1. Field of the Invention
This invention relates to continuous thickeners, clarifiers and similar gravititational settling devices for separating feed slurries or pulps into clarified liquid and sludge and is particularly concerned with a method and device for controlling the operation or design of such settling devices.
2. Description of the Prior Art
Continuous thickeners, clarifiers and similar gravitational settling devices are widely used in the chemical and metallurgical industries for the removal of liquids from slurries, metallurgical pulps, sewage and other liquid-solid suspensions. Such devices generally include a circular tank having a cylindrical center feedwell which extends downwardly into the vessel and is open at the bottom. The incoming slurry or pulp passes through a feed pipe or launder into the upper part of this central feedwell and is introduced into the surrounding liquid through the bottom of the feedwell in a manner designed to create a minimum of turbulence. This makes it possible to contain the bulk of the solids near the center of the unit. On leaving the feedwell, the liquid entering with the pulp or slurry tends to move outwardly in a radial direction and flow upwardly toward a peripheral overflow launder. The solids suspended in the slurry or pulp settle downwardly through the slow-moving liquid and accumulate on the bottom of the unit. These solids are compacted as they accumulate and are slowly moved toward a bottom sludge discharge opening by means of slowly rotating rakes suspended a short distance above the bottom. The rakes aid in compacting the sludge and reduce its liquid content.
During the normal operation of a thickener, decanter, clarifier or similar continuous gravity settling device of the type referred to above, a series of relatively well-defined, vertically-spaced zones exist within the settler. The uppermost of these zones comprises a layer of clear liquid from which most of the solids have settled out. Below this is an intermediate layer containing suspended solid particles which is generally referred to as the settling zone. The interface between the clear solution and the settling zone may be referred to as the upper boundary or slime level. At the bottom of the unit is a layer of settled sludge. Such a system is a dynamic one characterized by the movement of liquid and solid particles between the above zones. The levels of the three zones may vary considerably, depending upon the feed stream, operating conditions and other variables. To achieve maximum capacity with such a settling unit, it has generally been thought that the upper boundary should be maintained as close to the top of the unit as possible and that only a relatively thin layer of clarified solution be maintained above the floc layer.
It is conventional to add flocculants or coagulants to thickeners, decanters, clarifiers and similar settling devices to increase their capacity. These materials cause the suspended particles in the slurry or pulp to flocculate or agglomerate and thus settle more rapidly. The amount of flocculant or the like which is required at any particular moment depends in part upon the slurry or pulp feed rate, the solids content of the feed, the solids size range and distribution, the densities of the solid particles, and the temperature and other operating conditions. Under constant conditions, the amount of flocculant needed to achieve maximum capacity in a particular gravity settling unit is generally determined by trial and error. However, in actual practice the conditions may change due to variations in the amount and compositions of the solid suspended in the feed stream and other variables over which the operator of the unit may have relatively little or no control. Adjustments of the amount of flocculant added to the system is therefore necessary to compensate for the variations and maintain the desired capacity and degree of separation while at the same time keeping operating costs within acceptable bounds by eliminating overflocculation, and its related problems in downstream operations such as in the final polishing filtration.
It has been common practice to use the upper boundary of the settling zone within a settler as a measure of the settler's performance and to monitor this level as a means for determining the need for changes in the flocculant rate. In general, the higher the upper boundary, the more flocculant that is needed. This location of the upper boundary has generally been done manually by means of measuring sticks lowered into the vessel near the outer edge of the unit. Vacuum tubes, depth samplers, ultrasonic probes may also be used. The upper boundary is, however, not a direct measure of the settling characteristics of solids in the pulp or slurry and instead is the result of the combination of variables, including flocculant type and flow rate, solids feed rate, solids and liquid characteristics, mixing etc. There is normally a long time lag between the changes in the rate of addition of flocculant and corresponding changes in the upper boundary and hence the operator must estimate the amount of change in the rate of addition of flocculant which will be needed to produce a desired change in upper boundary. If he overestimates or underestimates the change in rate required, the unit may become unstable and eventually have to be shut down to avoid overloading or the carryover of solids. The upper boundary therefore provides at best a visible means for assessing the state of the thickener or clarifier operation and, if it increases progressively, it may serve as an delayed warning that the capacity of the settler has been exceeded.
Attempts have also been made to control the operation of a settler by sampling the incoming feed slurry to the feedwell at regular intervals downstream of the point at which flocculant is added to the feed slurry. The samples thus collected are passed to a laboratory sized gravity separation vessel where representative settling can take place. By sensing the interface level between the liquid and solid phases in the separation vessel and adjusting the rate of flocculant addition to the feed stream in accordance with variations in the level of the interface during operation of the system, it was hoped that the rate of addition of flocculant could be controlled automatically and that the flocculant consumption could be thereby substantially reduced. However, attempts to develop such a system in the past were abandoned because none was capable of providing reliable data necessary for the control and operation of a full size commercial settler.
One example of sampling equipment for measuring sedimentation rate is that described in Parker et al U.S. Pat. No. 4,318,296, issued Mar. 9, 1982. This system includes a sampling chamber for a sample to be tested, Elmer means for controlling a control means to stop the feed of suspension to the sampling chamber and means for retaining the height of the sample at a preselected level in the sampling chamber during a settling period. It also includes detector means for detecting when a boundary level defined by the settling solids in the sample in the sampling chamber reaches a further preselected level. The timer means determines the period of time elapsing between the stare of the settling period when the height of the sample is at the preselected level and the time when the detector means detects that the boundary level has reached the further preselected level.
Another control system is described in Valheim, "Flocculant Optimization Cuts Chemical Costs and Boosts Performance"; Process Control in Engineering; August 1990, pp. 34-35. That system measures continuously the concentration of suspended solids in the total flow of incoming slurry, the flow raze of the slurry and the turbidity of the material leaving the full size industrial settling unit. The turbidity is the control parameter for the flocculant dosage system.
In Eisenlauer et al "Z. Wasser Abwasser Forsch." 16, (1983) pp. 9-15 there is described a process for the control of flocculant to a settler which involves adding a varying amount of flocculant to a side stream of the suspension to be treated, and passing this mixture through a flow-through cell where the particle size distribution of the flocs is measured by a laser light scattering. This information determines the concentration at which flocculation begins, and the size and strength of the flocs.
Other control attempts have been made directly to full size settling devices such as that described in Chandler, U.S. Pat. No. 4,040,954, issued Aug. 9, 1977. This describes a process for controlling the settling rate by measuring continuously the turbidity of the suspension at a selected height in the full size settling vessel. The position corresponds to the upper limit of cloudy liquor or floc layer above the bottom layer of mud in a state of hindered settlement. This is done by measuring the light transmittance through a continuous sample withdrawn from the settling vessel, using a light beam from a light source directed through a curtain of liquor. When the turbidity is higher than the desired set point, indicating that the interface is going higher, the amount of flocculant is increased.
It is the object of the present invention to provide an improved testing system for measuring the settling characteristics of slurries and flocculant samples and using the results to either control a full size continuous industrial gravity settler or to construct such a full size settler.