Harris process

De ndng. The lead from the desilverizing contains 0.5—0.6% zinc which must be removed to meet standard specifications. This may be accompHshed by vacuum distillation, or oxidation using caustic soda, ie, the Harris process. 

In either case the Pb contains numerous undesirable metal impurities, notably Cu, Ag, Au, Zn, Sn, As and Sb, some of which are clearly valuable in themselves. Copper is first removed by liquation the Pb bullion is melted and held just above its freezing point when Cu rises to the surface as an insoluble solid which is skimmed off. Tin, As and Sb are next removed by preferential oxidation in a reverberatory furnace and skimming off the oxides alternatively, the molten bullion is churned with an oxidizing flux of molten NaOH/NaN03 (Harris process). The softened Pb may still contain Ag, Au and perhaps Bi. Removal of the first two depends on their preferential solubility in Zn the mixed metals are cooled slowly from 480° to below 420° when the Zn (now containing nearly all the Ag and Au) solidifies as a crust which is skimmed off the... 

The losses of lead as litharge (PbO) can be minimized if the impurity elements could be oxidized at an oxygen potential lower than that which causes the oxidation of lead. This can be achieved if the activities of the oxides of the impurity elements in the slag are decreased, for example, by the addition of an oxide which reacts very much more strongly with the oxides of the impurities than it does with lead oxide. Sodium hydroxide is a useful reagent for this purpose and sodium nitrate can be used as the source of oxygen. The reaction involved in this process, known as the Harris process, can be formally written as... 

The removal of small quantities of arsenic from metals and ores is a commercial problem which may be mentioned at this point. The Harris process of softening lead,6 used in several refineries, is based on the principle that such impurities as arsenic, antimony and tin may easily be oxidised and in the presence of certain alkali salts can he converted into arsenates, antimonates and stannates. Certain fluxes, such as sodium nitrate, sodium hydroxide, sodium chloride or lead oxide, are added to the molten lead, the presence of an oxidising agent and an alkali salt being essential. The alkaline slag obtained is fused 7 and... 

Method of purification Desilvering (Parkes process), electrolytic refining (Betts process), pyrome-tallurgical refining (Harris process). Bismuth is removed by Betterton-Kroll process. 

Antimony, arsenic, tin, and sulfur. Two methods of reducing/removing these elements from lead are available. The choice depends on the final level required in lead product. If the elements are to be removed from the bullion, the softening process is used. Alternatively, to reduce these elements to a specified level, caustic soda and/or sodium nitrate is stirred into the kettle held at about 500°C. This latter method is commonly referred to as the Harris process. 

In early lead-refining practices, further purification firstly involved the oxidation of bullion in shallow open pans. The dross formed contained antimony and arsenic and was skimmed off until the lead was softened . Oxidation softening in a reverberatory furnace was practised in the mid 1800s and was developed into a continuous operation in the early 1900s. The allemative Harris Process, for removal of arsenic, antimony and tin by the addition of caustic soda and sodium nitrate, was introduced in 1920. 

The Harris process removes arsenic, antimony and tin by oxidation with sodium nitrate and reaction with sodium hydroxide to form sodium arsenite, sodium antimonite and sodium stannate, which form a molten salt solution in excess sodium hydroxide. Sodium chloride can be added to extend the solvent (Paschen and Winterhagen, 1968). 

Reagent use for the Harris process (kilogram of reagent per kilogram of impurity). 

Treatment of caustic slags from the Harris process... 

A dilute water leach of the residue will extract tin and arsenic and leave antimony as insoluble sodium antimonite. This residue will also contain any tellurium, indium and selenium contained in the bulUon and if these elements are present in recoverable amounts then a two-stage Harris process can be employed (see below). 

Leach solutions may be combined for recovery of arsenic and tin by calcium precipitation. A generahsed process flow sheet for the treatment of Harris process slag is shown in Figure 12.7. 

Fig 12.7 - Process flow sheet for treatment of Harris process slags. 

Arsenic and antimony are also removed in accordance with the principles of the Harris process as detailed above, and this is particularly applicable if oxygen softening has been used, where higher residual levels of arsenic and antimony remain than from application of the Harris process. 

Antimony is oxidised preferentially to lead, and hence is removed by oxidation with air or oxygen in the softening process or by sodium nitrate used with sodium hydroxide in the Harris process. The litharge slag from softening can be reduced to form an antimonial lead alloy, and the caustic slag from the Harris process can be treated to produce sodium antimonate. 

Bullion commonly contains np to two per cent arsenic. Arsenic is preferentially oxidised in the softening or Harris processes, bnt more readily than antimony. Conseqnently when softener slag is reduced to form an antimonial lead alloy, arsenic can be retained in the residual slag from where it can be extracted by leaching and precipitation as arsenic trioxide or as calcium arsenite. Alternatively it can be extracted from canstic slags from the Harris process by leaching and precipitation with lime as calcium arsenite, which can contain around 20 per cent As. 

Selenium is normally separated by volatilisation during sintering or smelting operations and predominantly reports to fumes. Any present in lead bullion can be separated in caustic slags from the Harris process, but is relatively minor and of no real consequence. 

Tellurium primarily reports to lead bullion during smelting. It follows arsenic, antimony and tin into softener slags or caustic slag formed in the Harris process. It will concentrate in the first stage Harris... 

Tin tends to follow arsenic and antimony and is removed from lead bullion in softener slags or caustic slag from the Harris process. It is the most soluble of the three metals in caustic slags and can be selectively leached with water and precipitated from the resulting solution by the addition of lime to form calcium stannate containing around 40 per cent Sn. 

---