precipitate is small, it should be washed well and the manganese determined in it by the bismuthate method.

With ores containing as much as 2 per cent of manganese, a double precipitation of iron and aluminium hydroxides should be made.

Lime and Magnesia. Concentrate the filtrate from the manganese determination to half its volume, add 15 c.c. of ammonium oxalate solution, and NH4OH until alkaline, and heat just below the boiling point until the precipitate settles readily. If there is no immediate precipitation, concentrate the solution to about 100 c.c., before adding the NH4OH; boil for 15 minutes and filter. Wash the precipitate free from chlorides, ignite to constant weight with the blast lamp and weigh as CaO.

Cool the above filtrate; precipitate the magnesia by adding 10 c.c. of sodium phosphate solution and 10 c.c. of NH4OH, stir well and set aside for 12 hours. Filter on a 9-cm. paper; wash with the ammonia and ammonium nitrate solution; ignite at the lowest temperature necessary to burn off the paper, and weigh as Mg2P207.

If an accident happens to any of the above determinations, use the other half of the filtrate from the silica for another analysis.

PHENYLHYDRAZINE METHOD FOR ALUMINA

The alumina may be determined as follows: Dissolve several grams of the ore in HCl and filter, ignite the residue, decompose it with HF, evaporate to dryness with a drop of H2SO4, dissolve in HCl and add to the main solution. Dilute to 250 c.c., nearly neutralize, and reduce the iron with NH,HSO3. If the solution turns deep red (ferric sulfite), it is not acid enough, and a few drops of hydrochloric acid should be added, for the sulfite itself does not reduce ferric salts, at least not with rapidity. Now quickly bring to neutrality with ammonia, and then add several drops of dilute hydrochloric acid. If this last operation is done too slowly, the oxygen of the air helps to form a little ferric hydroxide, which does not always readily dissolve in the dilute acid. Finally, add from 1 to 3 c.c. of phenylhydrazine, according to the weight of the alumina to be precipitated. If too little has been used, a few drops added to the filtrate will disclose the mistake.

Stir until the precipitate has become sufficiently flaky and allow to settle. The supernatant liquid will now be plainly acid to litmus. One need not be disturbed if the precipitate has a brownish color, for it is not due to ferric hydroxide, but to the coloring matter contained in all phenylhydrazine which has not been freshly distilled. When the determinations are allowed to stand too long, the air increases this oxidation product, and a brown insoluble scum forms on the surface of the liquid and on the sides of the vessel, which is rather troublesome to the analyst. Fortunately, equilibrium appears to be established in a short time. The vessels need not stand more than an hour, at any rate. The precipitate is washed by a solution of phenylhydrazine sulfite made by adding cold saturated sulfurous acid to a little phenylhydrazine until the crystalline sulfite first formed dissolves in the excess. The solution has an acid reaction. Of this 5 to 10 c.c. are used in 100 c.c. of hot water.

Ignite the precipitate and weigh the Al2O3, TiO2, P2O5 and V2O4 if present. Determine the TiO2, P2O5 and V2O4 and obtain the Al2O3 by difference.

CHAPTER XLIV

DETERMINATION OF GOLD, SILVER AND PLATINUM IN ORES AND ALLOYS

These metals occur in ores as the free metals. Silver is found also as argentite (Ag2S), proustite (3Ag2SAs 2S3), pyrargyrite (3Ag2SSb2S3), cerargyrite (AgCl) and a number of other minerals. Gold does not form so many minerals as silver, the chief ones, beside native gold, being sylvanite (AuAgTe), calaverite (AuTe2) and other tellurides. Platinum occurs almost exclusively as the elementary metal alloyed with other of the noble metals. Its only mineral compound is the mineral sperrylite (PtAs2).

These noble metals occur usually in what is, chemically speaking, mere traces. An ore containing 1⁄2 oz. of gold per ton would be considered rich.

They are almost always determined by the methods of "fire assaying," although sometimes it is convenient to combine wet methods with dry methods.

Fire assaying consists in producing two immiscible liquids at high temperatures, one of which is molten lead and the other is a molten slag. In the molten lead the noble metals are extremely soluble, while they are not soluble in the molten slag. In the molten slag all of the oxidized minerals of the ore are soluble, while they are insoluble in the molten lead.

In order that the noble metals may dissolve in the molten lead, they must be freed from their compounds in intimate contact with metallic lead. In order that the gangue materials of the ore may be easily dissolved in the slag, they must be combined with the proper fluxes which produce compounds easily soluble in the molten slag at moderate temperatures.

