Metal reclamation is the process by which metals are produced from scrap. These reclaimed metals are not distinguishable from the metals produced from primary processing of an ore of the metal. However, the process is slightly different and the exposure could be different. The engineering controls are basically the same. Metal reclamation is very important to the world economy because of the depletion of raw materials and the pollution of the environment created by scrap materials.

Aluminium, copper, lead and zinc comprise 95% of the production in the secondary non-ferrous metal industry. Magnesium, mercury, nickel, precious metals, cadmium, selenium, cobalt, tin and titanium are also reclaimed. (Iron and steel are discussed in the chapter Iron and steel industry. See also the article “Copper, lead and zinc smelting and refining” in this chapter.)

Control Strategies

Emission/exposure control principles

Metal reclamation involves exposures to dust, fumes, solvents, noise, heat, acid mists and other potential hazardous materials and risks. Some process and/or material handling modifications may be feasible to eliminate or reduce the generation of emissions: minimizing handling, lowering pot temperatures, decreasing dross formation and surface generation of dust, and modifying plant layout to reduce material handling or re-entrainment of settled dust.

Exposure can be reduced in some cases if machines are selected to perform high-exposure tasks so that employees may be removed from the area. This can also reduce ergonomic hazards due to materials handling.

To prevent cross contamination of clean areas in the plant, it is desirable to isolate processes generating significant emissions. A physical barrier will contain emissions and reduce their spread. Thus, fewer people are exposed, and the number of emission sources contributing to exposure in any one area will be reduced. This simplifies exposure evaluations and makes the identification and control of major sources easier. Reclaim operations are often isolated from other plant operations.

Occasionally, it is possible to enclose or isolate a specific emission source. Because enclosures are seldom air tight, a negative draught exhaust system is often applied to the enclosure. One of the most common ways to control emissions is to provide local exhaust ventilation at the point of emission generation. Capturing emissions at their source reduces the potential for emissions to disperse into the air. It also prevents secondary employee exposure created by the re-entrainment of settled contaminants.

The capture velocity of an exhaust hood must be great enough to prevent fumes or dust from escaping the air flow into the hood. The air flow should have enough velocity to carry fume and dust particles into the hood and to overcome the disrupting effects of cross drafts and other random air movements. The velocity required to accomplish this will vary from application to application. The use of recirculation heaters or personal cooling fans which can overcome local exhaust ventilation should be restricted.

All exhaust or dilution ventilation systems also require replacement air (known also as “make-up” air systems). If the replacement air system is well designed and integrated into natural and comfort ventilation systems, more effective control of exposures can be expected. For example, replacement air outlets should be placed so clean air flows from the outlet across the employees, towards the emission source and to the exhaust. This technique is often used with supplied-air islands and places the employee between clean incoming air and the emission source.

Clean areas are intended to be controlled through direct emission controls and housekeeping. These areas exhibit low ambient contaminant levels. Employees in contaminated areas can be protected by supplied-air service cabs, islands, stand-by pulpits and control rooms, supplemented by personal respiratory protection.

The average daily exposure of workers can be reduced by providing clean areas such as breakrooms and lunchrooms that are supplied with fresh filtered air. By spending time in a relatively contaminant-free area, the employees’ time-weighted average exposure to contaminants can be reduced. Another popular application of this principle is the supplied-air island, where fresh filtered air is supplied to the breathing zone of the employee at the workstation.

Sufficient space for hoods, duct work, control rooms, maintenance activities, cleaning and equipment storage should be provided.

Wheeled-vehicles are significant sources of secondary emissions. Where wheeled-vehicle transport is used, emissions can be reduced by paving all surfaces, keeping surfaces free of accumulated dusty materials, reducing vehicle travel distances and speed, and by re-directing vehicle exhaust and cooling fan discharge. Appropriate paving material such as concrete should be selected after considering factors such as load, use and care of surface. Coatings may be applied to some surfaces to facilitate wash down of roadways.

All exhaust, dilution and make-up air ventilation systems must be properly maintained in order to effectively control air contaminants. In addition to maintaining general ventilation systems, process equipment must be maintained to eliminate spillage of material and fugitive emissions.

Work practice programme implementation

Although standards emphasize engineering controls as a means of achieving compliance, work practice controls are essential to a successful control programme. Engineering controls can be defeated by poor work habits, inadequate maintenance and poor housekeeping or personal hygiene. Employees who operate the same equipment on different shifts can have significantly different airborne exposures because of differences in these factors between shifts.

Work practice programmes, although often neglected, represent good managerial practice as well as good common sense; they are cost effective but require a responsible and cooperative attitude on the part of employees and line supervisors. The attitude of senior management toward safety and health is reflected in the attitude of line supervisors. Likewise, if supervisors do not enforce these programmes, employees attitudes may suffer. Fostering good health and safety attitudes can be accomplished through:

• a cooperative atmosphere in which employees participate in the programmes
• formal training and educational programmes
• emphasizing the plant safety and health programme. Motivating employees and obtaining their trust is necessary in order to have an effective programme.

