on the sides of the flask. The precipitate of hydrated chromic oxide is filtered on a small filter, washed with boiling, slightly ammoniacal water, and dried. The chromic oxide is then ignited and weighed. According to Fels, Calvert's, Britton's, and Dittmar's are the best methods for decomposing chrome iron ores. 2. Indirect assay.'-The ore is fused with potassium nitrate and sodium carbonate (in the same manner as arsenic, p. 238), and the alkaline chromates are lixiviated. The solution is saturated with acetic acid, and boiled, in order to remove the carbonic acid. It is then diluted with water (to prevent a separation of silver acetate), and a sufficient quantity of silver nitrate is added. The precipitate of silver chromate will then contain to one atom of chromium one-half atom of silver (100 Ag= 48.69 Cr= 70.92 Cr2O, 93.15 CrO3). The precipitate, together with the filter, is boiled in strongly diluted hydrochloric acid, the silver chloride formed is filtered off, smelted with lead, and cupelled, and the chromium calculated from the resulting quantity of silver. B. Volumetric assay.-1 gramme (15.43 grains) of the ore is powdered and ground as fine as possible. It is then fused with soda and saltpetre and converted into alkaline chromate (p. 244). It is then supersaturated with sulphuric acid and a weighed quantity of pure ferrous sulphate or ammonio-ferrous sulphate, when, the ferrous oxide becoming oxidized at the cost of the chromic acid, the latter is transformed into chromic oxide, and the color of the reddish-yellow solution becomes distinctly green (2CrO3+6FeSO4+6SO3 = Cr2S3O12+3Fe2S3O12); therefore, 6 equivalents of FeO correspond to 2 equivalents Cro1=3:1. The residue of ferrous oxide which has not 1 Plattner-Richter's Löthrohrprobirkunst, 1878, p. 651. been decomposed is titrated with solution of potassium permanganate, when the final reaction will be the more distinct the stronger the liquid has been acidulated with sulphuric acid, which causes the green color of the chromic oxide to become pale. XVII. MANGANESE. 70. ORES. Pyrolusite. MnO2, with 62.8 Mn and 37.2 O; braunite, Mn2O3, with 69.23 Mn and 30.77 O; hausmannite, MnO. Mn2O3, with 71.7Mn and 28.30; manganite, MnO3+ H2O, with 89.9 Mn2O3 and 10.1 H2O; varvicite, (MnO3+ H2O)+2MnO2, with 14.23 MnO, 80.79 O, and 4.98 H2O; psilomelane, (Mn.Ba.K2.Li2)O+4 MnO2, with 20 to 60 MnO2; wad, MnO.2MnO3+3H2O. 71. ASSAYS OF PYROLUSITE.' The percentage of manganese in an ore, which is of interest to the iron manufacturer, is generally determined by chemical analysis (Tamm2 has given a process of preparing manganese carbide, from pyrolusite), while the value of an ore for other technical purposes (manufacture of chlorine and of chloride of lime, the preparation of oxygen) is judged— 1. By the quantity of chlorine which the ore yields on treatment with hydrochloric acid (MnO2+4ClH=MnCl2 +2H2O+Cl2 =81.2 per cent. Cl), or, with common salt and sulphuric acid (MnO2 + 2NaCl + 2H2SO4 = MnSO + Na2SO4+2H2O+ 2C1 = 81.2 per cent.). 1 Muspratt's Chemie, iv. 1111. 2 Dingler, ccvi. 136. B. u. h. Ztg. 1873, p. 55. If an ore contains ferrous oxide, for instance in the form of spathic or magnetic iron, a part of the chlorine evolved from the hydrochloric acid is consumed in the higher oxidation of the ferrous oxide, and is therefore lost for technical use. Only so much of the chlorine as is actually obtained from pyrolusite, is of value to the buyer, and the assays which give this (for instance, Bunsen's and Gay-Lussac's) are to be preferred to those which give the total amount of chlorine without taking into consideration that, when ferrous oxide is present, the chlorine is not all available for practical purposes. 2. By the quantity of oxygen the pyrolusite yields when ignited or treated with sulphuric acid (MnO2+SO3= MnSO4+O 18.3 per cent.). 