When carbonated to the desired extent, the white magma, consisting of basic carbonate of lead and mother liquor, is drawn through the pipe r, by the pump s, into the filter press t.
The clear liquid flows through u, into the covered mother liquor tank v, whilst the press-cakes of white lead, after sufficient washing with water, are removed and dried in a suitable manner.
The washings are run off to a weak liquor tank (not shown) for concentration for use over again. Any inert gases accompanying the carbonic acid, or the latter alone in excess, pass from the carbonator through pipe w, into the catch-box or other condensing and absorbing apparatus x, containing either dilute acetic acid or cold water, in order to retain any ammonia carried over, and furnished with perforated trays or baffle-plates.
This catch-box is, if necessary, also connected with a further condenser, so as to remove all ammonia from the displaced air, or inert gases (if impure carbonic acid has been used) of the carbonator. It is a lead lined vessel preferably. The ammonia carried from each charge thus is tested by measuring the volume of the solution from the catch-box or other condensing and absorbing apparatus, and estimating the ammonia present in the solution. This amount of ammonia in a sufficiently concentrated form is then added to the charge in the carbonator, when cold, so as to produce a completely neutral solution. The ammonia in the catch-box is either added to the ammonia stock in the ammonia department, used for dilution of strong ammonia in making fresh acetate, or strengthened up to further absorption in the catch-box or condenser, until strong enough to add to a freshly run and cooled charge in the carbonator, to replace any ammonia driven off by heat.
The sludge in the filter-press m is washed with water to remove acetate liquors and ammonia, the weak liquors being run to a separate closed vessel similar to the carbonator o, but smaller, and not shown in the drawing.
Here white lead is precipitated by carbonic acid, and the product is passed into the filter-press t, the weak liquors associated with it serving to give a preliminary washing to a freshly received charge of white lead already in the press. The weak filtrate liquors thus obtained are preferably sent to the weak liquor tank already mentioned, for subsequent concentration, instead of to the stronger liquor tank v.
The sludge-cakes from the filter-press m, connected with the digestor, are suitably treated for the recovery of lead therefrom.
The mother liquors contained in tank v from the white lead filter-press t, are directly returned to the ammonium acetate tank y, by the pump z, for use over again.
The carbonators may be operated singly as described, or two or three may be connected together so as to be worked in rotation, the partially absorbed carbonic acid from one carbonator being completely, or more or less completely, absorbed in the one, or two, with which it is connected. The last of such series of carbonators would of course be connected with the catch-box arrangement previously described, or other condensing and absorbing apparatus.
Characters.—The advantages and disadvantages in the employment of white lead have been described pretty fully by Prof. Barff in one of the Cantor series of lectures which form such an important feature in the publications of the Society of Arts.