 

Work practice programmes cannot be simply “installed”. Just as with a ventilation system, they must be maintained and continually checked to insure that they are functioning properly. These programmes are the responsibility of management and employees. Programmes should be established to teach, encourage and supervise “good” (i.e., low exposure) practices.

Personal protective equipment

Safety glasses with side shields, coveralls, safety shoes and work gloves should be routinely worn for all jobs. Those engaged in casting and melting, or in casting alloys, should wear aprons and hand protection made of leather or other suitable materials to protect against the splatter of molten metal.

In operations where engineering controls are not adequate to control dust or fume emissions, appropriate respiratory protection should be worn. If noise levels are excessive, and cannot be engineered out or noise sources cannot be isolated, hearing protection should be worn. There should also be a hearing conservation programme, including audiometric testing and training.

Processes

Aluminium

The secondary aluminium industry utilizes aluminium-bearing scrap to produce metallic aluminium and aluminium alloys. The processes used in this industry include scrap pre-treatment, remelting, alloying and casting. The raw material used by the secondary aluminium industry includes new and old scrap, sweated pig and some primary aluminium. New scrap consists of clippings, forging and other solids purchased from the aircraft industry, fabricators and other manufacturing plants. Borings and turnings are by-product of the machining of castings, rods and forging by the aircraft and automobile industry. Drosses, skimmings and slags are obtained from primary reduction plants, secondary smelting plants and foundries. Old scrap includes automobile parts, household items and airplane parts. The steps involved are as follows:

• Inspection and sorting. Purchased aluminium scrap undergoes inspection. Clean scrap requiring no pre-treatment is transported to storage or is charged directly into the smelting furnace. The aluminium that needs pre-treatment is manually sorted. Free iron, stainless steel, zinc, brass and oversized materials are removed.
• Crushing and screening. Old scrap, especially casting and sheet contaminated with iron, are inputs to this process. Sorted scrap is conveyed to a crusher or hammer mill where the material is shredded and crushed, and the iron is torn away from the aluminium. The crushed material is passed over vibrating screens to remove dirt and fines.
• Baling. Specially designed baling equipment is used to compact bulky aluminium scrap such as scrap sheet, castings and clippings.
• Shredding/classifying. Pure aluminium cable with steel reinforcement or insulation is cut with alligator-type shears, then granulated or further reduced in hammer mills to separate the iron core and plastic coating from the aluminium.
• Burning/drying. Borings and turning are pre-treated in order to remove cutting oils, greases, moisture and free iron. The scrap is crushed in a hammer mill or ring crusher, the moisture and organics are volatilized in a gas- or oil-fired rotary dryer, the dried chips are screened to remove aluminium fines, the remaining material is magnetically treated for iron removal, and the clean, dried borings are sorted in tote boxes.
• Hot-dross processing. Aluminium can be removed from the hot dross discharged from the refining furnace by batch fluxing with a salt-cryolite mixture. This process is carried out in a mechanically rotated, refractory-lined barrel. The metal is tapped periodically through a hole in its base.
• Dry milling. In the dry-milling process, cold aluminium-laden dross and other residues are processed by milling, screening and concentrating to obtain a product containing a minimum aluminium content of 60 to 70%. Ball mills, rod mills or hammer mills can be used to reduce the oxides and non-metallics to fine powders. Separation of dirt and other non-recoverables from the metal is achieved by screening, air classification and/or magnetic separation.
• Roasting. Aluminium foil backed with paper, gutta-percha or insulation is an input in this process. In the roasting process, carboneous materials associated with aluminium foils are charged and then separated from the metal product.
• Aluminium sweating. Sweating is a pyrometallurgical process which is used to recover aluminium from high-iron-content scrap. High-iron aluminium scrap, castings and dross are inputs in this process. Open-flame reverberatory furnaces with sloping hearths are generally employed. Separation is accomplished as aluminium and other low-melting constituents melt and trickle down the hearth, through a grate and into air-cooled moulds, collecting pots or holding wells. The product is termed “sweated pig”. The higher-melting materials including iron, brass and oxidation products formed during the sweating process are periodically tapped from the furnace.
• Reverberatory (chlorine) smelting-refining. Reverberatory furnaces are used to convert clean sorted scrap, sweated pigs or, in some cases, untreated scrap into specification alloys. The scrap is charged to the furnace by mechanical means. Materials are added for processing by batch or continuous feed. After the scrap is charged a flux is added to prevent contact with and subsequent oxidation of the melt by air (cover flux). Solvent fluxes are added which react with non-metallics, such as residues from burned coatings and dirt, to form insolubles which float to the surface as slag. Alloying agents are then added, depending on the specifications. Demagging is the process which reduces the magnesium content of the molten charge. When demagging with chlorine gas, chlorine is injected through carbon tubes or lances and reacts with magnesium and aluminium as it bubbles. In the skimming step impure semi-solid fluxes are skimmed off the surface of the melt.
• Reverberatory (fluorine) smelting-refining. This process is similar to the reverberatory (chlorine) smelting-refining process except that aluminium fluoride rather than chlorine is employed.

 

Table 1 lists exposure and controls for aluminium reclamation operations.

