Magnesium drosses

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. 

Magnesium dross, wet or hot Mannitan tetranitrate Mannitol hexanitrate (dry)... 

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. 

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 . 

In the latter product, the fluxes accumulate, forming a scum on top of the molten bath, usually called dross or skim, that contains aluminum, oxides of aluminum and of elements such as magnesium and siheon, and other alloying elements present in the various alloys. The dross is removed or skimmed from the melt and processed, usually to recover the metallic aluminum occluded in the oxides. The aluminum can constitute 5 to 70 wt.% of the dross. Usually, the dross basic components are as follows ... 

The bismuth dross formed is removed from the surface of the lead on completion of the batch. Drosses normally contain between three and ten per cent Bi, and commonly around six per cent Calcium plus magnesium total around two per cent and the remainder is lead. Drosses can be upgraded by pressing or centrifuging to remove entrained lead. 

The majority of aluminum products are actually alloys, primarily with magnesium. Magnesium actually facilitates the formation of dross and will eventually cause breakaway oxidation where substantially more than 4% of the aluminum is oxidized, in some cases nearing 50%. 

One potential solution to this problem is the use of low levels of boron additives, which are known to dramatically inhibit the oxidation of aluminum alloys. This publication will review what is currently known about this borou additive effect. In particular, it will explore the chemistry of boron with magnesium since the conversion of periclase (MgO) to spinel (MgAl204> is the key step that leads to breakaway oxidation and significant dross formation. 

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