Process and plant for separating heavy metals from phosphoric starting material

Abstract

A process for separating heavy metals from phosphoric starting material comprises the following steps: (i) heating the starting material to a temperature of 600 to 1.200 C. in a first reactor (1) and withdrawing combustion gas; (ii) using the combustion gas of step (i) to preheat an alkaline source; and (iii) transferring the heated starting material of step (i) and the heated alkaline source of step (ii) to a second reactor (20), adding an elemental carbon source, heating to a temperature of 700 to 1.100 C. and withdrawing process gas and a product stream.

Claims

1. A process for separating heavy metals from phosphoric starting material comprising the following steps: (i) heating the starting material to a temperature of 600 to 1.200 C. in a first reactor and withdrawing combustion gas; (ii) using the combustion gas of step (i) to preheat an alkaline source; and (iii) transferring the heated starting material of step (i) and the heated alkaline source of step (ii) to a second reactor, adding an elemental carbon source, heating to a temperature of 700 to 1.100 C. and withdrawing process gas and a product stream.

2. The process according to claim 1, characterized in that the starting material is pre-heated in at least a first preheating stage to a temperature of 300 to 800 C. prior to step (i).

3. The process according to claim 1, characterized in that the starting material is pre-heated in multiple stages prior to step (i).

4. The process according to claim 1, characterized in that the alkaline source is pre-heated in a second preheating stage to a temperature of 200 to 500 C. prior to the introduction into the second reactor.

5. The process according to claim 1, characterized in that the alkaline source is selected from the group consisting of sodium carbonates, sodium hydroxides, potassium carbonates and potassium hydroxides or any combination thereof.

6. The process according to claim 5, characterized in that the alkaline source is soda ash.

7. The process according to claim 1, characterized in that the alkaline source is added in an amount of 2 to 80 wt.-% of the starting material.

8. The process according to claim 1, characterized in that the elemental carbon source is selected from the group consisting of pulverized lignites, dry sewage sludge, dry biomass, pulverized lignite, coal and coke or any combination thereof.

9. The process according to claim 1, characterized in that the elemental carbon source is added in an amount of 1 to 40 wt.-% of the starting material.

10. The process according to claim 2, characterized in that the combustion gases from step (i) are fed into a cyclone separator and subsequently into a Venturi section of the first preheating stage.

11. The process according to claim 1, characterized in that the combustion gases from step (i) are withdrawn from a cyclone separator, enter a Venturi section and are admixed with an alkaline source according to step (ii).

12. The process according to claim 1, characterized in that the process gas in step (iii) is cooled below the condensation temperature of the heavy metal compounds to allow for their precipitation and removal.

13. The process according to claim 12, characterized in that the heavy metal-free process gas is recycled into the first reactor.

Description

BRIEF DESCRIPTION OF THE OF THE DRAWING

(1) FIG. 1 is a simplified block diagram of a plant implementing the process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(2) In the plant shown in FIG. 1, a phosphorous-containing raw or starting material such as sewage sludge or biomass ash or rock phosphate is pneumatically conveyed from non-illustrated storage silos to a first preheating stage 2 comprising a Venturi section 2a and a cyclone separator 2b. Thereby, the starting material is intensively mixed with a hot combustion gas withdrawn from a first reactor 1 and heated to a temperature of 400 to 600 C., preferably about 575 C. In the cyclone separator 2b the solid material is separated from the gas and transferred via line 3 into the first reactor 1, which preferably is a fluidized bed reactor. In the first reactor 1, the preheated starting material is heated by the combustion of fuel, such as natural gas, biomass or sewage sludge supplied through fuel line 4 with air supplied through air line 5. The air may be introduced under elevated pressure via compressor 6. In the first reactor 1 the starting material is heated to a temperature of 700 to 1.100 C., preferably 900 to 1.000 C. and in particular about 950 C.

(3) The thus heated starting material is withdrawn from the first reactor 1 through line 7 and is fed to a second reactor 20, preferably a rotary kiln reactor. The level of the inventory in the first reactor 1 can be controlled by a seal pot 8 such as described in document WO 2008/104250 A, a dip leg seal or the like. The combustion gases are withdrawn from the first reactor 1 through line 9 into a cyclone separator 10 for separating the gas from the solid material. The solid material is withdrawn at the bottom of the cyclone separator 10 and transferred to the second reactor 20 through line 7. The hot combustion gases enter into the Venturi section 2a of the first preheating stage 2 for preheating the starting material.

(4) From the cyclone separator 2b of the first preheating stage 2 the gas is withdrawn at the top and enters a Venturi section 11a of a second preheating stage 11 where soda ash is added as a preferred alkaline source and mixed with the heating gas. The mixture then is transferred into a cyclone separator 11b for separating the solids from the gas. In the second preheating stage 11 the alkaline source is preheated to 300 to 400 C., preferably about 360 C., and then transferred into the second reactor 20 through line 16. The gas is withdrawn at the top of cyclone separator 11b and transferred into separator 13 via line 12, where the solids are separated from the gas after a suitable, calcium or sodium based, sorbent such as calcium hydrate, calcium carbonate or sodium hydrogen carbonate has been added. Finally, after passing through a filter 14, preferably an electrostatic precipitator and other suitable cleaning device, for recovering additional solids that may be introduced into the second reactor 20 though line 15, the clean gas is removed from the plant.

