“Burning off” the Silicons
The paper and contents, in a little crucible, are placed in a red-hot muffle furnace. The paper is such pure cellulose that it leaves no weighable ash. That which remains after burning is silicon oxide, which is a perfectly white, fine sand. This is very carefully weighed. (Ordinary sand is silicon oxide usually slightly colored with iron.)
The laboratory holds copies of the contracts upon which these materials were bought. If, upon comparison, the analysis obtained complies with the terms of the contract, an O. K. unloading slip is made out and the receiving department is given directions into what raw-material bin in the receiver building it shall be unloaded. If not fully up to the standard called for in the contract, the purchasing department is notified and the car is either rejected or accepted upon some proper terms of adjustment if it can be used without detriment to the product in which it is to be utilized.
Cars of coke, limestone, fluorspar, etc., are inspected, analyzed and treated in the same way, so that nothing is left to guess work. The compositions as determined by the laboratory serve not only as the basis for acceptance or rejection, but the analyses of accepted materials are forwarded at once to the metallurgists, who from them figure the mixtures to be used in the cupolas.
Having great stocks of analyzed raw materials in the labeled bins in the receiver building, the metallurgists who supervise the mixing and melting of the iron determine by mathematical calculation just what irons and how much of each must be taken to give molten iron of the best composition and properties for the castings.
Titrating the Sulphur
Sulphur is evolved from the drillings as a gas (hydrogen sulphide). This is absorbed in a solution of chloride of zinc. The amount of sulphur is measured by slowly running in from a burette a solution of iodine of very accurately known strength. The iodine unites with the sulphur compound as long as any of the latter remains, but the first drop added thereafter turns blue the whole solution because of the reaction of the excess of iodine with starch paste that has been added previously as an indicator. Accuracy is about .005 per cent of sulphur.
The total iron materials charged must have a definite amount of silicon, of manganese, of phosphorus and of carbon. For a 4,000–pound charge for soft cast iron, for instance, the total silicon in the materials which make up the charge must be somewhere near 118 pounds, the manganese and the phosphorus about 30 pounds each. The usual losses of these materials through oxidation are known, of course, and sufficient excess has been allowed that the desired final composition will result.
Titrating Phosphorus
A yellow precipitate containing the phosphorus is filtered out on filter paper. It is redissolved in alkali and titrated with a standard solution of nitric acid, similarly to the sulphur. The solution in the flask turns pink with the first drop added after the phosphorus has been measured.