Table 1. Engineering/administrative controls for aluminium, by operation

 

Copper reclamation

The secondary copper industry utilizes copper-bearing scrap to produce metallic copper and copper based alloys. The raw materials used can be classified as new scrap produced in the fabrication of finished products or old scrap from obsolete worn out or salvaged articles. Old scrap sources include wire, plumbing fixtures, electrical equipment, automobiles and domestic appliances. Other materials with copper value include slags, drosses, foundry ashes and sweepings from smelters. The following steps are involved:

• Stripping and sorting. Scrap is sorted on the bases of its copper content and cleanliness. Clean scrap may be manually separated for charging directly to a melting and alloying furnace. Ferrous components can be separated magnetically. Insulation and lead cable coverings are stripped by hand or by specially designed equipment.
• Briquetting and crushing. Clean wire, thin plate, wire screen, borings, turnings and chips are compacted for easier handling. The equipment used includes hydraulic baling presses, hammer mills and ball mills.
• Shredding. The separation of copper wire from insulation is accomplished by reducing the size of the mixture. The shredded material is then sorted by air or hydraulic classification with magnetic separation of any ferrous materials.
• Grinding and gravity separation. This process accomplishes the same function as shredding but uses an aqueous separation medium and different input materials such as slags, drosses, skimmings, foundry ashes, sweepings and baghouse dust.
• Drying. Borings, turnings and chips containing volatile organic impurities such as cutting fluids, oils and greases are removed.
• Insulation burning. This process separates insulation and other coatings from copper wire by burning these materials in furnaces. The wire scrap is charged in batches to a primary ignition chamber or afterburner. Volatile combustion products are then passed through a secondary combustion chamber or baghouse for collection. Non-specific particulate matter is generated which may include smoke, clay and metal oxides. Gases and vapours may contain oxides of nitrogen, sulphur dioxide, chlorides, carbon monoxide, hydrocarbons and aldehydes.
• Sweating. The removal of low vapour-melting components from scrap is accomplished by heating the scrap to a controlled temperature which is just above the melting point of the metals to be sweated out. The primary metal, copper, is generally not the melted component.
• Ammonium carbonate leaching. Copper can be recovered from relatively clean scrap by leaching and dissolution in a basic ammonium carbonate solution. Cupric ions in an ammonia solution will react with metallic copper to produce cuprous ions, which can be reoxidized to the cupric state by air oxidation. After the crude solution is separated from the leach residue, the copper oxide is recovered by steam distillation.
• Steam distillation. Boiling the leached material from the carbonate leaching process precipitates the copper oxide. The copper oxide is then dried.
• Hydrothermal hydrogen reduction. Ammonium carbonate solution containing copper ions is heated under pressure in hydrogen, precipitating the copper as a powder. The copper is filtered, washed, dried and sintered under a hydrogen atmosphere. The powder is ground and screened.
• Sulphuric acid leaching. Scrap copper is dissolved in hot sulphuric acid to form a copper sulphate solution for feed to the electrowinning process. After digestion, the undissolved residue is filtered off.
• Converter smelting. Molten black copper is charged to converter, which is a pear-shaped or cylindrical steel shell lined refractory brick. Air is blown into the molten charges through nozzles called tuyères. The air oxidizes copper sulphide and other metals. A flux containing silica is added to react with the iron oxides to form an iron silicate slag. This slag is skimmed from the furnace, usually by tipping the furnace and then there is a secondary blow and skim. The copper from this process is called blister copper. The blister copper is generally further refined in a fire refining furnace.
• Fire refining. The blister copper from the converter is fire refined in a cylindrical tilting furnace, a vessel like a reverberatory furnace. The blister copper is charged to the refining vessel in an oxidizing atmosphere. The impurities are skimmed from the surface and a reducing atmosphere is created by the addition of green logs or natural gas. The resulting molten metal is then cast. If the copper is to be electrolytically refined, the refined copper will be cast as an anode.
• Electrolytic refining. The anodes from the fire refining process are placed in a tank containing sulphuric acid and a direct current. The copper from the anode is ionized and the copper ions are deposited on a pure copper starter sheet. As the anodes dissolve in the electrolyte the impurities settle to the bottom of the cell as a slime. This slime can be additionally processed to recover other metal values. The cathode copper produced is melted and cast into a variety of shapes.

 

Table 2 lists exposures and controls for copper reclamation operations.

Table 2. Engineering/administrative controls for copper, by operation

 

Lead reclamation

Raw materials purchased by secondary lead smelters may require processing prior to being charged into a smelting furnace. This section discusses the most common raw materials which are purchased by secondary lead smelters and feasible engineering controls and work practices to limit employee exposure to lead from raw materials processing operations. It should be noted that lead dust can generally be found throughout lead reclamation facilities and that any vehicular air is likely to stir up lead dust which can then be inhaled or adhere to shoes, clothing, skin and hair.

Automotive batteries

The most common raw material at a secondary lead smelter is junk automotive batteries. Approximately 50% of the weight of a junk automotive battery will be reclaimed as metallic lead in the smelting and refining process. Approximately 90% of the automotive batteries manufactured today utilize a polypropylene box or case. The polypropylene cases are reclaimed by almost all secondary lead smelters due to the high economic value of this material. Most of these processes can generate metal fumes, in particular lead and antimony.

