Lead arsenate, Commercial

The use of plant extracts for insect control dates into antiquity the use of Paris green as an insecticide for control of the Colorado potato beetle in 1867 probably marks the beginning of the modern era of chemical control of injurious insects. The development of lead arsenate followed later in the nineteenth century for gypsy moth control. The commercial production of nicotine insecticides, the production of calcium arsenate at the time of the first world war, and the use of fluorine, arsenical, and cyanide compounds, as well as other inorganic chemicals for insect control, were important steps in pest control. These chemicals were applied largely by dilute high pressure sprays or dusts. 

Lead arsenate [Pb (AsO ) J is a toxic commercial insecticide and herbicide. 

Commercial lead arsenate usually consists mainly of the monohydrogen arsenate, but may also contain the normal arsenate. It is in great demand as an insecticide (see p. 301). Many methods of manufacture are described in the patent literature,14 some of the more recent being as follows (1) Metallic lead and arsenious oxide are added to a concentrated solution of arsenic acid containing nitric acid 15 lead arsenate is precipitated, the concentration of the arsenic acid remaining constant. At intervals the precipitate is removed and the arsenic acid solution again treated. (2) A solution of a soluble arsenate is treated... 

Pollution with arsenic can also be due to human, industrial activity, and most of the arsenic in the atmosphere is from the burning of fossil fuels. Arsenic has been used in fruit sprays, insecticides (lead arsenate), weedkillers (sodium or copper arsenite), rat poison, sheep dips, fly papers, wood preservative, and in glass-making. It is now also used in the semiconductor industry. Apparently, it was used as a pesticide by the Chinese centuries ago. Most of the arsenic used commercially in the world is probably contained in various pesticides. This inevitably leads to contamination of the environment. 

Arsenic is widely distributed about the earth and has a terrestrial abundance of approximately 5 g/t (4). Over 150 arsenic-bearing minerals are known (1). Table 2 fists the most common minerals. The most important commercial source of arsenic, however, is as a by-product from the treatment of copper, lead, cobalt, and gold ores. The quantity of arsenic usually associated with lead and copper ores may range from a trace to 2 —3%, whereas the gold ores found in Sweden contain 7—11% arsenic. Small quantities of elemental arsenic have been found in a number of localities. 

Lead is produced commercially from its principal ore, galena (PbS). The ore is associated with sulfides of several metals including iron, copper, zinc, silver, bismuth, arsenic, antimony and tin. The ore is crushed and ground. It then is selectively separated from gangue and other valuable minerals by one or more processes that include gravity separation and flotation. Selective... 

Arsenic. As, at wt 74.91 exists in three modifications all corresponding to the formula As4i mw 299.64. The most common form is the crystalline or a-form, known also as metallic arsenic. Its mp is 814° at 36 atm press, sublimation point 615, d 5 = 72 at RT (5.6-5.9 for commercial grade) and hardness 3.5 Mohs. Prepn props are given in Refs 1-6 toxicity, fire expln hazards of As dust are discussed in Ref 7. Arsenic is used in some alloys and for hardening lead shot... 

Preparation of Tin.—The reduction uf the ora is effected by roasting and smelting, in a manner similar to that which will he described further on, and the button so obtained is then treated in the following manner, to separate the metel from the arsenic, antimony, bismuth, zinc, lead, iron, and copper, one or more of which it may contain or ordinary commercial tin may be taken for this purpose. 

Preparation of Zinc.—To obtain perfectly pure zinc is a task of some difficulty, so much so that the commercial article is never met with in a puro state, hut rather with variable quantities of arsenic, cadmium, tin, lead, manganese, iron, cobalt, and nickel. In order to obtain perfectly pure metal, the purest commercial ziuo should be dissolved in dilute sulphuric acid, filtered from any residue, and the foreign metals thrown down by Immersing plates of zinc in the liquid till no further deposit takes place. After separating the precipitated metals, which may be tin, lead, copper, cadmium, antimony, and a trace of arsenic, the solution is treated with carbonate of soda, and the carbonate of zinc, which Falls after filtration and thorough washing, Is dried, heated to redness to expel carbonic acid, mixed with charcoal or lampblack, and introduced into a crucible as represented in Fig. 23, placed in a furnace,... 

The first commercially successfull synthetic nacreous pigments were hexagonal platelet crystals of lead salts Thiosulfate, hydrogen phosphate, hydrogen arsenate and most important nowadays, basic carbonate. Basic lead carbonate [1319-46-6] Pb(OH2) 2PbC03, Mr 775.7, is precipitated from aqueous lead acetate or lead propionate solutions with carbon dioxide ... 

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... 

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