1 (I) shows the condition when all of the pulp has entered zone D and compression of the slime flocs is going on. 1 show progressive stages of settling in which zones A and D are growing deeper, zone B is decreasing in depth, and zone C remains constant-a feature of this particular pulp. In this illustration zone B occupies the total depth. 1 (E) shows a cylinder freshly filled with pulp of a consistency of about 10 parts water to 1 part ore. The effect of the relations between the depths of zones B and C upon curves derived from settling tests will be discussed later.įig. Later this very fine material settles and the liquid becomes clear, although there are cases, especially when the liquid contains very little electrolyte, where it remains turbid for a long time. In the case of a very rapidly settling slime, particularly with material which has been roasted, zone A in the earlier stages may be turbid, due to finely divided matter remaining in suspension. Zone A, overlying zone is clear water or solution. Above C is zone B, of constant consistency of flocculated pulp and of the same consistency as the flocculated pulp in the feed pulp. In speaking of flocculated pulp, it is intended to eliminate from consideration the coarser portion of the contained sand which falls directly through the overlying zones into zone D. The pulp in zone C decreases in percentage of solids from the bottom, where the flocs enter zone D, to the top, where the consistency of the flocculated pulp is the same as that of the original pulp. Immediately above zone D is a transition zone C. After pulp enters zone D , further separation of liquid must come through liquid pressed out of the flocs and out of the interstitial spaces between the flocs. This we term zone D, which may be defined as that portion of the pulp wherein the flocs, considered as integral bodies, have settled to a point where they rest directly one upon another. Immediately following this and somewhat contemporaneously with the settling of the sand, the slime flocs nearest the bottom settle, filling the interstitial spaces between the sand particles, and build up, one upon another, in a zone of increasing depth. The first particles that reach the bottom of the cylinder are the coarser granular sand which may be present in the pulp. Methods for Determining the Capacities of Slime-Settling Thickenersįig.1- Experiment Showing Various Stages of Slime-SettlingĪfter a short time, however, it assumes a flocculent structure which, after settling a brief period of time, forms four distinct zones (A, C, and D), as indicated in Fig. cylinder, after thorough mixture, at least momentarily, it forms a homogeneous mass, as shown in Fig. If a thin pulp, of a dilution of, say, 10 to 1, is placed in a 1,000 c.c. The floes settle in the liquid medium according to certain laws. The electrolyte has the property of causing the colloidal portion of the slime to form aggregates known as floes, particles having a more or less definite size, consisting of water, colloidal material and usually fine granular material which has been entrapped. The chemical or chemicals present which exert an influence upon the subsequent settling behavior of the pulp are known as electrolytes. In this connection water is used as implying either ordinary water or water containing some chemical or chemicals in solution. In metallurgical practice, slime pulp consists of water, finely divided sand or granular particles, and colloidal material. The laws and principles controlling these conditions are not fully established, so that a complete discussion of them will not be attempted in this paper. However, it must be recognized that there is still a vast amount of work to be done upon certain phases of the subject therefore, at this time we will attempt only a discussion of the behavior of ore pulps during the process of settling or dewatering, under certain given conditions. We realize that certain of our preliminary conclusions may be subject to modification, but it seems probable that, with but few exceptions, the general laws enunciated will cover the settling behavior of all pulps encountered in metallurgical plants. Since it was desired to give the public the benefit of these methods at as early a date as possible, together with the knowledge of the general principles of slime settling necessary for a clear understanding of the laboratory tests, we have ventured to discuss the subject in so far as brought out by the work which we now have under way. In order to develop rational methods of measurement and laboratory testing, it becomes necessary to study the general phenomena of settling rates.
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