(5) The starting material supplied from the first reactor 1 through line 7 and the alkaline source supplied through lines 15, 16 is introduced into the second reactor 20 and heated therein to a temperature of 700 to 1.100, preferably 900 to 1.000 C. and in particular about 950 C. In addition to the starting material and the alkaline source an elemental carbon source, in particular pre-dried sewage sludge, biomass, pulverized lignite or coal and/or coke, is fed to the second reactor 20 through line 17. Air may be introduced through line 5. The compounds may be mixed before entering the second reactor 20 or supplied separately and mixed within reactor 20, preferably by rotation, Thereby the alkaline source decomposes into X+Y (where X is the alkaline ion and Y is a carbonate or hydrogen anion) and the elemental carbon source reduces the heavy metals to their elemental form. For soda ash the reaction is as follows:
Na.sub.2CO.sub.3+C.fwdarw.Na.sub.2O+2CO

(6) The heavy metals evaporate and leave the second reactor 20 with the process gas through line 21. The remaining, phosphorus rich solids leave the second reactor 20 through a gas-tight outlet and product line 22 and are cooled.

(7) The semi-product withdrawn through product line 22 is free from toxic heavy metals and conveyed to a finishing section (not shown) where it is manufactured to straight phosphorus or complex fertilizers.

(8) The process gas from the second reactor 20 contains the elemental heavy metals. Said process gas is transmitted via line 21 into separator 23 where it is quenched to about 200-400 C. with fresh air or water to condensate the heavy metal compounds to solid particles. These particles are captured in a baghouse filter(not shown) as filter dust. Alternatively, the solids may be separated from the gas by electrostatic precipitation, Until heavy metal recycling will be commercially viable, the filter dust will be deposited as secondary waste. Finally, the purified process gas is fed back into first reactor via line 24.

(9) At the point of leaving the thermo-chemical process, the semi-product already complies with the requirements of the fertilizer act. The concentration of toxic substances and particularly of cadmium and uranium is one to two orders of magnitude below the respective concentrations in phosphate rock based fertilizers.

(10) To comply with the phosphate concentration tolerances required by most fertilizer acts in the order of +/0.8 percentage points of total P.sub.2O.sub.5, a measured amount of a high grade straight phosphorus (P) carrier may be added to the semi-product. For this purpose, the semi-product is analyzed online for its concentration of P.sub.2O.sub.5 and one or several guiding heavy metals. Depending on the desired phosphate concentration in the final product, a measured quantity of triple-superphosphate (TSP) or phosphoric acid is admixed and homogenized. Alternatively and for the production of a phosphate fertilizer for organic farming, phosphate rock is used instead of TSP to adjust the P-concentration.

(11) As a first option the product is homogenized and granulated in a mixer-granulator anddepending on the final purposefinished as dust free powder or as final granules. From this stage, the product has become the final product of the plant that either will be sold to the agricultural product distributors or to fertilizer manufacturers.

(12) As a second option, the plant can be extended to manufacture complex fertilizers by admixture of additional nutrient carriers. This step requires additional silos/storing facilities and the corresponding design of the finishing section of the plant to handle the additional nutrient and fertilizer quantities. In this case, the product and additional nutrient carriers are conveyed and fed to the mixer-granulator in ratios determined by the target fertilizer type. By adding small amounts of water anddepending on the requirementsbinders and coating agents, complex fertilizer granules of homogenous composition and a determined corn size distribution are produced that comply with ail requirements in terms of threshold values, tolerances and nutrient solubility.

(13) The raw material, ash, does not contain combustible and halogenic-organic substances. It mainly consists of phosphate, calcium, silicon, iron and aluminum compounds.

(14) The starting material treatment capacity of the plant may be e.g. 4-10 tons per hour. Raw materials are heated by natural gas burners or by combusting sewage sludge or biomass, and energy is efficiently recycled within the plant. Process emissions are effectively controlled by a sequence of adsorption reactors and baghouse filters. The heavy metals are captured as dry filter dust and safely disposed of in a landfill.

(15) Application of the product as a phosphate fertilizer is more environment friendly than using either conventional mineral fertilizers or recycled organic fertilizers. In comparison to conventional mineral fertilizers, concentrations of cadmium and uranium are 1-2 orders of magnitude lower. In comparison to organic fertilizers, no risk of transfer of organic pollutants to the food and feed chain exists.

(16) Additional nutrient carriers are exclusively licensed fertilizers as ammonium sulfate, potassium chloride (MOP), potassium sulfate (SOP) and converter slag. Triple-superphosphate and the finished products will be stored in silos or as bulk material in covered warehouses. Binders and coating agents andon demandphosphorus and sulfuric acid are stored in compliance with legal requirements.

LIST OF REFERENCE NUMERALS

(17) 1 first reactor 2 first preheating stage 2a Venturi section 2b cyclone separator 3 line 4 fuel line 5 air line 6 compressor 7 line 8 dip leg seal 9 line 10 cyclone separator 11 first preheating stage 11a Venturi section 11b cyclone separator 12 line 13 separator 14 filter 15 line 16 line 17 line 20 second reactor 21 line 22 product line 23 separator 24 line