Counter Current Washer

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General Separator	Thickener	Classifier	Washer	Counter Current Washer	Underflow Washer

General Description

The Counter Current Washer is used to separate the solids from the liquids in the feed stream. This unit is similar to the Thickener and the Washer.

The counter current washer perfectly mixes the incoming wash water and side streams.
Then based on the feed stream (Mud), Scandrett efficiency and underflow solids content, the amount, temperature and composition of the underflow and overflow are calculated.
The Scandrett efficiency is based on the user defined species concentrations within the counter current washer.
The model allows reactions to be specified via a reaction file. The reactions occur to the Mud stream only, before mixing with the wash and side streams.
The EHX sub model applies to the Mud Stream only, before mixing with the wash and side streams.

Note: The Counter Current Washer Project, which is distributed with SysCAD in the Examples folder, demonstrates the use of this model.

Diagram

The diagram shows the default drawing of the Counter Current Washer, with all of the streams that can be connected to the unit.

The physical location of the streams connecting to the Counter Current Washer is unimportant. The user may connect the streams to any position on the unit.

Inputs and Outputs

Label	Required Optional	Input Output	Number of Connections		Description
Label	Required Optional	Input Output	Min	Max	Description
Mud Feed	Required	In	1	20	The slurry (or mud) feed to the Counter Current Washer.
Wash Water	Required	In	1	10	The dilute wash stream. Can also be used for Flocculant addition, miscellaneous dilution etc.
Overflow	Required	Out	1	1	Counter Current Washer overflow.
Underflow	Required	Out	1	1	Counter Current Washer underflow.
Vent	Optional	Out	0	1	Vent Stream (Vapour Only).

Behaviour when Model is OFF

If the user disables the unit, by un-ticking the On tick box, then the following actions occur:

All streams connected to the 'Mud Feed' inlet will flow out of the 'Underflow' outlet;
All streams connected to the 'Wash Water' inlet will flow straight out of the 'Overflow' outlet;
No mixing will occur.

So basically, the unit will be 'bypassed' without the user having to change any connections.

Model Theory

The Scandrett mixing efficiency [math]\displaystyle{ \mathbf{\mathit{\boldsymbol{\varepsilon}}} }[/math] is used to determine the concentration of the ALL aqueous species in the underflow and overflow from the unit.

[math]\displaystyle{ \mathbf{\mathit{\boldsymbol{\varepsilon}=\cfrac{C_{mud}-C_{underflow}}{C_{mud}-C_{overflow}}}} }[/math]

Where: C_mud - concentration of the user defined species in the mud stream

C_{under/over flow} - concentration of user defined species in the under and over flow respectively

The user must specify the Scandrett mixing efficiency and a representative aqueous species, e.g. Caustic, for the unit to perform the required mixing calculations.

The model will calculate the solid and liquid splits based on the Scandrett mixing efficiency and the user defined solids to underflow split.

References

H.F. Scandrett, "Equations for Calculating Recovery of Soluble Values in a counter current Decantation Washing System"

Limits and Notes

In the case where there is no solids present in the Feed, all flow will report to the Overflow.
In the case where there not enough solids to satisfy both overflow and underflow solids requirements, the Overflow solids conditions will be met first.
Limits apply to the overflow solids concentration, this equates to 5000g/L or 99% solid split to the overflow. A reasonable overflow solids concentration should be much lower than this.
Note that if the species used for calculation of Scandrett Efficiency is created or consumed in reactions within the CCWasher, the target Scandrett Efficiency value may not be achievable (or results may not be appropriate). In this case it is recommended to either use a different species, or to define reactions outside of the CCWasher (such as in the Underflow line).

Flowchart

The following shows the sequence of events if sub model options are switched on. See next heading for more information.

Note: Makeups also apply to the MUD stream.

Data Sections

The default access window consists of several sections,

CCWasher tab - Contains general information relating to the unit.
PC - Optional tab that is visible if EB.Solubility.On is enabled on the first tab.
MU - Optional tab, or multiple tabs if more than 1 Makeup is selected. Only visible if one of more Makeup is enabled in the Evaluation Block.
RB - Optional tab, only visible if the Reactions are enabled.
EHX - Optional tab, only visible if the EnvironHX is enabled.
Evap - Optional tab that is visible if Evaporator is enabled.
QUF - Available from Build 139. Optional tab, visible if ShowQUF is enabled. This and subsequent tab pages, e.g. QUF.. and Sp, shows the properties of the Underflow stream. The tags in the QUF tab are valid even when the ShowQUF option is not selected.
QOF - Available from Build 139. Optional tab, visible if ShowQOF is enabled. This and subsequent tab pages, e.g. QOF.. and Sp, shows the properties of the Overflow stream. The tags in the QOF tab are valid even when the ShowQOF option is not selected.
Info tab - contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents.
Links tab, contains a summary table for all the input and output streams.
Audit tab - contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.