Dewatering process and apparatus for slurry recovered from waste water

Abstract

Process and apparatus for dewatering slurry recovered from waste water, generated in the production and treatment of metals, are provided in which the flocculated slurry is first dewatered mechanically to provide a water output which is returned upstream for further treatment, and a sludge output containing about 25-32 percent by weight (w/o) solids and the remainder free liquid. The sludge is passed through a thermal drier and, after substantially all the free water has been removed, is disposed of as desired.

What is claimed is: 
     
       1. In a process for treating water recovered from aqueous solutions used in the treatment of metals and in which at least a portion of the waste water is a slurry containing solids, rhe steps of mechanically removing water from said slurry to provide a liquid output and an output in the form of a sludge having a substantially greater concentration of solids than said slurry, bringing said sludge into heat-exchange relationship with a hot drying agent so as to substantially dry said sludge and provide a primary stream of substantially dry solids and a secondary stream of said drying agent substantially free of solids, and recovering useful heat from said secondary stream. 
     
     
       2. The process as set forth in claim 1 in which said hot drying agent is a gas. 
     
     
       3. The process as set forth in claim 2 in which solids entrained in said secondary stream are separated therefrom after heat has been recovered from said secondary stream. 
     
     
       4. The process as set forth in claim 3 in which the solids in said slurry are first flocculated, then said slurry while being mechanically dewatered is filtered under substantial pressure so that said liquid output is a filtrate which is returned for further processing as waste water, and raising the temperature of said hot drying agent whenever the temperature of said secondary stream falls below a predetermined value. 
     
     
       5. In an apparatus for drying solids in a slurry from waste water recovered from aqueous metal-treating solutions, means for mechanically removing water from said slurry and providing a liquid output and a sludge output, said sludge output having a substantially greater concentration of solids than said slurry, drying means for thermally dewatering said sludge by placing it in heat-exchange relationships with a hot drying agent and for providing a primary stream made up substantially entirely of dry solids and a secondary stream of said drying agent substantially free of solids, and means for recovering useful heat from said secondary stream. 
     
     
       6. An apparatus as set forth in claim 5, in which said drying agent is a hot gas, and means for removing solids 
     
     
       7. An apparatus as set forth in claim 6, in which said means for mechanically removing water from said slurry is a high pressure belt filter press, said liquid output being the filtrate therefrom, said drying means is a rotary kiln dryer, and said heat recovering means is a boiler. 
     
     
       8. An apparatus as set forth in claim 6, in which means are provided for increasing the temperature of said hot drying agent in response to a drop in temperature of said secondary stream below a predetermined temperature.