3. By the foreign admixtures according to their quality and quantity. Substances soluble in acids (for instance, calcium and iron carbonates) are especially injurious. They increase unnecessarily the cost of manufacturing chlorine, and, by evolving considerable quantities of carbonic acid, exert a disturbing influence on the preparation of chloride of lime. For this reason, the quantity allowed is sometimes limited by contract to one per cent. The quantity of acid required for the decomposition is determined by finding the quantity of pure marble dissolved by a given quantity of hydrochloric acid; then allowing the same quantity of acid to act on a known quantity of the ore; then when chlorine ceases to be evolved, introducing the marble, and when the evolution of carbonic acid has ceased, removing the marble and weighing. The quantity of acid is ascertained from the difference in the loss of marble in the two 100 parts of marble saturate 70.5 parts of dry and 205 parts of aqueous hydrochloric acid of 1.17 specific gravity; and when the acid is of 1.09 specific gravity, corresponding to 18.2 per cent. of dry acid, 7.3 milligrammes (0.10 grain) of dry acid correspond to 1 gramme (15.43 grains) of dissolved marble. cases. 4. By the decomposability and the physical condition of the ore, as they require different quantities of acid, for instance, Spanish ore more than Nassau ore. Hydrochloric acid dissolves the ferrous oxide contained in manganese ores more readily than sulphuric acid, and differences may, therefore, arise in the yield of available chlorine from an ore, according as one or the other acid has been used, since the more ferrous oxide dissolved, the more chlorine is kept back (p. 247). 5. By the constitution of the ore, which requires also different quantities of acid. 1 atom pyrolusite requires 4 atoms hydrochloric acid to yield 2 atoms of chlorine (MnO,+4CIH = MnCl ̧+Cl,+2H ̧O); while braunite requires 6 atoms of the acid (Mn,O,+6C1H=2MnCl2+ Cl2+3H ̧0). 6. By the amount of hygroscopic water, which is sometimes considerable, and must be removed by drying, for instance, upon Fresenius' disk (Fig. 2, p. 26), at 100° or 110 to 115° C. (212° or 230 to 239° F.). It is customary to represent the commercial value of manganese ores, whether the available chlorine or oxygen is to be ascertained, in terms of manganese peroxide equiv alent to the chlorine or oxygen yielded (even if the ore does not contain it, as, for instance, braunite); namely, 2 atoms of chlorine (17), or 1 atom oxygen (16)=1 atom peroxide. Frequently 60 per cent. of peroxide is taken as the standard in commerce, and from a fixed price for this, the value of the ore is determined according to the higher or lower percentage of peroxide it contains. The following Table gives the theoretical yield of oxygen, chlorine, and peroxide of the different ores: A. Gravimetric assays.-These require but simple utensils, and can be easily executed by not very experienced operators, but the results, if certain substances are present, are inaccurate, or require control assays. 1. Method of Fresenius- Will.—2 to 5 grammes (30.87 to 77.16 grains) of very finely powdered ore are weighed out and placed in the flask A (Fig. 59), made of thin glass, and capable of holding about 120 cubic centimeters (7.32 cubic inches). To this are added two and a half times the quantity of powdered neutral potassium oxalate (5 to 12.5 grammes; 77.16 to 192.90 grains), and the flask is then filled to about one-third with water. The flask A is hermetically connected by the tube c (corks saturated with wax or paraffine, or caoutchouc stoppers should be used) with the flask B, half filled with English sulphuric acid. The tube a is closed with a plug of wax, or a caoutchouc tube with a glass rod, and the whole apparatus is weighed. The air is then aspirated at b, in order to create a partial vacuum in the flask B, by which the air above the liquid in flask A will likewise be somewhat rarefied, in consequence of which the acid 1 Ann. der Chemie u. Pharm. xlix. 137. Fresenius' Ztschr. i. 48 (Röhr) ; i. 110 (Kolb); i. 81, 110 (Kolbe). Dingler, clxxxvi. 210 (Lunge). |