Dewatering sludges from chemically contaminated hydrochloric acid pickling solutions

Abstract

Using a Girpomez (State All-Union Institute for the Planning of Metallurgical Plants) design, the Novolipetsk Metallurgical Combine has built a line to process dilute chemically contaminated wastes from its pickling department. The department employs hydrochloric acid for pickling. The new line also processes products from the regeneration of spent pickling solutions. In accordance with the flow diagram of the process (Fig. 1), the sludge formed in reactors 1 during neutralization of the wastes is pumped along pipes to FPAKM-25 filter presses 2 for dewatering. The filtrate obtained from this operation flows by gravity into the primary filtrate collection tanks 3. It is subsequently moved by centrifugal pumps to the clarifiers 4. The filtrate proceeds by gravity from the clarifiers to secondary collection tanks 5. From there, it is sent by centrifugal pumps to evaporators. The dewatered residue from the filter presses is sent on a belt conveyor 6 to a dock for shipment. In the filtration of sludges through one layer of TLF-5 cloth, the combine has achieved a productivity of 7-8 m 3 fi l trate/h on one filter press. Here, the content of suspended solids in the filtrate ranged up to 10 g/liter. It was established during operation of the system that the clarifiers do not perform up to standard, leaving the filtrate nearly unchanged as it proceeds to the secondary collection tanks. According to specifications, a filtrate with such a content of suspended matter cannot be processed on the evaporators. In connection with this, it was proposed that the filter presses be equipped with fil ter-cloth backings -pieces of filter cloth somewhat larger than the filter plates with loops on one end. The loops of the backing are placed over pins specially installed in the bracket of the press roller. The backing is then placed under the main filter cloth on top of a draining material, the free end of the backing being in the direction of motion of the cloth. Cloth of the "Belting-F" type is the most suitable for the backing, this cloth having the best filtering properties among the cloths used in the metallurgical industry to dewater sludges. In the operation of the filter presses with the backings, the quantity of suspended solids in the filtrate is 0.40.9 g/liter. Such a filtrate can be processed on evaporators without preliminary clarification. The use of the backings lowered filter-press productivity. This deficiency can be compensated for by reducing the quantity of sludge present through thickening. To intensify the neutralization of the wastes by milk of lime, a provision was made for heating of the suspension in a reactor. Heating of the neutralized sludge to +80~ has a significant effect on settling of the suspended matter. This process proceeds most intensively during the first 1.5 h, and the clarified layer reaches 30% of the overall height of the fluid column. When the neutralized sludge is allowed to settle at the ambient temperature without heating, settling is retarded, and only 25% of the column consists of clarified liquid after 4 h. Treatment of the sludge with different doses of polyacrylamide did not change the deposition rate, but the addition of 5 mg/li ter of active PAA significantly increased the transparency of the clarified part. Turbidity disappeared after 15 min of settling. Since the use of the cloth backings ensures the consistent production of quality filtrate suitable for processing on evaporators, there is no need for the two vertical 25 m a containers called for in the original design of the system. Three filter-presses designed to process sludge from three reactors have been installed in the dewatering facility. In accordance with the original design, the reactor lines and filter presses were installed so as to comprise three separate units. Sludge from one reactor can thus go only to one specific filter-press. This robs the system of some flexibility, since the entire line must be shut down if one component fails. Also, there is no capability of operating two filter presses from one reactor or, conversely, operating one filter press from two reactors. If three components from three different units were to malfunction, then the dewatering operation would have to be stopped even though 7090% of the remaining equipment would be in working order. The following recommendation was made to the combine: after some slight modifications, use the filtrate clarifiers as sludge thickeners. Here, the equipment is set up so that it is possible to deliver sludge to any thickener and send the sludge from any thickener t o a n y filter press.