The reduced metallic particles, being denser than the slag, settle to the bottom of the crucible if sufficient time is given. The lead, which has a high affinity for oxygen, is separated from the noble metals, which are not oxidizable by "cupellation," which is a process of oxidation at about 960°C.

The methods of fire assaying give extremely accurate results when due account is taken of certain losses and proper corrections made.

These losses result from a very slight solubility of the metals in the slag and from the vaporization of the metals during reduction and cupellation and absorption in the cupel. It is necessary to determine the extent of these losses and make proper correction.

There are two general methods of conducting fire assay, namely, the crucible method and scorification method. The crucible method is also carried out in two distinct ways, the excess litharge process and the excess iron processs. Litharge (PbO) is always added in the crucible assay, the reduction of the litharge to metallic lead in intimate contact with the ore being necessary to collect the noble metals and precipitate them to the bottom of the crucible, where they finally collect as a lead button which should weigh about as much as the ore used. In addition to this function, the litharge acts as an oxidizing agent to decompose the sulfides, arsenides, etc., thus liberating the noble metals, and it further acts as a flux to make an easily fusible slag. If the ore contains a large amount of sulfide minerals, such as pyrite, chalcopyrite, sphalerite, etc., the litharge is reduced to lead in excessive amount. Hence, high sulfur ores either are roasted before assaying or an excess of litharge over the amount necessary to furnish the proper sized button is not used. When the ore contains a large amount of sulfides, the sulfur is either removed by roasting, as mentioned above, previous to assaying, or the excess iron method is used. If the ore is roasted, the excess litharge process may be used. When the ore is not roasted, the sulfide minerals are decomposed by using in the crucible mixture a sufficient amount of metallic iron (usually about four 20-penny cut nails). Since the metallic iron reduces litharge when iron is used, only enough litharge can be used in the charge to supply the necessary amount of lead to collect the noble metals and make proper sized button. The decomposition of sulfides by iron may be represented by the following equations:

The decomposition of sulfides by litharge proceeds according to the following typical reaction:

FeS2+5PbO=FeO +2SO2 + 5Pb.

A gold or silver ore is practically nothing but rock carrying infinitesimal amounts of precious metals and varying amounts of the metallic minerals, like pyrite, chalcopyrite, galena, etc. The rock-forming minerals, such as quartz, feldspars, micas, limestones and numerous other silicates and carbonates, are infusible at the temperature at which fire assaying must be conducted. It is necessary, therefore, that the proper amount and kind of fluxes should be added to combine with these rock-forming minerals and produce a mixture of compounds which is

fusible at assaying temperature. The common fluxing materials are sodium carbonate, borax and litharge. Sodium carbonate melts at 852°C., borax (Na2B4O7, when dehydrated) melts at 878°C., litharge melts at 888°C. The sodium carbonate combines with silicates to form sodium silicate, sodium aluminate and other similar compounds, and litharge similarly forms lead silicate. These silicates, aluminates, titanates, phosphates, etc. are soluble in a molten mixture of sodium carbonate, borax and litharge, or in molten sodium carbonate or borax alone. Hence, the addition of sodium carbonate, borax or litharge to an ore changes the rock-forming minerals of the ore to more easily fusible compounds which, in turn, are completely soluble in a fused excess of these fluxes, provided they are present in sufficient amounts. The primary function of assaying, which is to make two immiscible liquids, in one of which (the slag) the gangue materials are held liquid and in the other (molten lead) the precious metals are dissolved, is thus accomplished.

A slag suitable for assay purposes should have the following properties:

1. It should have a comparatively low formation temperature readily attainable in assay furnaces. It should approximate a monosilicate, to sesquisilicate.

2. It should be pasty at and near its formation temperature, to hold up the particles of reduced lead until the precious metals are liberated from their mechanical or chemical bonds and are free to alloy with the lead.

3. It should be thin and fluid when heated somewhat above its melting point, so that shots of lead may settle through it readily.

4. It should have a low capacity for gold and silver, and should allow

a complete decomposition of the ore by the fluxes.

5. It should not attack the material of the crucible too violently.

6. Its specific gravity should be low, to allow a good separation of lead and slag.

7. When cold, it should separate readily from the lead and be homogeneous, thus indicating complete decomposition of the ore.

8. It should contain all the impurities of the ore and should be free from the higher oxides of the metals.

9. Except in the case of the iron-nail assay, it should be free from sulfides.

After the separation is complete, the contents of the crucible are poured into a mold and allowed to freeze, and, when cold, the slag is knocked off of the lead button. When this lead button is placed upon a "cupel," made usually of bone ash, and heated at about 960°C. in a