Counter Current Decantation (CCD)

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General Description

The Counter Current Decantation (CCD) model is used in Mass Balancing mode to quickly determine the concentration in the final over and under flow streams from a series of thickeners. The user is required to specify the number of stages in the system. The underflow from the model will have the same percentage solids as the feed to the system. Any solids in the wash stream will report to the overflow stream.

This model is designed to be used as a quick means of determining the effect of differing numbers of stages or wash water ratio in a CCD circuit. The user may also use a number of Counter Current Washer models in series to model a CCD circuit. By using individual Washer models the user can specify mixing efficiencies, which is not available with this model.

Diagram

The diagram shows the default drawing of the CCD model, with all of the streams that must be connected to the unit. The physical location of the streams connecting to the model 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.
Feed	In	1	10	The slurry feed to the CCD
Wash Water	In	1	10	The dilute wash stream
Overflow	Out	1	1	CCD overflow
Underflow	Out	1	1	CCD underflow

Model Theory

The concentration, Cn, in the final thickener in a CCD circuit containing n stages is calculated using the following formula:

_{File:Models-Counter-Current-Decantation-Image002.gif}

[math]\displaystyle{ \mathbf{\mathrm{C_n=\frac{C-l-C_w}{1+WR+WR^2+.......+WR^n}}} }[/math]

where n - Number of stages in the CCD circuit

C_n - Concentration in stage n

C_l - Concentration in the slurry feed to the CCD circuit

Cw - Concentration in the wash water

WR - Wash ratio = wash water flow / slurry liquor flow.

Assumptions

1. No material is leached from the solids in the process.
2. The efficiency of slurry and wash liquor mixing is 100% in all stages.
3. The flowrate of solution from each thickener underflow is equal to the incoming slurry flowrate and the wash water flow equals the thickener overflows.
4. There is no allowance for evaporation or the addition of any additional streams.
5. All solids from the feed will report to the underflow, all solids from the wash will report to the overflow.
6. The outlet temperatures are assumed to be the same as the weighted average of all inputting streams.

References

1. P.M.Page, 'A simple equation for CCD calculations' E/MJ Operating Handbook of Mineral Processing, October 1976.

Flowchart

Data Sections

The default CCD access window consists of two sections, the first tab has the same name as the CCD model tag, contains general information relating to the unit. The second tab 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:
Stages	Input	The required number of stages, or thickeners, in the CCD circuit. The default is three.
Results:
WashRatio	Calc	The ratio of wash water to the liquid in the feed stream.
UFSolids	Calc	Underflow percent solids (Note: all solids from feed stream report here.)
OFSolids	Calc	Overflow percent solids (Note: all solids from wash stream report here.)
UFSolConc25	Calc	The underflow solids concentration measured at 25°C.
OFSolConc25	Calc	The overflow solids concentration measured at 25°C.