Method and device for washing of lime mud
The method and an arrangement are for dewatering and washing lime slurry before the dewatered lime mud is passed to a lime mud kiln. The lime slurry is dewatered in a pressurized lime mud filter of disc or drum type. The pressurized lime mud filter is pressurized through a compressor drawing on its suction side (s) gas phase from the filtrate tank and pressurizing on the pressurized side (p) of the compressor the pressure vessel on the lime mud side of the filter. A certain pre-determined amount of gas phase in the gas circulation system is vented and an equivalent pre-determined amount of fresh air is added to the recycled gas phase with the aim of maintaining the partial pressure of oxygen gas above a certain minimum level.
1. A method for dewatering and washing a lime mud before dewatered lime mud is transported to a lime mud kiln, comprising:
dewatering the lime mud in a pressurized filter, connecting the pressurized filter to a closed gas circulation system,
connecting a filtrate tank to a filtrate side of the pressurized filter and where a fluid level of filtrate is established from the pressurized filter,
pressurizing the pressurized filter,
a compressor drawing on a suction side thereof a gas phase from the filtrate tank, and a pressurized side of the compressor pressurizing, via the gas circulation system, a lime mud side of the pressurized filter,
venting a pre-determined amount of the gas phase directly from the gas circulation system, and
adding an equivalent pre-determined amount of fresh air directly to the gas circulation system to maintaining a partial pressure of oxygen gas above a pre-determined minimum level.
2. The method according to claim 1 , wherein a temperature in the pressurized filter, including a temperature of the recycled gas phase, is maintained above 75° C.
3. The method according to claim 1 wherein an amount of residual white liquor in the lime mud does not exceed 10% of white liquor that is formed in a previous causticization step.
4. The method according to claim 1 wherein the lime mud that has been filtered out from the lime mud is dry-fed out from a disc filter for onward transportation to the lime mud kiln.
5. The method according to claim 1 wherein de-airing of the recycled gas phase is carried out on the pressurized side of the compressor via a de-airing device and an addition of fresh air is carried out by an air-supply device connected to the suction side of the compressor.
6. The method according to claim 1 wherein de-airing of recycled gas phase is carried out on the suction side of the compressor at a first distance from an inlet to the compressor via a de-airing device and an addition of fresh air is carried out through an air-supply device on the suction side of the compressor at a second distance from an inlet to the compressor, where the first distance is greater than the second distance.
7. The method according to claim 1 wherein an amount of recirculated gas phase that is exchanged lies within an interval 5-20%.
8. The method according to claim 1 wherein an amount of recirculated gas phase that is exchanged is regulated such that the amount depends on a detected process parameter.
9. The method according to claim 8 , wherein the detected process parameter is the partial pressure of oxygen gas in the pressurized filter.
10. The method according to claim 8 , wherein the detected process parameter is a flow rate of lime mud or dewatered lime mud, or parameters that are representative of these flow rates.
11. The method according to claim 1 wherein the pressurized filter is of a disc filter type.
12. A method for washing and dewatering a lime mud before dewatered lime mud is transported to a lime mud kiln, comprising:
dewatering the lime mud in a pressurized filter,
arranging a recirculation line for a gas phase from a filtrate side of a mud side,
connecting the pressurized filter to a gas circulation system that is essentially closed,
connecting a filtrate tank to a filtrate side of the pressurized filter and where a fluid level of a filtrate is established from the pressurized filter,
pressurizing the pressurized filter,
a compressor drawing on a suction side thereof a gas phase from the filtrate tank and a pressurized side of the compressor pressurizing, via the gas circulation system a lime mud side of the pressurized filter,
venting a certain pre-determined amount of gas phase directly from the gas circulation system, through a de-airing device, and
adding an equivalent pre-determined amount of fresh air directly to the gas circulation system through an air-supply device to maintain a partial pressure of oxygen gas above a pre-determined minimum level.
13. The method according to claim 12 , wherein the de-airing device is arranged at a position on the pressurized side of the compressor, and in that the air-supply device is arranged at a position on the suction side of the compressor.
14. The method according to claim 12 , wherein the de-airing device is arranged at a position on the suction side of the compressor at a first distance from the compressor, and the air-supply device is arranged at a position on the suction side of the compressor at a second distance from the compressor, wherein the first distance is greater than the second distance.
15. The method according to claim 12 wherein a control unit controls regulator valves.
16. The method according to claim 15 , wherein the control unit receives input signals from sensors.
17. The method according to claim 12 wherein the pressurized filter is of a disc filter type.