Twin shaft continuous mixing has undergone considerable development over time, as a function of the products mixed.

Summary of the principles of continuous mixing

A theory of continuous mixing does not exist, although the practice of this technology began some 70 years ago and has developed in very many countries.

Very briefly, an initial approach consists in quantifying the mixing energy by the following equation:

 W  »   n x t x s x k

 Where:

n = the number of revolutions per minute

t =  working time

s = shear surface area

k = a coefficient of form specific to the mixer

With the reservation that this function is not homogeneous.

Remarks concerning n

By definition, a continuous mixer rotates faster: 120 rpm (compared to 20 rpm for a batch mixer, that is to say 6 times faster). The mixing is vigorous, if not turbulent, and thus induces granular interaction. The helicoidal movement of the paddles favours inter-granular shearing and the linear advance of the material. However, it does not induce segregation.

 Is the transit time known? 

The tank of the mixer constitutes a retention zone in which the shearing action occurs. The greater the volume of the tank, the more retention time increases. Schematically, an average approach consists in observing that the theoretical volume of the tank of an 80320 mixer, for example, is 5.62 m3, but that its useful volume is of the order of 2.92 m3. For a nominal output of 300 m3/h, it follows that average retention time is:

2.92

------- x 3600 = 35 seconds = t

300

This pragmatic approach correlates tests carried out by the LCPC using radioactive traces which are injected at the head of the mixer and indicated at the tank outlet by a signal that can be measured.

The densities emitted occur after 40 to 50 seconds, and represent the so-called stagnant part of the mix which is situated outside the area of action of the paddles and surrounds the turbulent volume, described as perfectly stirred.

We grant that an average time of the order of 30 seconds is realistic. It is possible to vary t and increase it by creating a retention or recycling by means of a reverse paddle.

 Continuation