The Metallurgy of Lead and the Desilverization of Base Bullion ... Fourth edition

Front matter

THE METALLURGY OF LEAD AND THE Desilverization of Base Bullion BY H. O. HOFMAN. E.M., Ph.D. ASSOCIATE PROFESSOR Or MINING >ND METALLURGY IN THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY ; MEMBER OF THE AMERICAN INSTITUTE OF MINING ENGINEERS, etc., etc. FOUKTH EDITION. THE SCIENTIFIC PUBLISHING CO. NEW YORK and LONDON. 1895. Copyrighted, 1892, by The Scientific Publishing Company. Ferris Bros., Printers, 326 Pearl Street New Vort ERRATA. Page x. line 3, omit 192. Page x, line 23, for 303, 304, 306, 307, read 302, 303, 304, 305 306, 307. Page 23, line 6, for decreast *>. read increases. Page 43, line 39, for -Novemoer 23 and December 14 1890. read February 11, 1888. Page 139, line 6, for 2 KeOa, Si02, read 2 FeO, Si02. Page 158, line 41, for May 24, read March 24. Page 164, line 21, for In figure 79 is given the plan of two furnaces forming part of a series of six, read In figure 79 are given the plans of aroastinghouse containing six long-hearth slag-roasting furnaces.

Page 210, line 11, for 1 '4 read 5"26. Page 214, line 31, for 149 ounces, read 249 ounces. Page 214, line 38,/or 19"7 pounds, read 18 7 pounds. Page 214, line 39, for 34 "5 pounds, read 32 7 pounds. Page 214, line 39,/or 54*2 pounds, read 1*4 pounds. Page 214, line 40, for 72 pounds, read 70 pounds. Page 215, line 11. for 727 lbs. matte, read 69*9 lbs. matte. Page 216, line 30,/or Keo. read FeO. Patre 260, line 17,/or on , read matte. Page 276, line 13,/or ferrocyanide, read ferricyanide. Page 304, line 4, for 44,100, read 4,410. Page 367, line 35,/or May 27, 1887, read May 21, 1887. TO HIS WIFE This Book is Dedicated bt THE AUTHOR PEEFACE. In 1870 Dr. Percy published his great work, "The Metallurgy of Lead, Including Desilverization and Cupellation," which has become the standard book for England and America, and is also highly esteemed by the Germans and the French, into whose lan guages it has been translated.

It contains an exhaustive discus sion of smelting and desilverizing as carried on in the principal European works. During the past twenty years, however, so much that is new has been introduced in American practice that a book embodying the latest improvements seems almost a necessity for the student. It is the aim of the present work to supply this need. Thus, while European practice is not at all excluded from the book, the main object has been to make it a guide for Americans, and European methods have been discussed more in connection with their applicability in this country than from any other point of view. In the subject of the blast-furnace, for in stance, only such features have been brought out as seemed useful for America. Reverberatory-furnace practice, on the other hand, which has not made much progress as yet in this country, is given more in the European way, while the ore-hearth treatment follows both English and American methods, which supplement one an other.

Preface

TO THE SECOND EDITION. As the first edition of this book only appeared in December, it is impossible to do more for the second, which has been called for in an unexpectedly short time, than to add a list of errata. The author will be grateful to readers who will call his attention to any still nncorrected errors, whether of type or otherwise, or who will favor him with suggestions through which his work can be improved in each new edition. A treatise on any technical branch of science must necessarily represent to a great extent the personal experience and conclusions of the writer. Thus he is very liable to emphasize difficulties or advantages familiar to himself in a given mode of practice at the expense of others with which he has not happened to be brought in contact. It is only by compar ing the experiences and views of others with his own that any one can hope to produce in the end a book of general and lasting value. H. 0. H. Massachusetts Institute of Technology, January, 1893. TABLE OF CONTENTS. PAGK Preface iii List of Illustrations ix