In automotive battery breaking there is a potential for forming arsine or stibine due to the presence of arsenic or antimony used as hardening agents in grid metal and the potential for having nascent hydrogen present.

The four most common processes for breaking automotive batteries are:

1. high speed saw
2. slow speed saw
3. shear
4. whole battery crushing (Saturn crusher or shredder or hammer mill).

 

The first three of these processes involve cutting the top off of the battery, then dumping the groups, or lead-bearing material. The fourth process involves crushing the entire battery in a hammer mill and separating the components by gravity separation.

Automotive battery separation takes place after automotive batteries have been broken in order that the lead-bearing material can be separated from the case material. Removing the case may generate acid mists. The most widely used techniques for accomplishing this task are:

• The manual technique. This is used by the vast majority of secondary lead smelters and remains the most widely used technique in small to mid-sized smelters. After the battery passes through the saw or shear, an employee manually dumps the groups or lead-bearing material into a pile and places the case and top of the battery into another pile or conveyance system.
• A tumbler device. Batteries are placed into a tumbler device after the tops have been sawed/sheared off to separate the groups from the cases. Ribs inside the tumbler dump the groups as it slowly rotates. Groups fall through the slots in the tumbler while the cases are conveyed to the far end and are collected as they exit. Plastic and rubber battery cases and tops are further processed after being separated from the lead bearing material.
• A sink/float process. The sink/float process typically is combined with the hammer mill or crushing process for battery breaking. Battery pieces, both lead bearing and cases, are placed in a series of tanks filled with water. Lead bearing material sinks to the bottom of the tanks and is removed by screw conveyor or drag chain while the case material floats and is skimmed off the tank surface.

 

Industrial batteries which were used to power mobile electric equipment or for other industrial uses are purchased periodically for raw material by most secondary smelters. Many of these batteries have steel cases which require removal by cutting the case open with a cutting torch or a hand-held gas powered saw.

Other purchased lead-bearing scrap

Secondary lead smelters purchase a variety of other scrap materials as raw materials for the smelting process. These materials include battery manufacturing plant scrap, drosses from lead refining, scrap metallic lead such as linotype and cable covering, and tetraethyl lead residues. These types of materials may be charged directly into smelting furnaces or mixed with other charge materials.

Raw material handling and transport

An essential part of the secondary lead smelting process is the handling, transportation and storage of raw material. Materials are transported by fork-lifts, front-end loaders or mechanical conveyors (screw, bucket elevator or belt). The primary method of material transporting in the secondary lead industry is mobile equipment.

Some common mechanical conveyance methods which are used by secondary lead smelters include: belt conveying systems that can be used to transport furnace feed material from storage areas to the furnace charring area; screw conveyors for transporting flue dust from the baghouse to an agglomeration furnace or a storage area or bucket elevators and drag chains/lines.

Smelting

The smelting operation at a secondary lead smelter involves the reduction of lead-bearing scrap into metallic lead in a blast furnace or reverberatory.

Blast furnaces are charged with lead-bearing material, coke (fuel) limestone and iron (flux). These materials are fed into the furnace at the top of the furnace shaft or through a charge door in the side of the shaft neat the top of the furnace. Some environmental hazards associated with blast furnace operations are metal fumes and particulates (especially lead and antimony), heat, noise and carbon monoxide. A variety of charge material conveying mechanisms are used in the secondary lead industry. The skip hoist is probably the most common. Other devices in use include vibratory hoppers, belt conveyors and bucket elevators.

Blast furnace tapping operations involve removing the molten lead and slag from the furnace into moulds or ladles. Some smelters tap metal directly into a holding kettle which keeps the metal molten for refining. The remaining smelters cast the furnace metal into blocks and allow the blocks to solidify.

Blast air for the combustion process enters the blast furnace through tuyères which occasionally begin to fill with accretions and must be physically punched, usually with a steel rod, to keep them from being obstructed. The conventional method to accomplish this task is to remove the cover of the tuyères and insert the steel rod. After the accretions have been punched, the cover is replaced.

Reverberatory furnaces are charged with lead-bearing raw material by a furnace charging mechanism. Reverberatory furnaces in the secondary lead industry typically have a sprung arch or hanging arch constructed of refractory brick. Many of the contaminants and physical hazards associated with reverberatory furnaces are similar to those of blast furnaces. Such mechanisms can be a hydraulic ram, a screw conveyor or other devices similar to those described for blast furnaces.

Reverberatory furnace tapping operations are very similar to blast-furnace tapping operations.

Refining

Lead refining in secondary lead smelters is conducted in indirect fired kettles or pots. Metal from the smelting furnaces is typically melted in the kettle, then the content of trace elements is adjusted to produce the desired alloy. Common products are soft (pure) lead and various alloys of hard (antimony) lead.

Virtually all secondary lead refining operations employ manual methods for adding alloying materials to the kettles and employ manual drossing methods. Dross is swept to the rim of the kettle and removed by shovel or large spoon into a container.

Table 3 lists exposures and controls for lead reclamation operations.

