In grinding phosphorite and superphosphates, corrosive dust is produced. All grinding operations must, therefore, be carried out automatically in closed apparatus (ball mills, disintegrators, &c.). In making the phosphorite soluble by treatment with sulphuric acid, and subsequent drying of the product, corrosive hydrofluoric acid gas is developed, which requires to be carried away by an acid proof exhaust fan, and condensed in a tower by water (see fig. 58). The modern revolving drying machines are especially serviceable. [Illustration: FIG. 58.—Washing tower for hydrofluoric acid (_after Leymann_.)] In the production of _basic slag_ corrosive dust is given off, causing ulceration of the mucous membrane. Grinding and other manipulations creating dust must be carried on in apparatus under local exhaust ventilation. The following—somewhat shortened—are the German Imperial Regulations, dated July 3, 1909, for basic slag factories. BASIC SLAG REGULATIONS 1. Workrooms in which basic slag is crushed, ground, or stored (if not in closed sacks) shall be roomy and so arranged as to ensure adequate change of air. Floors shall be of impervious material allowing of easy removal of dust. 2. Preliminary breaking of the slag by hand shall not be done in the grinding rooms, but either in the open air or in open sheds. 3. Slag crushers, grinding mills, and other apparatus shall be so arranged as to prevent escape of dust as far as possible into the workrooms. They shall be provided with exhaust ventilation and means for collecting the dust if this cannot be done in the absence of dust. 4. Arrangements shall be made whereby barrows conveying material to the grinding mills shall be emptied directly into partially hooded hoppers provided with exhaust ventilation so as to prevent escape of dust into the workrooms. 5. The casing and joints of the grinding mills, ducts, dust collectors and sieves shall be airtight; if leaks are noticed they must be repaired forthwith. 6. Ducts, dust collectors and sieves shall be so arranged as to enable periodical cleansing to be undertaken from the outside. 7. Repairs of the plant mentioned in Para. 5 in which workers are exposed to inhalation of slag dust shall be entrusted by the occupier only to such workers as wear respirators supplied for the purpose or other means of protecting mouth and nostrils such as wet sponges, handkerchiefs, &c. 8. Emptying of slag powder from the grinding mills and dust collectors and transference to the store rooms shall only be done in accordance with special regulations designed to minimise dust. 9. Filling slag powder into sacks from the outlets of the mills, elevating and discharging it into receptacles shall only be done under efficient exhaust ventilation.