Part I

— INTRODUCTORY. Chapter I.— Historical and Statistical Notice. § 1. Introductory Remarks. — § 2. Lead in the United States. — § 3. The World's Production 1 1-13 Chapter II.— Properties of Lead and of Some of its Compounds. § 4. Lead.— § 5. Lead Oxide. — § 6. Lead Silicates.— § 7. Lead Sulphide.— § 8. Lead Sulphate. — § 9. Roasting- Lead Sulphide ; Reactions between Lead Sulphide, Lead Sulphate, and Lead Oxide.— § 10. Lead Carbonate.— § 11. Lead of Commerce 14-24 Chapter III.— Lead Ores. § 12. Introductory Remarks.— § 13. Galenite.— § 14. Anglesite, Cerussite.— § 15. Other Lead Minerals 25-29 Chapter IV.— Distribution of Lead Ores. § 16. Lead Ores of the United States: I. Lead Ores of the Atlantic Coast. § 17. Lead Ores of the Atlantic Coast.— II. Lead Ores of the Mississippi Valley. § 18. The Upper Mississippi Valley.— § 19.

The Mines of Missouri.— § 20. Other Occurrences.— III. Silver-Lead Ores of the Rocky Mountains. § 21. Colorado.— § 22. South Dakota— § 23. Montana.— § 24. New Mexico.— IV. Silver- Lead Ores of the Pacific. §25. Nevada.— §26. Utah.— § 27. Idaho. — § 28. Arizona and California 30-14 Chapter V.— Receiving, Sampling, Assaying and Purchasing of Ores, Fluxes and Fuels. §29. Receiving and Weighing of Ores.— § 30. The Moisture-Sample.— § 31. The Assay-Sample. — § 32. General Arrangement of Sampling Department.— § 33 Receiving and Sampling of Fluxes VI TABLE OF CONTENTS. P_OE and Fuels.— § 34. The Assaying of Lead-Silver Ores.— § 35. Purchasing of Lead-Silver Ores. — § 36. Purchasing of Non- Argentiferous Lead Ores.— § 37. Purchasing of Fluxes and Fuels 45-76

Part II

— THE METALLURGICAL TREATMENT OF LEAD ORES. § 38. Classification of Methods . . 78 Chapter VI — Smelting in the Reverberatory Furnace. § 39. Introductory Remarks. — § 40. Influence of Foreign Matter. — §41. Classification of Reverberatory Methods. — §42. TheCarinthian Method— § 43. The English Method.— §44. The Silesian Method. — § 45. Comparison of Reverberatory Methods 79-109 Chapter VII.— Smelting in the Ore-Hearth. §46. Introductory Remarks. — §47. Influence of Foreign Matter. — §48. Description of Ore-Hearths. — §49. Mode of Working in the Ore-Hearth. — § 50. Treatment of Slags. — §51. Recovery of Flue-Dust by the Lewis and Bartlett Bag Process 110-131 Chapter VIII. — Smelting in the Blast-Furnace. §52. Introductory Remarks. — §53. Lead Slags. — §54. Action ot Fluxes and Influence of Foreign Matter. — § 55.

Fuels Used in the Blast-Furnace.— §56. The Roasting of Ores.— §57. The Selection of a Furnace Site. — § 58. General Arrangement of a Smelting Plant. — § 59. The Blast-Furnace and its Accessory Apparatus. — §60. Chemistry of the Blast-Furnace. — §61. Calculation of Charge. General Smelting Operations. § 62. Blowing-In.— § 63. Regular Work on the Charging Floor. — §64. Regular Work on the Furnace Floor. — §65. Work on the Dump. — §66. Irregularities in the Blast-Furnace. — §67. Blowing-Out. — §68. Furnace Books. — §69. Furnace Assay Book. Furnace Products. § 70. Furnace Products.— § 71. Base Bullion.— § 72. Sampling and Assaying.— § 73. Speise.— §74. Analytical Determinations.— §75. Matte.— §76. Roasting of Matte.— § 77. Smelting of Roasted Matte.— §78. Nickel and Cobalt Matte.— § 79. Analvtical Determinations.— § 80.

