One or two minor uses have been suggested for titanium dioxide. Small quantities are fused with bauxite, silica, and ferric oxide in the preparation of abrasives,[664] whilst a mixture with carbon is suggested as a refractory body for linings, crucibles, etc., surface heating of this forming a layer of highly resistant carbide.[665] An interesting American patent protects the use of the dioxide for the preparation of phosphorus pentoxide from bone-ash or natural calcium phosphate.[666] The pulverised mixture of the phosphate and oxide is introduced at the upper end of an inclined rotating furnace, by means of a hopper and screw feed; fuel is fed in at the lower end, and an outlet is provided for the periodic removal of the calcium titanate, etc., formed. The silica and alumina of the impure phosphate, together with the titanium dioxide introduced, displace the phosphorus pentoxide, which, being volatile, escapes continuously through a special pipe; there is left a mixture of silicate, aluminate and titanate of calcium, which may be used as a source of titanium compounds. [664] Saunders, _U. S. P._ 954766, 954777, and 954778. [665] Becket, _U. S. P._ 1038827, September, 1912. [666] Peacock, _U. S. P._ 995897, June, 1911. ~Estimation of the Element.~--Owing to the difficulties of the separation from the acidic oxides, silica, zirconia, and the pentoxides of columbium and tantalum, and from the basic oxides, alumina and the oxides of iron and tin, the estimation of titanium in a mineral or a steel is usually a difficult and tedious process. Gravimetric as well as volumetric methods are employed. In the former, the element is isolated and weighed in the form of the dioxide; in the latter, standard solutions of suitable oxidising agents are employed, advantage being taken of the ease with which the element can be transformed from the trivalent to the tetravalent condition. The mineral or steel in which the element is to be estimated is usually fused with sodium hydrogen sulphate, which forms the sulphate. If thorium, uranium or rare earths are present, treatment in the cold with hydrofluoric acid is often more suitable; the acidic oxides are taken into solution, leaving the more positive elements in the form of the insoluble fluorides. Trautmann finds that steels or ferro-titaniums of high silicon content are attacked only very slightly by fused sodium bisulphate; he recommends[667] ignition to the oxides, evaporation with hydrofluoric acid to remove silicon as the volatile tetrafluoride, and fusion of the residue with bisulphate. [667] _Zeitsch. angew. Chem._ 1911, ~24~, 877.