When separating light components from a liquid suspension or the like, the separated particles will float on the liquid. The supernatant layer thus obtained can have different characteristics, in particular in respect of its stability and viscosity. This will, inter alia, depend on the character of the components and of the carrier liquid, the presence of air bubbles, etc.
It is sometimes possible to remove such a floating layer simply over a weir, but generally auxiliary means are required for removing this layer. To that end often scrapers are used which comprise a plurality of blades which can be circulated by means of endless cables or chains guided by rollers or sprockets, and this in such a manner that these blades are fully or partly immersed into the floating layer during a part of the cycle, and move substantially parallel to the liquid surface, so that the supernatant layer is displaced by these blades towards the discharge side of the separation device. At this discharge side an overflow weir is provided which generally extends above the liquid, but which, near the liquid surface, joins either gradually or angularly an inclined or curved surface along which the lower ends of the scraper blades are movable, so that the floating layer will be discharged by the circulating blades over the overflow. The shapes of this surface and of the trajectory of the scraper blades should be mutually adapted.
Such scrapers have disadvantages. It has appeared that such scrapers cannot fulfill their double task, viz., on the one hand, laterally displacing the floating layer and, on the other hand, shoving the supplied floating matter in an optimal manner over the overflow, in particular when modifications of the coherence, the composition and other characteristics of the floating layer occur. It has appeared that this layer, especially when the discharge falls behind the supply, can escape again for a substantial part below the blades. This will lead to undesirable disturbances of this layer which will unfavorably influence the separation effect and may even destroy it in a very considerable degree. If, on the other hand, the supply falls behind the discharge, it will be possible that near the discharge end, instead of the floating matter, also the underlying carrier liquid, insufficiently thickened floating matter or airy foam will be removed which is undesirable too, since it is, in fact, often desired to discharge the floating matter as concentrated as possible.
Broadly speaking, prior art devices for skimming a floating layer from the surface of a liquid can be classified into two main catagories, the first catagory is exemplified by: U.S. Pat. No. 3,635,349 issued Jan. 18, 1972 to Weiss et al.; U.S. Pat. No. 3,890,289 issued June 17, 1975 to Johnson; U.S. Pat. No. 3,756,418 issued Sept. 4, 1973 to Pentz et al.; and, U.S. Pat. No. 3,872,005 issued Mar. 18, 1975 to Baker.
The skimming devices in this first catagory are characterized in that they use a single blade for pushing the floating layer in a continuous motion toward the discharge weir.
Typically, the floating layer exhibits some resiliency or compressability, and as the floating layer is compressed between the single blade and the discharge weir it tends to escape by passing beneath the blade. If the blade is extended deeper into the liquid to prevent escape of the floating layer, excessive disturbance of the liquid will be caused, resulting in re-mixing of the already separated components, and liquid will be pushed over the weir.
The second catagory of prior art skimming devices is exemplified by: U.S. Pat. No. 3,286,844 issued Nov. 22, 1966 to Juell; U.S. Pat. No. 2,880,876 issued Apr. 7, 1959 to Dujardin; and, U.S. Pat. No. 2,697,384 issued Dec. 21, 1954 to Craig et al. The devices in this second catagory are characterized in that they employ a plurality of blades, instead of a single blade, affixed to a flexible endless belt. As the belt is driven, the blades are drawn across the surface of the liquid in a continuing succession. Although the use of a plurality of blades tends to relieve to some extent the problems encountered with single-blade devices, the skimmers in this second catagory generally tend to produce considerable splashing and disturbance of the floating layer as the blades enter the liquid, resulting in some re-mixing of the separated components. The disturbance is caused by the blade entering the liquid with a substantial forward velocity in most cases and at an angle with the liquid surface.
The structure of the present invention bears a superficial resemblance to features found in; U.S. Pat. No. 3,796,315 issued Mar. 12, 1974 to Chapman et al.; U.S. Pat. No. 3,966,592 issued June 29, 1976 to Herbert; and, U.S. Pat. No. 1,098,812 issued June 2, 1914 to Lowden. These patents disclose devices in which a rake with a plurality of blades is driven in a reciprocating raking motion.
These last three patents all relate to classifiers in which, as in known in the art, agitation is used to separate larger particles from smaller particles. In each case, the material being classified is intentionally agitated to produce the desired result, and in each case, the material classified is resting on an underlying surface, rather than floating on the upper surface of a liquid. Consequently, escape of the material beneath the blade is not a problem. Thus, the devices disclosed in these last three patents achieve a different result from the present invention and operate on a different principle. These patents do not teach or suggest the use of their devices for skimming a floating layer from a liquid.