Slag— §81. Analytical Determinations. —§82. Wall-Accretions.— § 83. Hearth Accretions or Sows.— §84. Furnace Cleanings and Furnace Refuse.— §85. Flue-Dust or Chamber-Dust.— § 86. Treatment of Flue-Dust.— *< 87. Analytical Determinations. — §88. Losses in Smelting.— §89. Cost of Smelting 132-294 TABLE OF CONTENTS. VII PAGE

Part III

-DESILVERIZATION OF BASE BULLION. §90. Introductory. 296 Chapter IX.— Pattinson'r Process. § 91. Introductory Remarks. — § 92. Description of Plant and Mode of Conducting the Process. — § 93. Luce and Rozan's Process (Steam-Pattinson Process.) 298-309 Chapter X.— Parkes' Process. §94. Introductory Remarks. — §95. Outline of Plant and Process. — §96. Receiving Base Bullion. — §97. Softening Base Bullion. — §98. Furnaces.— §99. Mode of C inducting the Process.— § 100. Desilverizing Softened Bullion. — § 101. Desilverizing Kettles. — §102. Liquating Apparatus. — § 103. Mode of Conducting the Desilverization. — §104. Mode of Liquating. — §105. Refining Desilverized Lead. — § 106. Refining in the Reverberatory Furnace. — §107. Refining m the Kettle.— §108. Moulding from the Refining Furnace. — § 109.

Moulding from the Merchant Kettle. — §110. Labor, Fuel, Output of Lead.— §111. Treatment of Zinc- Crusts.— § 112. Flach's Process.— § 113. Distillation of Zinc- Crusts.— §114. Furnaces. — §115. Condensers.— §116. The Method of Working.— §117. Tools.— §118. Results.— §119. Roesing's Suggested Improvements. — §120. Comparison of the Two Processes. — §121. Treatment of By-Products. —§122. Softening Furnace Dross and Skimmings.— § 123. Tin Skimmings. — §124. Kettle Dross.— § 125. Refining Skimmings and Polings. — §126. Rich Lead and Metallic Zinc. — §127. Retort Dross and Blue Powder. — § 128. Litharge.— § 129. Old Retorts, Cupel Bottoms, etc.— § 130. Table of Desilverization.— § 131. General Remarks. — § 132. Relative Advantages of Parkes' and Pattinson's Processes 310-368 Chapter XL— Cupellation. §133.

Introductory Remarks 369 A. German Cupellation. §134. Characteristics.— § 135. The Furnace.— § 136. Plattner's Cupelling Furnace. — § 137. Mode of Conducting the Process. B. English Cupellation. § 138. Characteristics.— § 139. The Furnace.— § 140. Test-Rings.— §141. Test-Ring Supports.— § 142. TheBlast.— §143. The Tools.— § 144. Mode of Conducting the Process.— § 145. Comparison of Methods 369-102 LIST OF ILLUSTRATIONS. FIGURES PAGE 1-6. Quartering 49 7. Brunton's Quartering-Shovel 50 8. Pipe Ore-Sampler 53 9-10. Brunton's Simpler 54, 55 11. Sampling-Mill with Brunton's Sampler 56 12-14. Bridgman's Sampler 57, 59, 61 15-16. Sampling-Mill with Bridgman's Sampler 63 17-18. Bridgman's Laboratory Sampler 65 19-20. Bridgman's Mixer and Divider 66 21-24. Reverberatory Furnace at Raibl, Carinthia 85 25-26.

Air-Furnace 90, 91 27-32. Reverberatory Furnace at Stiperstones, England, 92, 93, 94, 95, 96, 97 33-46. Tools Used with the Reverberatory Furnace of Stiperstones . . 98 47-50. Reverberatory Furnace at Tarnowitz, Silesia 101, 102, 103, 104 51-53. Scotch Ore-Hearth at Howden, England Ill 54-55. American Water-Back Ore-Hearth 113, 114 56-59. Moffet Ore-Hearth 115 60-64. Tools Used with the Moffet Ore-Hearth 116 65- 67. Slag- Eye Furnace of the Lone-Elm Works 121, 123, 124 68-70. Cooling-Pipes of the Lewis and Bartlett Bag-Process . .125, 126, 129 71. Bag-House of the Lewis and Bartlett Bag-Process 131 72. Crystalline Forms of Lead-Slags 137 73-78. Roasting Furnace with Fuse-Box 163 79. Plan of Roaster-Building 164 80-81. Omaha and Grant S. and R. Co., Denver, Col., Plan and Section of Works 170, 171 82-83.

