MINERALS & METALLURGICAL PROCESSING VOL. 20, NO. 2 • MAY 2003 57 Thickening in the 20 th century: a historical perspective F. Concha Professor, Department of Metallurgical Engineering, University of Concepción, Chile R. Bürger Scientific assistant, Institute of Applied Analysis and Numerical Simulation, University of Stuttgart, Stuttgart, Germany Abstract This paper reviews the development of thickening theory and practice during the 20 th century. It starts with the early design of the Dorr thickener and describes the progress made in understanding the process in the laboratory and in the plant. The major contributions of Coe and Clevenger, Comings and Kynch are given in detail, and the recent developments by several authors, leading to the phenomenological theory of sedimen- tation, are presented from the authors’ perspective. The known methods of thickener design are mentioned, and the contribution of the authors in this matter is expressed in detail. Finally, some new research and developments, such as high-capacity high-density thickeners and new tools to study and design thickeners, are indicated. The invention of the Dorr thickener Clarification and thickening involve the settling of one sub- stance in solid particulate form through a second substance in liquid form. While clarification deals with very dilute suspen- sions, thickening is performed to produce pulps that are more concentrated. Maybe this is the reason clarification was the first of these operations amenable to mathematical descrip- tion. The work of Hazen in 1904 was the first analysis of factors affecting the settling of solid particles from dilute suspensions in water. Hazen’s analysis shows that detention time is not a factor in the design of settling tanks, but rather that the portion of solid removed is directly proportional to the sur- face area of the tank, is directly proportional to the settling prop- erties of the solid matter and is inversely proportional to the flow rate through the tank. The invention of the Dorr thickener in 1905 can be men- tioned as the starting point of the modern thickening era. It made the continuous dewater- ing of dilute pulp possible, whereby a regular discharge of a thick pulp of uniform density took place concurrently with overflow of clarified solution. Scraper blades or rakes, driven by a suitable mechanism, rotating slowly over the bottom of the tank, which usually slopes gently toward the center, move the material as fast as it settles without enough agitation to interfere with the settling (Dorr, 1915) (Fig. 1). The first reference on variables affecting sedimentation was made in 1908. Authors such as Nichols (1908), Forbes (1912), Clark (1915), Free (1916) and Ralston (1916) studied the effect of solid and electrolytes concentration, degree of flocculation and temperature in the process. Mishler (1912, 1918), an engineer and superintendent at the Tigre Mining Co.’s con- centrator in the Sonora desert of Mexico, was the first to show by experiment that the rate of settling of slimes is different for dilute and for concentrated suspensions. While the settling speed of dilute slimes is usually inde- pendent of the depth of the settling column, the thick slimes-sedimentation rate in- creases with the depth of the settling column. Mishler devised the follow- ing formula by means of which laboratory settling test results Invited feature number 02-326. Original manuscript submitted August 2002. Revised manuscript received and accepted for publication February 2003. Discussion of this peer-reviewed and approved paper is invited and must be submitted to SME Publications Dept. prior to Nov. 30, 2003. Copyright 2003, Society for Mining, Metallurgy, and Exploration, Inc. SPECIAL INVITED FEATURE Figure 1 — Original Dorr thickener (Dorr, 1936).