Table 3. Engineering/administrative controls for lead, by operation

 

Zinc reclamation

The secondary zinc industry utilizes new clippings, skimmings and ashes, die-cast skimmings, galvanizers’ dross, flue dust and chemical residue as sources of zinc. Most of the new scrap processed is zinc- and copper-based alloys from galvanizing and die-casting pots. Included in the old scrap category are old zinc engravers’ plates, die castings, and rod and die scrap. The processes are as follows:

• Reverberatory sweating. Sweating furnaces are used to separate zinc from other metals by controlling the furnace temperature. Scrap die-cast products, such as automobile grilles and licence plate frames, and zinc skins or residues are starting materials for the process. The scrap is charged to the furnace, flux is added and the contents melted. The high-melting residue is removed and the molten zinc flows out of the furnace directly to subsequent processes, such as melting, refining or alloying, or to collecting vessels. Metal contaminants include zinc, aluminium, copper, iron, lead, cadmium, manganese and chromium. Other contaminants are fluxing agents, sulphur oxides, chlorides and fluorides.
• Rotary sweating. In this process zinc scrap, die-cast products, residues and skimmings are charged to a direct-fired furnace and melted. The melt is skimmed, and zinc metal is collected in kettles situated outside the furnace. Unmeltable material, the slag, is then removed prior to recharging. The metal from this process is sent to distillation or alloying process. Contaminants are similar to those of reverberatory sweating.
• Muffle sweating and kettle (pot) sweating. In these processes zinc scrap, die-vapour-cast products, residues and skimmings are charged to the muffle furnace, the material sweated and the sweated zinc is sent to refining or alloying processes. The residue is removed by a shaker screen which separates the dross from the slag. Contaminants are similar to those of reverberatory sweating.
• Crushing/screening. Zinc residues are pulverized or crushed to break down physical bonds between metallic zinc and contaminant fluxes. The reduced material is then separated in a screening or pneumatic classification step. Crushing can produce zinc oxide and minor amounts of heavy metals and chlorides.
• Sodium carbonate leaching. Residues are chemically treated to leach out and convert zinc to zinc oxide. The scrap is first crushed and washed. In this step, the zinc is leached out of the material. The aqueous portion is treated with sodium carbonate, causing zinc to precipitate. The precipitate is dried and calcined to yield crude zinc oxide. The zinc oxide is then reduced to zinc metal. Various zinc salt contaminants can be produced.
• Kettle (pot), crucible, reverberatory, electric induction melting. The scrap is charged to the furnace and fluxes are added. The bath is agitated to form a dross that can be skimmed from the surface. After the furnace has been skimmed the zinc metal is poured into ladles or moulds. Zinc oxide fumes, ammonia and ammonium chloride, hydrogen chloride and zinc chloride can be produced.
• Alloying. The function of this process is to produce zinc alloys from pre-treated scrap zinc metal by adding to it in a refining kettle fluxes and alloying agents either in the solidified or molten form. The contents are then mixed, the dross skimmed, and the metal is cast into various shapes. Particulates containing zinc, alloying metals, chlorides, non-specific gases and vapours, as well as heat, are potential exposures.
• Muffle distillation. The muffle distillation process is used to reclaim zinc from alloys and to manufacture pure zinc ingots. The process is semi-continuous which involves charging molten zinc from a melting pot or sweating furnace to the muffle section and vaporizing the zinc and condensing the vaporized zinc and tapping from the condenser to moulds. The residue is removed periodically from the muffle.
• Retort distillation/oxidation and muffle distillation/oxidation. The product of the retort distillation/oxidation and muffle distillation/oxidation processes is zinc oxide. The process is similar to retort distillation through the vaporization step, but, in this process, the condenser is bypassed and combustion air is added. The vapour is discharged through an orifice into an air stream. Spontaneous combustion occurs inside a refractory vapour-lined chamber. The product is carried by the combustion gases and excess air into a baghouse where the product is collected. Excess air is present to insure complete oxidation and to cool the product. Each of these distillation processes can lead to zinc oxide fume exposures, as well as other metal particulate and oxides of sulphur exposure.

 

Table 4 lists exposures and controls for zinc reclamation operations.

Table 4. Engineering/administrative controls for zinc, by operation

 

Magnesium reclamation

Old scrap is obtained from sources such as scrap automobile and aircraft parts and old and obsolete lithographic plates, as well as some sludges from primary magnesium smelters. New scrap consists of clippings, turnings, borings, skimmings, slags, drosses and defective articles from sheet mills and fabrication plants. The greatest danger in handling magnesium is that of fire. Small fragments of the metal can readily be ignited by a spark or flame.

• Hand sorting. This process is used to separate magnesium and magnesium-alloy fractions from other metals present in the scrap. The scrap is spread out manually, sorted on the basis of weight.
• Open pot melting. This process is used to separate magnesium from contaminants in the sorted scrap. Scrap is added to a crucible, heated and a flux consisting of a mixture of calcium, sodium and potassium chlorides is added. The molten magnesium is then cast into ingots.

 

Table 5 lists exposures and controls for magnesium reclamation operations.