Montana Smelting Co., Great Falls, Mont., Plan and Section of Works , 173,175 84-88. Omaha and Grant S. and R. Co., Denver, Col., Blast-Furnace 177 89-95. Globe S. and R. Co., Denver, Col., Blast-Furnace 178, 179, 180 96-98. Colorado Iron Co., Denver, Col., Blast-Furnace LIST OF ILLUSTRATIONS. X FIGURES PAGE 99-120. Montana Smelting Co., Great Falls, Mont., Blast-Furnace, 183, 186, 192, 197 121-124. Murray's Tuyere-Box 198 125-128. Fraser and Chalmers' Ordinary Slag-Pot 199, 200 129-131. Fraser and Chalmers' Slag-Pot with Roller Bearings 200 132-133. Terhune's Sectional Slag-Pot 201 134-139. Murray's Slag-Pot , 203 140-144. Nesmith's Dumping-Car 204 145. Distribution of Charge on Feed-Floor 228 146-149. Wall- Accretions in the Blast-Furnace 237 150-152. Bars of Base Bullion 245 153. Davies' Converter for Desilverizing Speise 251 154-157.

Pribram Roasting-Stalls 257, 258 158-163. Montana Smelting Co., Matte-Roasting Reverberatory Furnace 262 164. Suspended Sheet-Iron Flue 283 165-166. Sheet-Iron Flues with Triangular Projections 284 167-168. Water-Cooled Flue of Sheet-Lead 285 169-170. Flue with Freudenberg Plates 287 171-177. Montana Smelting Co , Flue and Dust-Chamber ... 288, 289 178-179. Montana Smelting Co., Blast-Furnace Stack 290 180-195. Pribram, Luce-Rozan Desilverizing Plant 303, 304, 306, 307 196-197. Parkes' Desilverizing Plant 315 198-204. Softening Furnace with Water-jackets 321 205-234. Desilverizing Kettles with Brick-Work and Castings 330 235. Steitz Lead Siphon 343 236. Lead-Moulding Apparatus for Refining Furnace 343 237-239. Lead-Mould for Refined Lead 344 240-243. Lead-Moulding Apparatus for Merchant-Kettle 343 244-248.

Faber du Faur's Retorting Furnace 349, 350, 351 249-251. Tatham's Retorting Furnace 352 252-255. Pribram, German Cupelling Furnace 369 256-260. English Cupelling Furnace 381 261-269. Test-Rings with Fixed Test 383, 384 270-272. Fraser and Chalmers' Test-Carriage 386 273-274. Lynch's Test-Support 387 275. (261, 262). Test-Carriage with Swinging Test 383,

Chapter I

HISTORICAL1 AND STATISTICAL2 NOTICE. § 1. Introductory Remarks. — Lead was probably known at a very early date. The oldest people of whom we have any record — the Egyptians — used it in glossing their pottery, and the abun dance of silver among the ancients suggests the presence of argen tiferous lead ores in many places. We know that lead was mined in considerable quantities by the Greeks* and Romans. The mines of Laurion, in Attica, opened up again by a French company in 1863, flourished especially in the fifth century B.C. The Romans extracted large amounts of ore from the mines in the southeastern part of Spain, which had been opened in the third century B.C. by Hannibal and which form the main lead district of Spain to-day. They also carried on mining operations in England and along the Rhine from Bale to Cologne.

