Process and apparatus for calcining solids or sludges

1. A process for calcining solids or sludges in a multiple-deck furnace having a firing region, which has at least one upper heated hearth and at least one lower heated hearth, and having a cooling region, which is provided with one or more unheated hearths, said process comprising the step of:

passing a main portion of a material to be calcined into the firing region and then into the cooling region, and

delivering a bypass portion of the material to be calcined to at least one of the lower heated hearth or the cooling region, bypassing the at least one upper heated hearth of the firing region, wherein the at least one of the lower heated hearth or the cooling region has the main portion of the material already present therein at a temperature of at least 400° C.

2. A process according to claim 1 , further including the step of calcining the bypass portion of the material to be calcined by the heat of the main portion of the material coming out of the firing region.

3. A process according to claim 1 , wherein said delivering step delivers the bypass portion of the material to be calcined to the cooling region, bypassing the firing region.

4. A process according to claim 1 , wherein said delivering step initially delivers at least one sub-portion of the bypass portion of the material to be calcined to a pre-heating region of the furnace upstream of the firing region.

5. A process according to claim 4 , further including the step of providing a remainder of the bypass portion of the material to be calcined which is not initially delivered to the pre-heating region with a smaller particle size than the main portion of the material delivered to the pre-heating region.

6. A process according to claim 1 , wherein the bypass portion of the material to be calcined amounts to from 3 to 30% of all the material to be calcined.

7. A process according to claim 6 , wherein the bypass portion of the material to be calcined amounts to from 5 to 15% of all of the material to be calcined.

8. A process according to claim 1 , wherein the at least one of the lower heated hearth or the cooling region has the material already present at a temperature of at least 600° C.

9. A process according to claim 1 , wherein the bypass portion of the material to be calcined that is delivered to the cooling region is fed into one or more hearths of the cooling region.

10. A process according to claim 1 , wherein said delivering step includes the step of discharging a sub-portion of the main portion of the material to be calcined from the multiple-deck furnace from at least one of any hearth of a) the pre-heating region b) the firing region or c) the cooling region.

11. A process according to claim 1 , wherein the material to be calcined is at least one of magnesite, dolomite or lime.

12. A process according to claim 1 , wherein the bypass portion of the material to be calcined is fresh material.

13. A process according to claim 1 , further comprising the steps of:

initially passing the main portion to a pre-heating region of the furnace upstream of the firing region; and

providing the bypass portion of the material to be calcined with a smaller particle size than the main portion of the material delivered to the pre-heating region.

14. An apparatus for calcining solids or sludges comprising:

a multiple-deck furnace having

a firing region, which has at least one upper heated hearth and at least one lower heated hearth, and

a cooling region, which is provided with one or more unheated hearths, and

a delivery means for delivering a main portion of a material to be calcined to the firing region, and

a bypass delivery means for delivering a bypass portion of the material to be calcined to at least one of the lower heated hearth or the cooling region, bypassing the at least one upper heated hearth of the firing region, wherein the at least one of the lower heated hearth or the cooling region has the main portion of the material already present therein heated to a temperature of at least 400° C.

15. An apparatus according to claim 14 , further comprising a preheating and/or drying region having one or more hearths arranged upstream of the firing region.

16. An apparatus according to claim 15 , further comprising means for discharging a portion of the main portion of the material to be calcined from at least one of the preheating region or the firing region.