In the days before digital computers, sizing distributions were often simplified to a single value, typically the size in microns through which 50% or 80% of the material passed. This was good enough for Bond’s work on circuit design but not good enough for understanding breakage mechanisms or identifying how problems in breakage occurred. Complete size distributions were needed, and it was Paul Rosin and Erich Rammler in Germany who showed that sieve analyses of pulverized coal could be fitted by a simple negative exponential equation.
During World War I the combustion of hard coal dust had been developed in the USA and put into large scale operation. After the war the industrial countries of Europe followed suit. In Germany the mining of brown coal and hard coal had been greatly stepped up. Efforts were therefore made to introduce the combustion of brown coal dust. In this connection Dr-Ing Paul Rosin, Head of the Department of Heat Economy of the Staatlichen Blaufarbenwerke in Freiberg and unsalaried lecturer at the Bergakademie Freiberg, set himself the task of investigating the special problems of the combustion of brown coal dust, in particular for metallurgical furnaces. (Schubert and Wachtler 1987)
Rosin started his research work on grinding and combustion processes in 1923, at the same time that Curt von Grueber was installing Maxecon mills in the Moabit power station in Berlin. Rosin set up several mills and air classifiers in a pilot plant in a smelter near Freiberg in an effort to understand how fine particles were generated during size
reduction and to develop equations of size distributions. In March 1925, after graduating from the Freiberg Mining Academy with an outstanding academic record, Erich Rammler joined Rosin.
In 1925, construction began on the first brown coal power station in Germany, which was near Leipzig and Rosin’s experimental plant. Inevitable start-up problems were encountered because unwanted fines were present when the power station came online, which emphasized the importance of fine particle studies. Rosin set up a program in which feed and product size distributions were measured and changes in the operating variables were related to changes in the size distributions. In 1933, Rosin and Rammler published the equation for the size distribution of broken coal: x = 100. exp(-b. xn), where x is the oversize and b, n are variables that describe particle characteristics (Rosin and Rammler 1933). This equation has been found to be applicable to many broken products and has become a tool for grinding studies, even being used for assessing the efficiency of dentures.
Rosin and Rammler extended their brown coal research to drying and firing and became consultants in brown coal processing. In 1937, Rosin emigrated to England, and, in 1945, Rammler became director of a research program on briquetting and degasifying brown coal at the Bergakademie Freiberg, where he remained for many years.