About the year 1000 a.d. the cele brated German silver-lead mines of the Hartz Mountains, Saxony and Silesia, and those of Austria were discovered. § 2. Lead in the United States. — At the present day the lead mines of the United States occupy the first place. In this country lead mining dates as far back as the beginning of the seventeenth century, when lead was mined and smelted near Falling Creek, 1 Pulsifer, W. H., "Notes for a History of Lead," New York, 1888. 2 C. Kirchhoff, in "United States Geological Survey: Mineral Re sources of the United States, 1882," and following, Washington, 1883. METALLURGY OF LEAD. Va. During colonial times lead mines were operated mainly in North Carolina, New York, and the New England States, but on a small scale and not very successfully. The mines principally mentioned are the Washington mine, Davidson County, N. C.;the Rossie mine, St. Lawrence County, N. Y.; and the mines near Middletown, Conn., and Southampton, Mass.

None of them were worked continuously, and to-day the East produces hardly any lead ore. The lead ores of Missouri were discovered by Le Sueur in 1700 or 1701, and were first worked in 1720; of these the Mine La Motte, of Madison County, which is worked to-day, was the first discov ered. Rumors of lead in the upper Mississipi Valley were afloat as early as 1766, and in 1788 Dubuque obtained from the Indians a grant for a lead mine on the place where the city named after him now stands. From these two districts the bulk of the lead of the United States came until the great mines of the West were opened up in 1867. The total product of the United States in 1825 was only 1,500 net tons. This increased steadily until 1848, when 28,000 tons were produced. After falling off considerably for a little over twenty years, the production of the Mississippi Valley increased again, and reached 31,351 tons in 1890.

As the total product of the United States for the same year was 161,754 tons, this district produces 19.36 per cent. #of the entire output. All the rest came from the Western States and Territories, which produce argentiferous lead ores, while those of the Mississippi Valley are practi cally free from silver. Argentiferous lead was first produced, according to O. H. Hahn, in 1866 or 1867, near Helena, Mont., and at Oreana, Nev. In 1870 the mines of Eureka, Nev., discovered in 1864, were reopened and the treatment of ores was begun. Next came Utah, where smelters were erected in 1870, followed by Colorado, which came into prominence in 1878. Later Idaho, New Mexico, Arizona, and California were added to the list. Colorado is to-day the great est producer. Then follow Idaho and Utah, furnishing togeth er about half as much as Colorado; the three together produce about 60 per cent, of all the argentiferous lead of the United States. § 3.

The World's Production.— The world's production in 1884 ' was as follows: 1 " Mineral Resources of the United States," 1885, p. 264. HISTORICAL AND STATISTICAL NOTICE. 13 Countries. Metric Tons. Spain 116,293 Germany 102,584 England 40,716 France 7,500* Italy 14,000* Greece 8,000* Belgium 8,000* Austria 11,391 \ Hungary 1,800* Russia 500* Sweden 400* United States 126,907 Mexico 1,000* Turkey 600* Australia 5,000* South America 2,000* Total 446,691 * Estimated. t Including litharge, estimated at 80 per cent. lead. The price of common pig-lead in New York averaged for the years 1886 to 1890 inclusive per pound avoirdupois 4.32 cents. The two other principal lead markets are St. Louis and Chicago, where the lead is cheaper than in New York by the cost of trans portation.

Chapter II

PROPERTIES1 OF LEAD AND OF SOME OF ITS COMPOUNDS. § 4. Lead. — Lead has a bluish-gray color; on a freshly-cut sur face it shows a considerable lustre, but loses it quickly when ex posed to atmospheric air. It does not crystallize readily. When it is cooled slowly, as in the Pattinson process (§ 91), bundles of small, imperfect octahedrons form. Also, when refined lead is poured at the correct temperature into a warm mould and is al lowed to cool, fern-like crystalline aggregates appear at the sur face. It is the heaviest of all base metals. Reich's figure, 11.37, as specific gravity for pure lead at 0° C. (water at 4° C. being unity), is the one generally accepted. The specific gravity of lead will vary slightly, according as it is cooled quickly or slowly, ham mered or rolled. Commercial lead has a lower specific gravity than 11.37 on account of the impurities contained in it.