Table 5. Engineering/administrative controls for magnesium, by operation

 

Mercury reclamation

The major sources for mercury are dental amalgams, scrap mercury batteries, sludges from electrolytic processes that use mercury as a catalyst, mercury from dismantled chlor-alkali plants and mercury-containing instruments. Mercury vapour can contaminate each of these processes.

• Crushing. The crushing process is used to release residual mercury from metal, plastic and glass containers. After the containers are crushed, the contaminated liquid mercury is sent to the filtering process.
• Filtration. Insoluble impurities such as dirt are removed by passing the mercury-vapour bearing scrap through a filter media. The filtered mercury is fed to the oxygenation process and the solids which do not pass through the filters are sent to retort distillation.
• Vacuum distillation. Vacuum distillation is employed to refine contaminated mercury when the vapour pressures of the impurities are substantially lower than that of mercury. Mercury charge is vaporized in a heating pot and the vapours are condensed using a water-cooled condenser. Purified mercury is collected and sent to the bottling operation. The residue remaining in the heating pot is sent to the retorting process to recover the trace amounts of mercury that were not recovered in the vacuum distillation process.
• Solution purification. This process removes metallic and organic contaminants by washing the raw liquid mercury with a dilute acid. The steps involved are: leaching the raw liquid mercury with dilute nitric acid to separate metallic impurities; agitating the acid-mercury with compressed air to provide good mixing; decanting to separate the mercury from the acid; washing with water to remove the residual acid; and filtering the mercury in a medium such as activated carbon or silica gel to remove the last traces of moisture. In addition to mercury vapour there can be exposure to solvents, organic chemicals and acid mists.
• Oxygenation. This process refines the filtered mercury by removing metallic impurities by oxidation with sparging air. The oxidation process involves two steps, sparging and filtering. In the sparging step, contaminated mercury is agitated with air in a closed vessel to oxidize the metallic contaminants. After sparging, the mercury is filtered in a charcoal bed to remove the solid metal oxides.
• Retorting. The retorting process is used to produce pure mercury by volatilizing the mercury found in solid mercury-bearing scrap. The steps involved in retorting are: heating the scrap with an external heat source in a closed still pot or stack of trays to vaporize the mercury; condensing the mercury vapour in water-cooled condensers; collecting the condensed mercury in a collecting vessel.

 

Table 6 lists exposures and controls for mercury reclamation operations.

Table 6. Engineering/administrative controls for mercury, by operation

 

Nickel reclamation

The principal raw materials for nickel reclamation are nickel-, copper- and aluminium-vapour based alloys, which can be found as old or new scrap. Old scrap comprises alloys that are salvaged from machinery and airplane parts, while new scrap refers to sheet scrap, turnings and solids which are by-products of the manufacture of alloy products. The following steps are involved in nickel reclamation:

• Sorting. The scrap is inspected and manually separated from the non-metallic and non-nickel materials. Sorting produces dust exposures.
• Degreasing. Nickel scrap is degreased by using trichloroethylene. The mixture is filtrated or centrifuged to separate the nickel scrap. The spent solvent solution of trichloroethylene and grease goes through a solvent recovery system. There can be solvent exposure during degreasing.
• Smelting (electric arc or rotary reverberatory) furnace. Scrap is charged to an electric arc furnace and a reducing agent added, usually lime. The charge is melted and is either cast into ingots or sent directly to a reactor for additional refining. Fumes, dust, noise and heat exposures are possible.
• Reactor refining. The molten metal is introduced into a reactor where cold-base scrap and pig nickel are added, followed by lime and silica. Alloying materials such as manganese, columbium or titanium are then added to produce the desired alloy composition. Fumes, dust, noise and heat exposures are possible.
• Ingot casting. This process involves casting the molten metal from the smelting furnace or the refining reactor into ingots. The metal is poured into moulds and allowed to cool. The ingots are removed from the moulds. Heat and metal fume exposures are possible.

 

Exposures and control measures for nickel reclamation operations are listed in table 7.

Table 7. Engineering/administrative controls for nickel, by operation

 

Precious metals reclamation

The raw materials for the precious metal industry consist of both old and new scrap. Old scrap includes electronic components from obsolete military and civilian equipment and scrap from the dental industry. New scrap is generated during the fabrication and manufacturing of precious metal products. The products are the elemental metals such as gold, silver, platinum and palladium. Precious metal processing includes the following steps:

