Dross removal

When utilizing a lead-free solder in a wave process, the machine and process must be modified to ensure the alloy selected is compatible with the internal parts of a wave machine. Lead-free solders are not drop in replacements , for eutectic Sn-Pb solder. Issues such as lead contamination, flux chemistry compatibility, dross removal equipment, and dissolution of coatings on the surfaces of a wave machines internal parts into the solder alloy must be considered. 

Before this treatment, the cassiterite content of the ore is increased by removing impurities such as clay, by washing and by roasting which drives off oxides of arsenic and sulphur. The crude tin obtained is often contaminated with iron and other metals. It is, therefore, remelted on an inclined hearth the easily fusible tin melts away, leaving behind the less fusible impurities. The molten tin is finally stirred to bring it into intimate contact with air. Any remaining metal impurities are thereby oxidised to form a scum tin dross ) on the surface and this can be skimmed off Very pure tin can be obtained by zone refining. 

The lead and 2inc are separated into two molten phases by progressive cooling. Following ammonium chloride treatment to remove dross, the lead is returned to the condenser. Zinc is cast into ingots after dissolved lead is removed by cooling. 

Dressing. The impure lead bulhon, produced from any of the smelting processes, is cooled to remove dissolved copper prior to the refining operation. The operation is referred to as copper drossing, and is performed in one or two 250 t cast-iron ketdes. The process consists of skimming off the dross, stirring the lead, and reskimming. 

In the fumace/ketde batch process, a charge of drossed blast furnace buUion is treated in a reverberatory furnace or a kettie (see Fig. 12). Oxygen is supphed in the form of compressed air or as lead oxide blown into the bath through submerged pipes. The formation of lead oxide serves by mass action to assure the removal of the impurities to the desired low concentrations. The softening reactions are... 

In the Betterton-KroU process the dezinced lead is pumped to the debismuthizing kettie, in which special care is taken to remove drosses that wastefuUy consume the calcium and magnesium. The skimmed blocks from the previous debismuthizing kettie are added to the bath at 420°C and stirred for a short time to enrich the dross with the bismuth being extracted from the new charge. This enriched dross is skimmed to blocks and sent to the bismuth recovery plant. 

FoUowing the removal of the enriched dross, the required quantities of calcium, as a lead—calcium alloy and magnesium in the form of metal ingots, are added. The bath is stirred about 30 min to incorporate the reagents and hasten the reaction. The molten lead is cooled gradually to 380°C to permit the precipitate to grow and soHdify. The dross is skimmed for use with the next lot of lead to be treated. 

The lead contains residual calcium and magnesium that must be removed by chlorination or treatment with caustic and niter. The molten lead is pumped or laundered to the casting kettles in which it is again treated with caustic and niter prior to mol ding, After a final drossing, the refined lead is cast into 45-kg pigs or 1- and 2-t blocks. 

Precipita.tlon, In the simplest case, the solubihty of an impurity in the Hquid metal changes with temperature. Thus the impurity may precipitate as a sohd phase upon cooling. For instance, the removal of iron from tin and of copper from lead are achieved by precipitation. When the soHd is lighter than the Hquid, it floats as a dross on the surface of the melt where it is easily removed by scraping. The process is called dressing. 

Precipitation can also occur upon chemical reaction between the impurity and a precipitating agent to form a compound insoluble in the molten metal. The refining of cmde lead is an example of this process. Most copper is removed as a copper dross upon cooling of the molten metal, but the removal of the residual copper is achieved by adding sulfur to precipitate copper sulfide. The precious metals are separated by adding zinc to Hquid lead to form soHd intermetaHic compounds of zinc with gold and silver (Parkes process). The precious metals can then be recovered by further treatment (see Lead). 

