Counter Current Washer

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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 assumes there are no solids in the overflow.

The model allows reactions to be specified via a reaction file. The reactions occur before washing and any solids generated by the reaction(s) report to the underflow. The temperature of the solids and liquids is calculated after any reactions and/or heat loss has occurred.

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 Side Stream does not have to be connected for the unit to operate.

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	Input / Output	No. of Connections		Description
		Min	Max.
Mud	In	1	20	The slurry (or mud) feed to the Counter Current Washer
Wash Water	In	1	20	The dilute wash stream
Side Stream	In	0	10	Any input other than the Mud stream or the wash water. These streams are combined with the wash streams.
Overflow	Out	1	1	Counter Current Washer overflow (liquids only)
Underflow	Out	1	1	Counter Current Washer underflow
Vent	Out	0	1	Vent Stream (Vapour Only)

Model Theory

The Scandrett wash efficiency (e)¹ is used to determine the concentration of the user defined species, e.g. Caustic, in the underflow and overflow from the unit.

[math]\displaystyle{ \mathbf{\mathit{\boldsymbol{\varepsilon}=\frac{C_{mod}-C_{underflow}}{C_{mod}-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 efficiency and either the % solids, or the user defined species concentration, in the underflow from the unit. The model will calculate the solid and liquid splits based on these values.

References

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

Flowchart

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

Data Sections

The default access window consists of two sections,

a) The first tab has the same name as the model tag, contains general information relating to the unit.

b) Optional tab RB, or Reaction Block, fully described in Reaction Block (RB).

c) Optional tab Environmental Heat Exchanger (EHX)

a) Audit, fully described in Audit Section. See Model Examples for enthalpy calculation Examples.

First Section - Tag Name

Tag / Symbol	Input / Calc	Description
Common First Data Section
Requirements:
On	Check Box	This allows the user to enable or disable the unit. If the unit is disabled, the material will all flow out of the underflow.
RqdSpecie	Input	The selected species is used to calculate the Scandrett efficiency. (Eg, for Alumina industry, this would be Caustic.)
ScandrettEff	Input	The required Scandrett efficiency of the unit. At 100% efficiency, all of the incoming streams will be perfectly mixed.
Method	List Box	Options are: %_Solids or g/L_Solids
RqdUFSol Conc25	Input	Visible with g/L_Solids method - user specified solids concentration in the underflow stream
RqdUFSolids	Input	Visible with %_Solids method - user specified percentage solids in the underflow stream.
Results:
UFSolids	Calc	The actual % solids in the under flow
UFSolConc25	Calc	The under flow solids concentration measured at 25°C.
ScandrettEff	Calc	The calculated Scandrett Efficiency of the unit.
UFSpecie	Calc	The concentration of the user defined species in the under flow.
OFSpecie	Calc	The concentration of the user defined species in the over flow.
Reactions	Tick Box	This can be used to switch on the Reaction Block (RB). If this box is checked then the associated page, RB becomes visible and may be configured. Note: The user does not have to configure a reaction file, even if this block is checked.
EnvironHX	Tick Box	This can be used to switch on Environmental Heat Exchanger (EHX). If this box is checked then the associated page, EHX may become visible and may be configured. Note: The user does not have to configure an environmental heat exchange, even if this block is checked.

Hints and Comments

a)There may be mass entering or leaving the unit via the Reaction Block source or sink, so if the outgoing mass is not the same as the incoming mass, this may be a place to check first.

b)The Scandrett efficiency is based on the concentration of Liquid or Aqueous species.

c)The model assumes no solids reports to the overflow stream.

d)If vapour is present in the feed streams, or is produced in the reaction block, the vent line must be connected to collect the vapour. Otherwise, the vapour will be lost resulting in mass imbalance.