Lead is very soft, especially when allowed to cool and solidify slowly. It is harder if cooled quickly and if it contains slight admixtures of other metals, such as copper, arsenic, antimony, zinc, etc. The grade of commercial lead is often approximately determined by the resistance it offers to scratches with the finger-nail and the facility with which it makes a gray streak on paper. Lead is very malleable ; it is rolled into sheets and hammered into foil. In the form of filings it becomes a solid mass if subjected to a pressure of thirteen tons to the square inch, and liquefies at two and one-half times this pressure (Roberts-Austin 2). Lead is not sufficiently ductile to be drawn into fine wire ; its tenacity, according to Karmarsch, is inferior to that of most ductile 1 Condensed for the most part from Percy, " Metallurgy of Lead " London, 1870, pp. 8-93. 2 Engineering and Mining Journal, November 3, 1888 ; Scientific American Supplement, numbers 675, 676, 677. metals.

It fuses at 325° C. (Le Chatelier l), boils at between 1450 and 1600° C. (Carnelly and Williams2), but cannot be distilled. The latent heat of lead is 5.369 ; the coefficient of cubical dilation for 1° C, 0.000089 ; the linear coefficient about one-third of the cubical. The specific heat between 10° and 100° C. is 0.0314 ; with silver as 100, the conductivity for heat at 12° C. is 8.5, and for electricity 10.7. Lead undergoes no change in perfectly dry air nor in water that is free from air ; its surface becomes, however, dull by oxidation when it is exposed to the atmospheric air on account of the moisture which it contains. Similarly, it is oxidized by water that is not free from air. If melted in contact with air it oxidizes and becomes covered with an iridescent pellicle, said to be the suboxide, Pb20 ; this gradually changes to the oxide, PbO, and if the heat ing be prolonged sufficiently, the red oxide, Pb304, is obtained.

The other two oxides which lead forms are the sesquioxide, Pb203, and the peroxide, Pb02. The best solvent of lead is dilute nitric acid. Dilute hydrochloric aud sulphuric acids have little or no action ; boiling concen trated hydrochloric acid and sulphuric acid of 66° B. dissolve it slowly. Organic acids — acetic, tartaric, citric acids — attack it in contact with air. § 5. Lead Oxide, Pb O. — Of the different oxides enumerated, the oxide is the one that is metallurgically interesting and of im portance. It is obtained on a large scale as massicot and litharge, which have different physical properties. Massicot, an amorphous yellow powder, is formed by heating lead on a flat hearth to a low red heat, removing the film of suboxide as fast as it forms, and oxidizing it to yellow oxide. If the temperature be raised to the melting-point — that is, to a bright red heat — and the fused oxide cooled, it solidifies as crystalline litharge.

On a large scale litharge is obtained by cupelling argentiferous lead. It crystallizes in orthorhombic octahedrons, and is soft and greasy to the touch. While molten it is transparent and orange-colored ; when cold it is opaque, and its color varies from yellow to red according to the rate at which it has cooled ; quick cooling promotes the yellow, slow cooling the red color. Yellow litharge is produced on a large scale by allowing it to run from the furnace over an iron plate and 1 Engineering and Mining Journal, October 11, 1890. 8 Journal of the Chemical Society, xxxv., p. 563. METALLURGY OF LEAD. chilling it with water, if necessary ; it is thus obtained in small lumps. The red, flaky variety is formed by allowing the running litharge to collect in front of the furnace in cakes of from one to one and one-half tons in weight, and to cool slowly.

The inner part of a cake will swell up and form flakes of red litharge ; the outer and lower parts, having cooled quickly, will remain solid and have a yellow color. This swelling is caused by the giving off of oxygen, which molten litharge absorbs. In solidifying quickly in small lumps the oxygen only makes the surface uneven ; in cooling slowly in large lumps the outer solid crust obstructs the passage of the oxygen. This prevents the inner part from solidifying firmly, and causes instead the formation of loose flakes. The flakes and lumps are separated by sifting. Both varieties, when ground, have a reddish-yellow color. Litharge melts at 954° C. (Honsell a) ; it is a good conductor of electricity when molten. It is volatilized at a white heat. It is only slightly soluble in water (1 part in 12,000 parts), but readily so in nitric acid and acetic acid.