Scrap that is unsuitable for recycling into products by the primary aluminum producers is used in the secondary aluminum industry for castings that have modest property requirements. Oxide formation and dross buildup are encountered in the secondary aluminum industry, and fluxes are employed to assist in the collection of dross and removal of inclusions and gas. Such fluxes are usually mixtures of sodium and potassium chlorides. Fumes and residues from these fluxes and treatment of dross are problems of environmental and economic importance, and efforts are made to reclaim both flux and metal values in the dross. 

In hquidation, tin is heated on the sloping hearth of a small reverberatory furnace to just above its melting point. The tin mns into a so-called poling ketde, and metals that melt sufficiently higher than tin remain in the dross. Most of the iron is removed in this manner. Lead and bismuth remain, but arsenic, antimony, and copper are partly removed as dross. 

AHoy scrap containing tin is handled by secondary smelters as part of their production of primary metals and alloys lead refineries accept solder, tin drosses, babbitt, and type metal. This type of scrap is remelted, impurities such as iron, copper, antimony, and zinc are removed, and the scrap is returned to the market as binary or ternary alloy. The dross obtained by cleaning up the scrap metal is returned to the primary refining process. 

The melting pot is heated either electrically or by gas to 427—524°C. The pot capacity is typically over 100 kg of lead alloy, and periodically the top of the molten metal must be skimmed to remove the dross. The pot fumes must be removed by adequate ventilation (forced suction). When the molten metal has reached the proper temperature and flow characteristics, it is transported by pump to the grid mold. 

This bismuth—calcium—magnesium dross also contains lead that must be removed. The dross is heated in a ketde to free any entrapped lead that melts and forms a pool under the dross. This lead is cast and returned to the bismuth separation cycle. The dross is then melted and treated with chlorine and/or lead chloride to remove the calcium and magnesium. The resulting molten metal is an alloy of bismuth and lead, high in bismuth which is then treated to produce refined bismuth metal. 

Betts Electrolytic Process. The Betts process starts with lead bullion, which may carry tin, silver, gold, bismuth, copper, antimony, arsenic, selenium, teUurium, and other impurities, but should contain at least 90% lead (6,7). If more than 0.01% tin is present, it is usually removed from the bullion first by means of a tin-drossing operation (see Tin AND TIN ALLOYS, detinning). The lead bullion is cast as plates or anodes, and numerous anodes are set in parallel in each electrolytic ceU. Between the anodes, thin sheets of pure lead are hung from conductor bars to form the cathodes. Several ceUs are connected in series. 

It has been seen that iron has an adverse effect because it forms a second phase (insoluble) material in the alloy which acts as an effective local cathode. Sequestering is the technique of adding an alloying addition that will cause an alternative intermetallic compound with iron to form. This compound might form a dross to be removed mechanically. Alternatively the new intermetallic compound could be a less effective cathode in which case removal would not be necessary. 

Both silicon and aluminium are added to zinc to control the adverse effects of iron. The former forms a ferro-silicon dross (which may be removed during casting). Aluminium forms an intermetallic compound which is less active as a cathode than FeZn,] . Similarly in aluminium and magnesium alloys, manganese is added to control the iron . Thus in aluminium alloys for example, the cathodic activity of, FeAl, is avoided by transformation of FeAlj to (Fe, Mn)Al/. This material is believed to have a corrosion potential close to that of the matrix and is, therefore, unable to produce significant cathodic activity . 

The purity of the zinc used in the galvanising bath is not critical. Grades which contain just over 1% lead are usually used indeed, lead is essential to avoid operational problems. Lead is soluble in molten zinc up to about 1%, but slab zinc containing a higher percentage of lead is helpful as the excess lead separates and prevents dross from sticking to the bottom of the bath and thus aids its removal. Aluminium is often deliberately added in very small quantities (about 0.005%) to brighten the appearance of the work... 

The dross is removed and fed into a dross furnace for recovery of the nonlead mineral values. To enhance copper recovery, dressed lead bullion is treated by adding sulfur-bearing materials, zinc, and/or aluminum, lowering the copper content to approximately 0.01%. 

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