Validity of water industry wastes in cement industry

Abstract

This paper examines the validity of several water industry wastes to be reused in different stages of cement production. The wastes considered are: a drinking water treatment plant sludge (DWTP), a sewage sludge (SS) and a spent activated carbon. Different procedures of drying of wastes using a novel dry spray system or subjected to thermal drying or stabilization and dried with lime are analysed.

The spray drying process was successfully used with the DWTP sludge (but not with SS). The material was not found to be suitable as a supplementary material in blended cements. Despite this, the spray dried sludge did show good potential as partial or full substitute for clay as a raw material in cement clinker manufacture.

A novel thermally drying process (Turbo-drying RINA-JET) was applied to produce dried sewage sludge. The dry sludge consisted of 56 % organic matter and has a high calorific value (8291 J / g), making it valid as an alternative fuel. The ash (35% of dry SS) contains Ca, Fe, P, Si and Al as main inorganic elements which are incorporated into the clinker phases. The burnability of the raw mixture containing this SS sludge was better than in the control raw mix.

Furthermore, the paper describes an industrial process for stabilising sewage sludge (SS) with lime and evaluates the viability of the stabilised product, referred to as “Neutral”, as a raw material in the production of Portland cement clinker for the cement industry. The proposed process for sewage sludge treatment has a number of advantages over traditional treatments. In the Basic plant design, the chemical energy in the reagents generates sufficient thermal energy for the moisture to evaporate. This makes the process more energy-efficient than others. The validity of the “Neutral” product as a starting material in raw mixes for the production of cement clinker by substitution of limestone is demonstrated.

Regarding the validity of spent activated carbon as an alternative to pet coke as fuel in Portland cement kilns, the carbon differed substantially from the coke, having lower calorific value (18 % less). However, the qualities of the spent coke were still sufficient for the intended use. The ash fraction of the spent activated carbon was mainly composed of anhydrite and quartz (SO₃ = 14.1 %) and it is included into the clinker phases during the burning. The SO₃ from the ashes promotes a very large growth in alite crystals in the clinker.