Litharge is a strong base and quickly corrodes acid furnace material, with which it forms a silicate. It is an excellent flux, forming fusible compounds with oxides that are infusible alone. They do not always enter into chemical combination with it, but often are simply held in igneous solution by an excess of litharge. Thus fusible mixtures are formed with lime, baryta, magnesia, and alumina. The following table shows the proportion of litharge required to form fusible compounds with the principal metallic oxides: Litharge, being easily reduced to the metallic state, forms an important oxidizing agent. This is seen by its behavior with S, Te, As, Sb, Sn, Bi, Cu, Zn, Fe. They become wholly or partly oxidized, and the oxides either volatilized or scorified by the sur plus of litharge, a corresponding amount of lead, which combines with any unoxidized part, having been reduced. § 6.

Lead Silicates. — Lead oxide and silica begin to combine at a temperature where the oxide becomes soft. In fact, it is dis- 1 Bergu. Hiittenmannische Zeitung, 1866, p. 106. 1 part of Cu^O CuO ZnO Fe30. Fe.O- MnO SnO. St__3 Sb02 As203 As20B Requires parts of 1.5 1.8 4 10 10 13-13 fusible in all proportions. 5 0.4-0 8 0.25-1 advantageous to raise the temperature quickly if a silicate is to be formed. This can be seen in slag-roasting a galena ore to which fine sand has been added. If the time at which the roasted ore is pasty be shortened and the fusion urged, uncombined silica will be found with combined when the roast is decomposed in nitro hydrochloric acid. All silicates that do not contain more silica than is required to form the tri-silicate, Pb2Si308 (2Pb0.3Si02), are fusible at a low temperature, forming a transparent vitreous mass ; the singulo-silicate, Pb2Si04 (2PbO.Si02), is as fluid as water.

If the proportion of silica be raised above that of the tri silicate, the compound becomes less fusible; thus 2Pb09.Si02 gives a porcelain-like mixture, and Pb0.18Si02 fritts only to a porous mass. All fusible lead silicates are yellow ; they become darker in proportion to the quantity of lead they contain. They change their color, if they are contaminated with other metallic oxides, as can be seen if lead is slagged in a scorifier ; e.g., iron colors brown ; copper, green ; manganese, purple-black ; nickel, brownish-yellow ; cobalt, blue ; tellurium, yellowish-red, the colors growing dark in proportion to the oxide added. The lead from silicates is not readily liberated by the ordinary reducing agents. Sulphur decomposes the singulo-silicate to some extent, but it has less effect on the bi-silicate ; iron sulphide throws down some lead, a double silicate of lead and iron being the result ; carbon reduces from a bi-silicate part of the lead.

In order to extract all the lead, it must be first set free from its com bination with silica by a basic flux ; thus metallic iron decomposes all fusible lead silicates at a bright-red heat, provided enough is added to form a singulo-silicate. The singulo-silicate and bi-silicate of lead are readily decom posed by nitric acid, the tri-silicate is not completely decomposed ; the more acid the silicate, the less soluble it is. § 7. Lead Sulphide, PbS. — This occurs native as galenite. It is formed by heating lead and sulphur, or lead oxide with an excess of sulphur, or by reducing lead sulphate with carbonaceous matter. The artificial sulphide has the same properties as the mineral. The existence of subsulphides of lead (Pb2S,Pb4S) is denied by Percy, who shows that lead sulphide and lead can be melted together in all proportions, and that the properties of the resulting compound will resemble galena or lead according to the predomi nance of one or the other compound.

