processed at a customized processing copper smelter. Magnetic, sink-float, and hand-sorting methods have been unsuccessful in’ rejecting aluminum’and iron from shredded scrap to organize an enriched, easily smelted product.