• Hand sorting and shredding. Precious metal-bearing scrap is hand sorted and crushed and shredded in a hammer mill. Hammer mills are noisy.
• Incineration process. Sorted scrap is incinerated to remove paper, plastic and organic liquid contaminants. Organic chemicals, combustion gases and dust exposures are possible.
• Blast-furnace smelting. Treated scrap is charged to a blast furnace, along with coke, flux and recycled slag metal oxides. The charge is melted and slagged, producing black copper which contains the precious metals. The hard slag that is formed contains most of the slag impurities. Dust and noise may be present.
• Converter smelting. This process is designed to further purify the black copper by blowing air through the melt in a converter. Slag-containing metal contaminants are removed and recycled to the blast furnace. The copper bullion containing the precious metals is cast into moulds.
• Electrolytic refining. Copper bullion serves as the anode of an electrolytic cell. Pure copper thus plates out on the cathode while the precious metals fall to the bottom of the cell and are collected as slimes. The electrolyte used is copper sulphate. Acid mist exposures are possible.
• Chemical refining. The precious metal slime from the electrolytic refining process is chemically treated to recover the individual metals. Cyanide-based processes are used to recover gold and silver, which can also be recovered by dissolving them in aqua regia solution and/or nitric acid, followed by precipitation with ferrous sulphate or sodium chloride to recover the gold and silver, respectively. The platinum-group metals can be recovered by dissolving them in molten lead, which is then treated with nitric acid and leaves a residue from which the platinum-group metals can be selectively precipitated. The precious metal precipitates are then either melted or ignited in order to collect the gold and silver as grains and the platinum metals as sponge. There can be acid exposures.

 

Exposures and controls are listed, by operation, in table 8 (see also “Gold smelting and refining”).

Table 8. Engineering/administrative controls for precious metals, by operation

 

Cadmium reclamation

Old cadmium-bearing scrap includes cadmium-plated parts from junked vehicles and boats, household appliances, hardware and fasteners, cadmium batteries, cadmium contacts from switches and relays and other used cadmium alloys. New scrap is normally cadmium vapour bearing rejects and contaminated by-products from industries which handle the metals. The reclamation processes are:

• Pre-treatment. The scrap pre-treatment step involves the vapour degreasing of alloy scrap. Solvent vapours generated by heating recycled solvents are circulated through a vessel containing scrap alloys. The solvent and stripped grease are then condensed and separated with the solvent being recycled. There can be exposure to cadmium dust and solvents.
• Smelting/refining. In the smelting/refining operation, pre-treated alloy scrap or elemental cadmium scrap is processed to remove any impurities and produce cadmium alloy or elemental cadmium. Products of oil and gas combustion exposures and zinc and cadmium dust may be present.
• Retort distillation. Degreased scrap alloy is charged to a retort and heated to produce cadmium vapours which are subsequently collected in a condenser. The molten metal is then ready for casting. Cadmium dust exposures are possible.
• Melting/dezincing. Cadmium metal is charged to a melting pot and heated to the molten stage. If zinc is present in the metal, fluxes and chlorinating agents are added to remove the zinc. Among potential exposures are cadmium fumes and dust, zinc fumes and dust, zinc chloride, chlorine, hydrogen chloride and heat.
• Casting. The casting operation forms the desired product line from the purified cadmium alloy or cadmium metal produced in the previous step. Casting can produce cadmium dust and fumes and heat.

 

Exposures in cadmium reclamation processes and the necessary controls are summarized in table 9.

Table 9. Engineering/administrative controls for cadmium, by operation

 

Selenium reclamation

Raw materials for this segment are used xerographic copying cylinders and scrap generated during the manufacture of selenium rectifiers. Selenium dusts may be present throughout. Distillation and retort smelting can produce combustion gases and dust. Retort smelting is noisy. Sulphur dioxide mist and acid mist are present in refining. Metal dusts can be produced from casting operations (see table 10).

Table 10. Engineering/administrative controls for selenium, by operation

 

The reclamation processes are as follows:

• Scrap pre-treatment. This process separates selenium by mechanical processes such as the hammer mill or shot blasting.
• Retort smelting. This process purifies and concentrates pre-treated scrap in a retort distillation operation by melting the scrap and separating selenium from the impurities by distillation.
• Refining. This process achieves a purification of scrap selenium based on leaching with a suitable solvent such as aqueous sodium sulphite. Insoluble impurities are removed by filtration and the filtrate is treated to precipitate selenium.
• Distillation. This process produces a high vapour purity selenium. The selenium is melted, distilled and the selenium vapours are condensed and transferred as molten selenium to a product formation operation.
• Quenching. This process is used to produce purified selenium shot and powder. The selenium melt is used in producing a shot. The shot is then dried. The steps required to produce powder are the same, except that selenium vapour, rather than molten selenium, is the material which is quenched.
• Casting. This process is used to produce selenium ingots or other shapes from the molten selenium. These shapes are produced by pouring molten selenium into moulds of the proper size and shape and cooling and solidifying the melt.

 

Cobalt reclamation

The sources of cobalt scrap are super alloy grindings and turnings, and obsolete or worn engine parts and turbine blades. The processes of reclamation are:

• Hand sorting. Raw scrap is hand sorted to identify and separate the cobalt-base, nickel-base and non-processable components. This is a dusty operation.
• Degreasing. Sorted dirty scrap is charged to a degreasing unit where vapours of perchloroethylene are circulated. This solvent removes the grease and oil on the scrap. The solvent-oil-grease vapour mixture is then condensed and the solvent is recovered. Solvent exposures are possible.
• Blasting. Degreased scrap is blasted with grit to remove dirt, oxides and rust. Dusts can be present, depending on the grit used.
• Pickling and chemical treatment process. Scrap from the blasting operation is treated with acids to remove residual rust and oxide contaminants. Acid mists are a possible exposure.
• Vacuum melting. Cleaned scrap is charged to a vacuum furnace and melted by electric arc or induction furnace. There can be exposure to heavy metals.
• casting. Molten alloy is cast into ingots. Heat stress is possible.

