Counter Current Decantation (CCD)

From SysCAD Documentation
Jump to navigation Jump to search

Navigation: Main Page -> Models -> Mass Separation Models


General Description

The Counter Current Decantation (CCD) model is used in ProBal 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

Models-Counter-Current-Decantation-Image001.gif

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 Required
Optional
Input
Output
Number of Connections Description
Min Max.
Feed 1 Required In 1 20 The slurry feed to the CCD
Wash Water 1 Required In 1 10 The dilute wash stream
Overflow Required Out 1 1 CCD overflow
Underflow Required Out 1 1 CCD 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 '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 concentration, Cn, in the final thickener in a CCD circuit containing n stages is calculated using the following formula:

[math]\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

Cn - Concentration in stage n
Cl - 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

Models-Counter-Current-Decantation-Image004.gif

Data Sections

  1. CCD_Circuit tab - Contains general information relating to the unit.
  2. Info tab - Contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents.
  3. Links tab, contains a summary table for all the input and output streams.
  4. Audit tab - Contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.


Counter Current Decantation Page

Unit Type: CCD_Circuit - The first tab page in the access window will have this name.

Tag (Long/Short) Input / Calc Description/Calculated Variables / Options
Common Data on First Tab Page

Requirements

On Tick Box This allows the user to enable or disable the unit. If the unit is disabled, this is equivalent to bypassing the unit, as the following behaviour will occur:
  • all streams connected to the '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.
Stages Input The required number of stages, or thickeners, in the CCD circuit. The default is three.
OperatingP...
Method Atmospheric outlet streams will be at Atmospheric Pressure. The atmospheric pressure is calculated by SysCAD based on the user defined elevation (default elevation is at sea level = 101.325 kPa). The elevation can be changed on the Species tab page of the Plant Model.
AutoDetect If there are any liquids AND no vapours present in the feed, outlet streams will take the highest pressure of the feeds. Else (eg. some vapours present) outlet streams will take the lowest pressure of the feeds.
LowestFeed outlet streams will take the lowest pressure of the feeds.
HighestFeed outlet streams will take the highest pressure of the feeds.
RequiredP outlet streams will be at the user specified pressure.
IgnoreLowQm Tick Box This option is only visible if the AutoDetect, LowestFeed or HighestFeed methods are chosen. When calculating the outlet pressure and temperature of the tank, SysCAD will ignore the low flow feed streams should this option be selected. The low flow limit is set in the field below.
LowQmFrac Input This field is only visible if the IgnoreLowQm option is selected. This is the amount any stream contributes to the total flow. For example, if the total feed to the tank is 10 kg/s, and this field is set to 1%. Then any feed streams with less than 0.1 kg/s will be ignored in the pressure calculations.
PressureReqd / P_Reqd Input This field is only visible if the RequiredP method is chosen. This is user specified pressure.
Result Display The actual pressure used for the sum of the feeds which will also be the outlet pressure (unless further model options change the pressure).
 

Results

WashRatio Calc The ratio of Wash Water to the Liquid in the feed stream.
SolidWashRatio / SolWashRatio Calc The ratio of Wash Water to the Solids in the feed stream.

Separation Results

The following table displays the flows and compositions of the Underflow and Overflow streams.
MassFlow / Qm Display The total mass flow in each of the Underflow and Overflow streams.
VolFlow / Qv Display The total volume flow in each of the Underflow and Overflow streams.
SolidMassFlow / SQm Display The solids mass flow in each of the Underflow and Overflow streams.
LiquidMassFlow / LQm Display The liquid mass flow in each of the Underflow and Overflow streams.
SolidFrac / Sf Display The solid fraction in each of the Underflow and Overflow streams.
Density / Rho Display The total density of each of the Underflow and Overflow streams.
SolidConc Display The solid concentration (mass of solids/density of slurry) in each of the Undeerflow and Overflow streams.
SolidConc25 Display The solid concentration at 25°C (mass of solids/density of slurry at 25°C) in each of the Underflow and Overflow streams.
GridDisplay Tick Box This is a global selection, if this is selected, then where available, variables will display in a table format, if not selected, they will be listed in separate groups as a single list.

Adding this Model to a Project

Insert into Configuration file

Sort either by DLL or Group.

  DLL: Separation.dll Units/Links Separation: Thickener - Washers(CCD Circuit)
or Group: Mass Separation Units/Links Separation: Thickener - Washers(CCD Circuit)

See Project Configuration for more information on adding models to the configuration file.


Insert into Project

  Insert Unit Separation Thickener - Washers(CCD Circuit)

See Insert Unit for general information on inserting units.