Mejora higiénica del agua: ACIDIFICACIÓN DEL AGUA DE BEBIDA 135

Rotary kilns were first developed in the 1890s, and became widespread in the early part of the 20^th century. They were a great improvement on the earlier shaft kilns, giving continuous production and a more uniform product in larger

quantities.

Figure 4.3 A dry-process kiln, showing the raw meal silo (A), the pre-heater tower (B) and the kiln (C ). (Picture courtesy Castle Cement.)

In a modern works, the blended raw material passes from the silo (A in Figure 4.3) to the preheater tower (B) and then to the kiln (C). In the preheater tower, hot gases from the kiln, and often the cooled clinker at the far end of the kiln, are used to heat the raw meal. As a result, the raw meal is already hot before it enters the kiln.

We’ll look first at the general principle of the rotary kiln (Figure 4.4). Ignore the whimsical wheelbarrow – not standard equipment in a modern kiln.

A rotary kiln is basically a long cylinder rotating about its axis about 1-3 times a minute. At one end of the kiln is a flame. The kiln is inclined at a slight angle, the end with the flame being lower. The rotation of the kiln causes the raw meal to gradually pass along the kiln from where it enters at the cool end, to the hot end where it eventually drops out as clinker and cools.

Figure 4.4 General principle of a rotary kiln.

Wet process kilns

The original rotary cement kilns were called ‘wet process’ kilns and in their basic form, they were relatively simple compared with new modern kilns. The raw meal was fed into the kiln at ambient temperature in the form of a slurry.

A wet process kiln may be up to 200 m long and 6 m in diameter. It has to be long because a lot of water has to be evaporated and the process of heat transfer in a wet process kiln is not very efficient.

Clearly, slurry contains water that has to be evaporated, perhaps 35% or more of the total feed. This takes a lot of energy and various developments of the ‘wet process’ were aimed at reducing the water content of the raw meal. An example of this is the ‘filter press.’ Imagine a musical accordion 10-20 metres long and several metres across and you’ll get the concept. Such adaptions were described as ‘semi-wet.’

Figure 4.5 Basic principle of a wet-process kiln. The raw meal is fed into the kiln unheated (eg: at 20 C) as slurry.

All the drying and heating takes place in the kiln.

The wet process has survived for over a century because many raw materials are suited for blending as slurry. Also, for many years, it was technically difficult to get dry powders to blend adequately.

Quite a few wet process kilns are still in operation, usually now with higher-tech bits bolted on. However, most new cement kilns are of the ‘dry process’ type and we will focus on these.

Dry process kilns

A dry process kiln is much more thermally efficient than a wet process kiln.

Firstly, and most obviously, this is because the meal is a dry (or nearly dry) powder and so there is no water that has to be evaporated.

Secondly, and less obviously, the process of transferring heat is much more efficient in a dry process kiln.

Figure 4.6 Basic principle of a dry process kiln with

precalciner. This type of kiln has two burners. The raw meal is a fine powder and is more-or-less dry. It is heated by hot gases from the first burner, or precalciner. Also (not shown in the diagram) the meal is heated by hot gases from both the kiln and the clinker cooler before passing into the kiln at a temperature of 900 C – 1000 C.

The basic dry process system consists of the kiln and a “suspension preheater”.

The suspension preheater is a heat exchanger housed in a tall tower, consisting of a series of cyclones in which fast-moving hot gases from the kiln keep the meal powder suspended in air. All the time, the meal gets hotter and the gas gets cooler until the meal is at almost the same temperature as the gas. The high open structure made of steel girders (B) in Figure 4.3 is a preheater tower containing the cyclones.

The raw meal is fed in at the top of the preheater tower and passes through the series of cyclones in the tower before entering the kiln. Hot gas from the kiln and, often, hot air from the clinker cooler are blown through the cyclones. Heat is transferred efficiently from the hot gases to the raw meal.

The heating process is efficient because the meal particles have a very high surface area in relation to their size and because of the large difference in temperature between the hot gas and the cooler meal. Typically, 30%-40% of the meal is decarbonated before entering the kiln.

A development of this process is the ‘precalciner’ kiln (Figure 4.7). Most new cement kilns are of this type. The principle is similar to that of the dry process preheater kiln but with the addition of another burner, called the precalciner. This is placed in the preheater tower, either as a separate unit or within the ‘riser duct,’ conveying kiln gas from the kiln to the last preheater cyclone. The meal, which is already hot, reaches about 900 C in a few seconds and about 85%-95%

of the meal is decarbonated before it enters the kiln.

Figure 4.7 Basic diagram of a dry process kiln with a suspension preheater showing three cyclones and a precalciner (P).

In practice, a preheater tower is likely to have four to six stages; only three are shown in Figure 4.7 for simplicity. Some preheater designs are much more

complex but all share the principle of maintaining the feed particles in suspension in a flow of hot gases, while transferring heat from the gases to the feed.

Optimising the efficiency of this process will entail some elegant mathematics and applied physics to determine airflows and the best shape for the ducting. In a preheater tower, just small parts of the total system are visible at each floor level; only in drawings can they be seen in their entirety and their graceful form appreciated.

Since meal enters the kiln at about 900 C, rather than 20 C in a wet process kiln, a dry process kiln can be shorter and of smaller diameter for the same output. This reduces the capital costs of a new cement plant. A dry process kiln might be only 70 m long and 6 m wide but produce a similar quantity of clinker (usually measured in tonnes per day) as a wet process kiln of the same diameter but 200m in length.

At ‘A’ in Figure 4.7, most of the meal is decarbonated and the feed enters the kiln at about 950 °C. At ‘B’ the feed has reached about 1100 °C – 1200 °C and

consists largely of belite, free lime and intermediate minerals. At ‘C’, the burning zone, the feed temperature reaches 1400 °C – 1500 °C; clinker nodules form and most of the remaining free lime reacts with belite to form alite. The next chapter will go into this in more detail.

The kiln is made of a steel casing lined with refractory bricks. There are many different types of refractory brick and they have to withstand not only the high temperatures in the kiln but reactions with the meal and gases in the kiln, abrasion and mechanical stresses induced by deformation of the kiln shell as it rotates.

Figure 4.8 View inside a kiln, showing the burner pipe and flame. (Picture courtesy Rugby Cement.)

Bricks in the burning zone are in a more aggressive environment compared with those at the cooler end of the kiln (the ‘back end’), so different parts of the kiln are lined with different types of brick.

Periodically, the brick lining, or part of it, has to be replaced. Refractory life is reduced by severe changes in temperature, such as occur if the kiln has to be stopped suddenly. As the cost of refractories is a major expense in operating a cement plant, kiln stoppages are avoided as far as possible.

As the meal passes through the burning zone, it reaches clinkering temperatures of about 1400 C – 1500 C. Clinker nodules form as the burning zone is

approached; when the clinker has passed the burning zone, it starts to cool, slowly at first, then much more quickly as it passes over the ‘nose ring’ at the end of the kiln and drops out into the cooler.

Fuels

The most common fuels used in a cement kiln are probably coal and petroleum coke. Gas is also used.

Recently, particularly in the last ten years or so, many cement producers have reduced their consumption of fossil fuels by supplementing them with other

“greener” types of fuel (Chapter 12). Examples of these substitute fuels include bonemeal, car tyres (chopped into small pieces, or “chipped”) and waste paper.

Hazardous waste materials such as solvents are also burned.