 

See table 11 for a summary of exposures and controls for cobalt reclamation.

Table 11. Engineering/administrative controls for cobalt, by operation

 

Tin reclamation

The major sources of raw materials are tin-plated steel trimmings, rejects from tin-can manufacturing companies, rejected plating coils from the steel industry, tin drosses and sludges, solder drosses and sludges, used bronze and bronze rejects and metal type scrap. Tin dust and acid mists can be found in many of the processes.

• Dealuminization. In this process hot sodium hydroxide is used to leach aluminium from tin-can scrap by contacting the scrap with hot sodium hydroxide, separating the sodium aluminate solution from the scrap residue, pumping the sodium aluminate to a refining operation to recover soluble tin and recovering the dealuminized tin scrap for feed.
• Batch mixing. This process is a mechanical operation which prepares a feed suitable for charging to the smelting furnace by mixing drosses and sludges with a significant tin content.
• Chemical detinning. This process extracts the tin in scrap. A hot solution of sodium hydroxide and sodium nitrite or nitrate is added to dealuminized or raw scrap. Draining and pumping the solution to a refining/casting process are performed when the detinning reaction is complete. The detinned scrap is then washed.
• Dross smelting. This process is used to partially purify drosses and produce crude furnace metal by melting the charge, tapping the crude furnace metal and tapping the mattes and slags.
• Dust leaching and filtration. This process removes the zinc and chlorine values from flue dust by leaching with sulphuric acid to remove zinc and chlorine, filtering the resulting mixture to separate the acid and dissolved zinc and chlorine from the leached dust, drying the leached dust in a dryer and conveying the tin and lead rich dust back to the batch mixing process.
• Settling and leaf filtration. This process purifies the sodium stannate solution produced in the chemical detinning process. Impurities such as silver, mercury, copper, cadmium, some iron, cobalt and nickel are precipitated as sulphides.
• Evapocentrifugation. The sodium stannate is concentrated from the purified solution by evaporation, crystallization of sodium stannate and recovery of sodium stannate is by centrifugation.
• Electrolytic refining. This process produces cathodic-pure tin from the purified sodium stannate solution by passing the sodium stannate solution through electrolytic cells, removing the cathodes after the tin has been deposited and stripping the tin from the cathodes.
• Acidification and filtration. This process produces a hydrated tin oxide from the purified sodium stannate solution. This hydrated oxide can either be processed to produce the anhydrous oxide or smelted to produce elemental tin. The hydrated oxide is neutralized with sulphuric acid to form the hydrated tin oxide and filtered to separate the hydrate as filter cake.
• Fire refining. This process produces purified tin from the cathodic tin by melting the charge, removing the impurities as slag and dross, pouring the molten metal and casting the metallic tin.
• Smelting. This process is used to produce tin when electrolytic refining is not feasible. This is accomplished by reducing the hydrated tin oxide with a reducing agent, melting the tin metal formed, skimming the dross, pouring the molten tin and casting the molten tin.
• Calcining. This process converts the hydrated tin oxides to anhydrous stannic oxide by calcining the hydrate and removing and packaging the stannic oxides.
• Kettle refining. This process is used to purify crude furnace metal by charging a preheated kettle with it, drying the dross to remove the impurities as slag and matte, fluxing with sulphur to remove copper as matte, fluxing with aluminium to remove antimony and casting molten metal into desired shapes.

 

See table 12 for a summary of exposures and controls for tin reclamation.

Table 12. Engineering/administrative controls for tin, by operation

 

Titanium reclamation

The two primary sources of titanium scrap are the home and titanium consumers. Home scrap which is generated by the milling and manufacturing of titanium products includes trim sheets, plank sheet, cuttings, turnings and borings. Consumer scrap consists of recycled titanium products. The reclamation operations include:

• Degreasing. In this process sized scrap is treated with vapourized organic solvent (e.g., trichloroethylene). Contaminant grease and oil are stripped from the scrap by the solvent vapour. The solvent is recirculated until it can no longer has an ability to degrease. Spent solvent can then be regenerated. The scrap can also be degreased by steam and detergent.
• Pickling. The acid-pickling process removes oxide scale from the degreasing operation by leaching with a solution of hydrochloric and hydrofluoric acids. The acid treatment scrap is washed with water and dried.
• Electrorefining. Electrorefining is a titanium scrap pre-treatment process which electro-refines scrap in a fused salt.
• Smelting. Pre-treated titanium scrap and alloying agents are melted in a electric-arc vacuum furnace to form a titanium alloy. The input materials include pre-treated titanium scrap and alloying materials such as aluminium, vanadium, molybdenum, tin, zirconium, palladium, columbium and chromium.
• Casting. Molten titanium is poured into moulds. The titanium solidifies into a bar called an ingot.

 

Controls for exposures in titanium reclamation procedures are listed in table 13.

Table 13. Engineering/administrative controls for titanium, by operation

 

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