Also, if such an apparent subsulphide be heated carefully, comparatively pure lead will eliquate and a residue of hard crystalline sulphide remain behind. Galena is not as fusible as lead, but it is very fluid when melted, and penetrates the fire-brick of the furnaces in which it is treated; often a net-work of small veins of bright crystalline galena is found in furnace-linings. When melted, galena begins to volati lize without being decomposed, if free oxygen be excluded. On the walls of lead blast-furnaces crystals of sublimed galena are of common occurrence. Lead sulphide is isomorphous with metallic sulphides, as Ag2S, Cu2S, ZnS, FeS. Such mixtures of sulphides are found in lead matte and copper matte obtained in smelting sulphide ores. With the electro-negative sulphides of antimony and arsenic it forms sulpho-salts.

Quite a number of these occur as minerals -,1 others can be artificially prepared in the dry and wet ways. Galena as well as matte is a good conductor of electricity.2 Iron decomposes galena better than any other metal. For instance, copper that has a greater affinity for sulphur than iron decomposes lead sulphide only partially, as it alloys too readily with the liber ated lead, and the cuprous sulphide formed combines with the remaining lead sulphide to matte. Zinc decomposes galena partly, but the zinc sulphide formed is so refractory that a separation of the liberated lead is not effected; the result is a black, porous mass containing particles of lead and galena. The reaction that takes place between iron and galena is generally expressed thus: PbS-|-Fe=FeS-f-Pb, and forms the basis of what is called the precipitation or iron reduction process.

In reality, however, the iron sulphide retains some undecomposed lead sulphide, and Xolte's 3 formula, 3PbS+ 3Fe= 2Pb+ (PbS-)-Fe2S+FeS), might be truer to the actual facts, although he presupposes the existence of an iron subsulphide. In decomposing galena in furnace-work by means of iron, enough has to be present so as to have lFe for lPbS. If less is added, the resulting matte remains too rich in lead; if an excess is given, it is wasted. It may in fact be a disadvantage in decomposing argentiferous galena, as on account of the affin ity the silver sulphide has for iron sulphide, more silver will o-o into the matte than can be accounted for by the amount of lead pres- 1 See Dana, " System of Mineralogy," New York, 1880, p. 84. 2 Kiliani, Bergu. Hiittenmannische Zeitung, 1883, pp. 237, 366, 378. 3 Berg-und Hiittenmannische Zeitung, 1860, p. 165. ent.

In addition to having a correct amount of iron present to decompose galena, the temperature is of great importance; the higher it is, within reasonable limits, the better will be the decomposition. A basic ferrous silicate (4FeO.Si02) will decompose galena readily; the singulo-silicate (2FeO.Si02) shows little effect. In practice, the amount of iron that is in excess of that required to flux the silica will be available for the decomposition of the lead sulphide. Lime as well as baryta has a decomposing action on galena. If air has access, the following reaction takes place (Rivot1): 4PbS+4CaO = 3CaS+CaSO.-i-4Pb. If the air be excluded and carbon present, the following occurs (Berthier2): 2PbS+CaO+C=Pb+(PbS.CaS)-(-CO. § 8. Lead Sulphate, PbSO^— This occurs as anglesite. It is formed in roasting lead sulphide (§ 9).

Of all metallic sulphates it is the only one that is not decomposed upon ignition at a bright-red heat; it softens at a white heat and loses some of its sulphuric acid, forming a basic salt. Silica readily decomposes the sulphate, forming a lead silicate, while the sulphuric acid is driven off, being split into sulphurous acid and oxygen. In this way lead sulphate, obtained in roasting a siliceous galena, is decomposed. The operation goes by the name of slag-roasting. Lead sulphate is a poor conductor of electricity (Kiliani 3). Carbon, if present in sufficient quantity, reduces the sulphate completely to sulphide at a dark-red heat : PbS04+2C=PbS-f2CO!!. If there is not enough carbon present, only part of the sulphate will be reduced : 2PbS04-(-C=PbS04-|-PbS-|-2C02, and 4PbS04+2C=3PbS04+PbS+2COi, At a cherry-red heat the resulting sulphate and sulphide will react upon each other, as shown in the next paragraph.