Zinc dross

The processing of bauxite aluminium ore (hydrated alumina oxides) exemplifies these processes. In the Bayer process hot concentrated sodium hydroxide dissolves alumina which is subsequently separated and crystallized as aluminium hydroxide (hydrometallurgy). This is calcined to anhydrous alumina (pyrometallurgy) before being reduced in the Hall process (electrometallurgy) wherein alumina serves as the electrode from which elemental aluminium is deposited on the cathode. 

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Zinkasche, /. zinc ash, zinc dross (oxide of zinc). 

Secondary zinc Medium-grade zinc drosses, Zinc oxide fumes... 

Raw Materials. The source of zinc can be zinc oxide from a smelter, zinc dross or sweepings, ammonium chloride slag from hot dip galvanizing, or liquid waste such as pickle liquors from galvanizing plants. Variations in the price of zinc have a large effect on the economics of zinc sulfide pigments. 

The zinc vapor and the CO gas are then oxidized to zinc oxide and carbon dioxide above the reaction bed or at the furnace exit. Various zinc-containing materials are used, e.g., zinc concentrates, metallization residues, byproduct zinc hydroxide, and above all zinc dross from casting furnaces or galvanizing. The dross must first be treated to remove chloride and lead by heating at ca. 1000 °C in rotary kilns. 

Zinc melts at a relatively low temperature (907°C) and is removed from its zinc oxide ores pyrometallurgically by being distilled and condensed, or by being roasted, leached, then electrodeposited. Zinc residues and zinc drosses can be generated from these, as well as remelting processes. 

Zinc oxide or zinc dross can be tested for lead by direct spotting of the sample with sodium rhodizonate and buffer solution. A rapid method of detecting lead in zinc oxide is especidly useful if the material is to be used in the preparation of glass, because lead often enters glass in this way. A little (0.5 --1 mg) of the sample is fumed with sulfuric acid the residue is extracted with water filtered, and the undissolved material on the paper tested with the reagent and buffer solutions. 

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. 

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). 

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. 

In the Parkes desilvering process, 1—2% zinc is added to molten lead where it reacts with any gold, silver, and copper to form intermetaUic compounds which float as cmsts or dross that is skimmed (see Lead and lead alloys). 

Ref. 66. Zinc dust, gross weight waste and scrap, gross weight dross and skimmings = zinc content. ... 

Melting Zinc oxide fume Careful skimming of dross... 

Use only zinc not lower in quality than high grade per ASTM Specification B-6. Heat zinc to a range allowing pouring at 950°F (510°C) to 975°F (524°C). Skim off any dross accumulated on the surface of the zinc bath. Pour molten zinc into the socket basket in one continuous pour if possible. Tap socket basket while pouring. 

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%. 

As mentioned above, approximately 7% of the total sulfur present in lead ore is emitted as S02. The remainder is captured by the blast furnace slag. The blast furnace slag is composed primarily of iron and silicon oxides, as well as aluminum and calcium oxides. Other metals may also be present in smaller amounts, including antimony, arsenic, beryllium, cadmium, chromium, cobalt, copper, lead, manganese, mercury, molybdenum, silver, and zinc. This blast furnace slag is either recycled back into the process or disposed of in piles on site. About 50 to 60% of the recovery furnace output is slag and residual lead, which are both returned to the blast furnace. The remainder of this dross furnace output is sold to copper smelters for recovery of the copper and other precious metals. 

Splash condenser dross residue. The treatment of steel production pollution control sludge generates a zinc-laden residue, called dross. This material, generated from a splash condenser in a high-temperature metal recovery process, is known as a splash condenser dross residue. Because this material contains 50 to 60% zinc, it is often reclaimed, reused, or processed as a valuable recyclable material. Facilities commonly handle this material as a valuable commodity by managing it in a way that is protective of human health and the environment, so U.S. EPA excluded this residue from the definition of solid waste. 

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