•president's address — SECTION B. 103 



The formation of substantial quantities of sulphuric anhydride has 

 been assumed on the analogy of the blister furnace. The point does 

 not appear to have been investigated, but the assumption is open to 

 doubt. The formation of the anhydride is only likely at fairly low 

 temperatures. It decomposes at the very heat of its own formation 

 when it is the direct result of the oxidation of sulphur, so that, except 

 possibly towards the end of the copper finish, its existence is jeopardised. 



Nature and Importance of Rde Played hy Silica. — The action of 

 silica, supplied by the lining or other\vise, is an exceedingly important 

 one. It has already been stated that it must be in the free state to 

 act at all. No pneumatic process involving the maintenance of the 

 solid products in a state of fusion is possible if it requires the removal 

 of essential quantities of iron, as will no doubt always be the case, unless 

 free silica is present to regulate the oxidation and to act as a balance- 

 wheel in a combined physical and chemical sense. In the simultaneous 

 presence of free silica and free air or oxygen, under the general con- 

 ditions of pneumatic smelting, the iron will only burn to protoxide, and 

 will at once unite with the silica to a silicate of a definite composition, 

 forming a slag which, in the presence of matte, will readily seek a 

 supernatant position, and so facilitate the mechanical separation of 

 the two. The determining influence of silica is not often referred to, 

 but its action follows fixed laws which one cannot over-ride and which 

 narrowly circumscribe the pneumatic processes. What we call silica 

 is in reality simply the anhvdride of an exceedingly strong acid, which 

 becomes active only in an igneous condition, but then seeks its base 

 the same as any other mineral acid. As far as its chemical relations 

 to iron are concerned under the conditions existing in the converter, 

 that element (iron) behaves exactly the same as it does at ordinary 

 temperatures in the presence of ordinary free acids. Hydrogen sul- 

 phide will not precipitate iron in the presence of hydrochloric acid, 

 for instance, and, similarly, iron will not form sulphides at high tem- 

 peratures in the presence of free silica. Conformably, therefore, sulphur 

 already combined with iron can be driven out and burnt in an oxidising 

 atmosphere when uncombined silica is present, the iron and silica 

 uniting. One of the most striking features of the converting operation 

 is that this union follows set lines, controlled by the heat generated 

 within the converter and by the formation-heats of definite silicates. 

 Only that particular silicate of iron will form which can form at the 

 temperature evolved, and no other. Against popular expectation, the 

 formation of a silicate approaching a bi-silicate is indicative of a lower 

 temperature than the production of a singulo-silicate. For the sake 

 of the best utilisation of the lining, or, in other words, its consumption 

 to the smallest possible extent, a basic slag should be looked forward 

 to. Fortunately, the thermal energy of copper-converting is vigorous 

 enough to permit of the generation of the singulo-silicate of iron. Its 

 formation temperature has been synthetically determined as about 

 1,270° C, a figure which agrees with direct measurement of the tempera- 

 ture in the converter, as also with calculation of same from the analyses. 

 The presence of scorifiable bases other than iron modifies the resultant 



