LEAD
Occurrence. Lead is found in nature chiefly as the sulphide (PbS), called galena; to a much smaller extent it occurs as carbonate, sulphate, chromate, and in a few other forms. Practically all the lead of commerce is made from galena, two general methods of metallurgy being in use.
Metallurgy. 1. The sulphide is melted with scrap iron, when iron sulphide and metallic lead are formed; the liquid lead, being the heavier, sinks to the bottom of the vessel and can be drawn off:
PbS + Fe = Pb + FeS.
2. The sulphide is roasted in the air until a part of it has been changed into oxide and sulphate. The air is then shut off and the heating continued, the reactions indicated in the following equations taking place:
2PbO + PbS = 3Pb + SO2,
PbSO4 + PbS = 2Pb + 2SO2.
The lead so prepared usually contains small amounts of silver, arsenic, antimony, copper, and other metals. The silver is removed by Parkes's method, as described under silver, and the other metals in various ways. The lead of commerce is one of the purest commercial metals, containing as a rule only a few tenths per cent of impurities.
Properties. Lead is a heavy metal (den. = 11.33) which has a brilliant silvery luster on a freshly cut surface, but which soon tarnishes to a dull blue-gray color. It is soft, easily fused (melting at 327°), and quite malleable, but has little toughness or strength.
It is not acted upon to any great extent by the oxygen of the air under ordinary conditions, but is changed into oxide at a high temperature. With the exception of hydrochloric and sulphuric acids, most acids, even very weak ones, act upon it, forming soluble lead salts. Hot, concentrated hydrochloric and sulphuric acids also attack it to a slight extent.
Uses. Lead is employed in the manufacture of lead pipes and in large storage batteries. In the form of sheet lead it is used in lining the chambers of sulphuric acid works and in the preparation of paint pigments. Some alloys of lead, such as solder and pewter (lead and tin), shot (lead and arsenic), and soft bearing metals, are widely used. Type metal consists of lead, antimony, and sometimes tin. Compounds of lead form several important pigments.
Compounds of lead. In nearly all its compounds lead has a valence of 2, but a few corresponding to stannic compounds have a valence of 4.
Lead oxides. Lead forms a number of oxides, the most important of which are litharge, red lead or minium, and lead peroxide.
1. Litharge (PbO). This oxide forms when lead is oxidized at a rather low temperature, and is obtained as a by-product in silver refining. It is a pale yellow powder, and has a number of commercial uses. It is easily soluble in nitric acid:
PbO + 2HNO3 = Pb(NO3)2 + H2O.
2. Red lead, or minium (Pb3O4). Minium is prepared by heating lead (or litharge) to a high temperature in the air. It is a heavy powder of a beautiful red color, and is much used as a pigment.
3. Lead peroxide (PbO2). This is left as a residue when minium is heated with nitric acid:
Pb3O4 + 4HNO3 = 2Pb(NO3)2 + PbO2 + 2H2O.
It is a brown powder which easily gives up a part of its oxygen and, like manganese dioxide and barium dioxide, is a good oxidizing agent.
Soluble salts of lead. The soluble salts of lead can be made by dissolving litharge in acids. Lead acetate (Pb(C2H3O2)2·3H2O), called sugar of lead, and lead nitrate (Pb(NO3)2) are the most familiar examples. They are while crystalline solids and are poisonous in character.
Insoluble salts of lead; lead carbonate. While the normal carbonate of lead (PbCO3) is found to some extent, in nature and can be prepared in the laboratory, basic carbonates of varying composition are much more easy to obtain. One of the simplest of these has the composition 2PbCO3·Pb(OH)2. A mixture of such carbonates is called white lead. This is prepared on a large scale as a paint pigment and as a body for paints which are to be colored with other substances.
White lead. White lead is an amorphous white substance which, when mixed with oil, has great covering power, that is, it spreads out in an even waxy film, free from streaks and lumps, and covers the entire surface upon which it is spread. Its disadvantage as a pigment lies in the fact that it gradually blackens when exposed to sulphur compounds, which are often present in the air, forming black lead sulphide (PbS).
Technical preparation of white lead. Different methods are used in the preparation of white lead, but the old one known as the Dutch process is still the principal one employed. In this process, earthenware pots about ten inches high and of the shape shown in Fig. 89 are used. In the bottom A is placed a 3% solution of acetic acid (vinegar answers the purpose very well). The space above this is filled with thin, perforated, circular pieces of lead, supported by the flange B of the pot. These pots are placed close together on a bed of tan bark on the floor of a room known as the corroding room. They are covered over with boards, upon which tan bark is placed, and another row of pots is placed on this. In this way the room is filled. The white lead is formed by the fumes of the acetic acid, together with the carbon dioxide set free in the fermentation of the tan bark acting on the lead. About three months are required to complete the process.
Fig. 89
Lead sulphide (PbS). In nature this compound occurs in highly crystalline condition, the crystals having much the same luster as pure lead. It is readily prepared in the laboratory as a black precipitate, by the action of hydrosulphuric acid upon soluble lead salts:
Pb(NO3)2 + H2S = PbS + 2HNO3.
It is insoluble both in water and in dilute acids.
Other insoluble salts. Lead chromate (PbCrO4) is a yellow substance produced by the action of a soluble lead salt upon a soluble chromate, thus:
K2CrO4 + Pb(NO3)2 = PbCrO4 + 2 KNO3.
It is used as a yellow pigment. Lead sulphate (PbSO4) is a white substance sometimes found in nature and easily prepared by precipitation. Lead chloride (PbCl2) is likewise a white substance nearly insoluble in cold water, but readily soluble in boiling water.
Thorium and cerium. These elements are found in a few rare minerals, especially in the monazite sand of the Carolinas and Brazil. The oxides of these elements are used in the preparation of the Welsbach mantles for gas lights, because of the intense light given out when a mixture of the oxides is heated. These mantles contain the oxides of cerium and thorium in the ratio of about 1% of the former to 99% of the latter. Compounds of thorium, like those of radium, are found to possess radio-activity, but in a less degree.