Dewatering process and apparatus for slurry recovered from waste water

This invention relates to a process for treating waste water recovered from aqueous solutions used in the treatment of metals; and, more particularly, to such a system which is especially well suited for dewatering the slurry recovered from water clarifiers used in such processes and the sludge generated therefrom.

Waste water resulting from the manufacture and treatment of metals contains quantities of water soluble agents including salts, acids, detergents or the like. In the treatment of such waste water so as to put it into condition for return to a water supply such as a river, active agents are neutralized and undesirable solids removed. The solids are removed, usually by means of one or more clarifiers, to provide a waste water effluent returnable to a water supply and a slurry having a relatively low concentration of solids, e.g., about 1.5-2% by weight (w/o). The high free liquid content of the slurry requires further processing before the solids may be recycled, if desired, or disposed of as in a land fill.

While many systems have been proposed utilizing mechanical and/or thermal means for dewatering a wide variety of materials such as flour mill wastes, wet coal, manure or sewerage mud, none are concerned with the problems associated metals industry waste water treatment plants.

It is therefore a principal object to provide a method and apparatus for dewatering the slurry recovered from the treatment of waste water from metal-treating processes and then drying the resulting sludge to provide an end product of solids having a free water content below the maximum tolerable for land fill disposal.

In accordance with the present invention, the slurry recovered from the clarifiers of a plant for treating the waste water from metal-treating processes is first dewatered mechanically to provide a water output which is returned upstream for further treatment and a sludge output, containing about 25-32 w/o solids and the remainder free liquid. The sludge is passed through a thermal drier in heat-exchange relationship with a hot fluid. As the fluid and dried solids exit from the dryer they are separated into a primary and a secondary stream. The primary stream is made up of substantially all of the dried solids except for fines or dust which may remain entrained in the secondary stream. The secondary stream includes the major part of the fluid, and before separating out the dust and/or fines and disposing of the fluid, useful heat is extracted from it and recycled. The primary stream made up of the bulk of the dried solids is lead to a suitable storage bin where the solids are stored pending disposal thereof by recycling, in a land fill or as may be desired.

The foregoing as well as further objects and advantages of the present invention will be apparent from the following description of a preferred embodiment thereof and the accompanying drawing in which the single FIGURE shows diagrammatically a preferred slurry-sludge system in accordance with the present invention.

Referring now to the drawing, a preferred dewatering or slurry-sludge drying system 10 in accordance with the present invention comprises flocculating or coagulating the slurry from one or more waste water treatment plant clarifiers. The waste water treatment plant is one in which waste water from processing and rinsing metals are processed so that neutralized waste water can be recycled or returned to a water supply after undesired solids in the form of a slurry have been removed therefrom. The slurry obtained from the clarifiers contains such a high percentage of liquid, about 98 w/o, as to preclude ready disposal thereof. As shown, in the tank 18, an anionic polymer flocculating agent is added to the slurry from the waste water clarifiers. The amount and rate at which the agent is required may vary depending upon the variations in the slurry but is readily determined. Good results have been obtained with "HERCOFLOC" No. 1018 trademark of and a product of Hercules Inc., Wilmington, Del. Another product that may be used is CDP90088 polymer, a product of BETZ Laboratories, Inc., Trevose, Pa. From tank 18 the flocculated slurry is fed to a high pressure belt filter press 11. The belt filter press provides a sludge output to feed hopper 12, containing about 25-32 w/o solids and a filtrate output which as indicated in the drawing is returned upstream for further processing.

From hopper 12 the sludge is carried by a conveyor 13 to rotary dryer 14, preferably a rotary kiln dryer, connected by conduit 15 to a source of high temperature, relatively dry gas. As shown, conduit 15 is connected through a one way damper 15A to a source of waste heat such as is available from annealing or soaking ovens or the like utilized in the metals industry. To insure a continuous supply of hot gas or air at the required temperature, a branch of conduit 15 is also connected through one way damper 15B to an auxillary heat source which may be a gas fired heater. The temperature at which dryer 14 is operated, for best results, is not less than the stoichiometric balance point which depends, as is well known, upon such parameters as the quantity and rate at which the material is fed through and the amount of water required to be carried off as vapor. Tumbling of the sludge by the rotation of the dryer 14 brings the sludge and hot gas into intimate heat exchange and drying contact with the result that most of the remaining water, preferably 90 w/o or more, is removed from the sludge. The thus dried sludge makes up a primary output stream 14A from the dryer conveyed along pneumatic conveyor 16. The hot gas exits from the dryer 14 as a secondary stream 14B in which fines or dust from the dried sludge may be entrained and is lead into a heat recovery boiler 21. In boiler 21, heat from the secondary stream 14B is extracted as steam which is fed from boiler 21 by pipe 22 to provide a source of heat for any suitable heat utilization device or system. After passing through boiler 21 where heat is extracted from it, the relatively cool secondary stream passes through impeller or blower 23 to a separator such as bag house 24 where the dust and fines are filtered out and collected as in containers 25 while the gas is permitted to escape to the atmosphere as indicated at 24A or otherwise disposed of as may be desired or required. A normally closed damper 28 is opened for venting heat when blower 23 is shut down.

Pneumatic conveyor 16 receives the dried solids from the dryer 14 through a rotary lock 16A. Rotary lock 16A is preferrably positioned sufficiently distant from the dryer to ensure that the temperature of the dried solids has dropped to 250° F. or better yet 150° F. when the solids leave the rotary lock 16A. Conveyor 16 delivers the primarY stream of dried solids to a silo 17 or other place of storage pending recycling or disposal, as may be desired. When the dryness of the sludge is such that handling or transporting the sludge to a remote site, as in trucks, results in objectionable loss of fines or dust to the atmosphere, a suitable dust suppressant may be added, if desired, as the dried solids pass through silo hopper 20.

To insure minimum consumption of fuel by heater 27, the rate at which fuel is consumed is modulated in response to the temperature of the gas in the dryer 14. To that end, a thermocouple 29 or other heat responsive device is mounted near the exit opening of the dryer and is connected by lead 30 to a controller 31 which in turn is activated to increase or decrease the fuel flow rate as the temperature falls below or rises above the set point by a selected amount.

The process and apparatus of the present invention is highly efficient and cost effective in treating the slurry resulting from clarification of the waste water up to about 5000 pounds of sludge per hour containing about 32% by weight solids and 68% by weight water are readily converted to dry solids containing about 98% to 100% by weight solids. To that end the temperature of the hot gas delivered by pipe 15 to the dryer 14 is maintained high enough so that the temperature of the secondary stream after leaving the dryer and before entering the heat recovery boiler 21 is about 1200° F., the recovery boiler thereby being maintained at about 500° F.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.