Batch Powder Mixer discharge time / emptying time

How to optimize emptying duration of a dry mixer

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Section summary
Process Parameters influencing Mixing time
Typical mixing time of common mixers

Powder Mixing (main page)
# Types of mixers
# Mixing mechanisms
# Mixing time
# Mixing speed
# Mixing volume
# Mixing power
#Mixer loading # Discharge time
# Froude Number
# Coefficient of Variation
# Homogeneity
# Tracer
# Segregation / demixing
# Tip speed
# Sampling
# ATEX speed
# Mixer Bearing seals
# Mixer discharge valve

1. Discharge time

The discharging time of a bulk solids mixer is the time needed to empty it to the point it is ready for another batch. It is important to remark that the definition does not precise to "empty totally", indeed it is not always necessary to totally discharge a mixer working in batch.

2. Powder mixer discharged weight as function of time

Graph 1 :Discharge time profile

Batch powder mixer discharge time profile

The weight discharged increases sharply at the beginning of the discharge, but then follows an asymptote, thus the last kg to be removed from the mixer takes proportionally longer than the rest of the batch.

Note : the graph above is relevant for mixers having a discharge valve, flap or round. For mixers having bomb doors opening in the whole length of the mixer, the asymptote will not really exist.

3. Optimization discharge time batch powder mixers

To optimize the cycle time, it can be interesting to decide to stop the discharge while few kg of product are still remaining in the mixer. If it is sufficiently low, it is likely the remaining product will not affect the next mix homogeneity, but this will help to gain several seconds that will translate in additional capacity. The discharge can be governed according to a timer, but it is better to verify it through weight as well by having the discharge hopper or IBC on load cells.

The mixer should be run at low speed, below ATEX speed, to promote the discharge. An optimum is to be found as running too quick may in the contrary slow down the end of the discharge.

Bomb doors will give the quicker discharge time. For applications where bomb doors are not applicable, the discharging valve can be doubled to allow more flow.

4. Orders of magnitude discharge time mixers

For a 1000 l mixer

Table 1 : Usual discharge time

Discharge type Discharge time
Bomb doors 20 s
Single flap / valve 1-2 min
Double flap / valves 45-60 s

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