Abstract:
A method of treatment of wood ash residue. A first step involves wetting the wood ash residue. A second step involves reacting the wetted wood ash residue with carbon dioxide gas. Mineral oxides and hydroxides are converted to carbonates, thereby reducing the caustic nature of the wood ash residue. A third step involves continuing to react the wetted wood ash residue until the resulting ash residue is substantially carbonated.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of treatment of wood ash residues, so that they can be mixed with soil without detrimental effect to plant growth. 
     BACKGROUND OF THE INVENTION 
     Carbonaceous materials, such as wood, contain mineral components, known as ash, that do not combust when burned in air, regardless of combustion temperature. Alkaline minerals, formed from alkali or alkali earth metals, form one part of the ash. Due to the elevated temperatures achieved during combustion, normally exceeding 850° C., a large proportion of the alkaline minerals are converted from their carbonate form to their oxide form through the loss of carbon dioxide. Wood ash containing primarily the oxide form of alkaline minerals is normally extremely alkaline, with pH values of 12.0 or more, and heavily laden in soluble salts, as evidenced by electrical conductivity values of 20 deciSiemens per meter (dS/m) or more. 
     Wood ash is applied on land to increase soil pH, buffer soil against decreases in pH due to acid addition, add calcium and magnesium for improved plant growth and soil structure, and increase the supply of micronutrients. Most plants grow optimally in soil having a pH range of from 6.0 to 8.0. The limitations of using wood ash residue, either directly as a soil amendment or as a major component of other fertilizers, are its excessively high pH and high content of soluble salts. For example, wood ash has pH values as high as 13.0, which can cause i) deficiencies in micronutrients such as iron, copper and zinc; ii) ‘toxic shock’ to young seedling plants; and iii) localized mineralogical changes in the soil environment where it is placed. Soluble salt contents of wood ash, measured as electrical conductivity (EC), can reach 40 dS/m. Depending on the amount of wood ash applied per year, these elevated levels of pH and soluble salts can be detrimental to soil quality, crop production, and water quality. 
     In an article published in Resources, Conservation and Recycling 38 (2003) 301-3116 entitled “Drying of granulated wood ash by flue gas from saw dust and natural gas combustion”, S. L. Homberg, T. Claesson, M. Abul-Milh, and B. M. Steenari; the authors investigated how drying by flue gas affected the chemical composition and properties of wood ash. The conclusion of the Homberg et al is that drying by flue gas was an environmentally acceptable way to dry granules in terms of effects on hardening and the chemical composition of the granules to make them less reactive. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a method of treatment of wood ash residue. A first step involves wetting the wood ash residue. A second step involves reacting the wetted wood ash residue with carbon dioxide gas. Mineral oxides and hydroxides are converted to carbonates, thereby reducing the caustic nature and the soluble salt content of the wood ash residue A third step involves continuing to react the wetted wood ash residue until the resulting ash residue is substantially carbonated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings. The drawings are for the purpose of illustration only and are not intended to in any way limit the scope of the invention to the particular embodiment or embodiments shown, wherein: 
         FIG. 1  is a block diagram illustrating an example of the type of equipment and flow scheme of the preferred embodiment to be used in practicing the present invention which hereafter will be described as “continuous carbonation” methodology. 
         FIG. 2  is a block diagram illustrating an example of the type of equipment and flow scheme of an alternative embodiment to be used in practicing the present invention which hereafter will be described as “batch carbonation” methodology. 
         FIG. 3  is a graph illustrating pH and electrical conductivity over time of dry dilute carbonation. 
         FIG. 4  is a graph illustrating pH and electrical conductivity over time of thin film carbonation. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In  FIG. 1 , dry alkaline wood ash is loaded into bin  10  and is discharged by dry ash feeder  12  at a predetermined rate into mixer  16  along with water from pump  14  fed to the mixer at a rate sufficient to achieve the desired degree of wetting. After sufficient residence time in mixer  16  to ensure intimate contact between the alkaline ash and the water, the wetted mixture is discharged from mixer  16  into conditioning bin  18  and stored for a period of time sufficient to ensure that substantially all of the water has reacted with the alkaline ash. 
     After conditioning, wetted material is then discharged at a predetermined rate from bin  18  by feeder  20  into a solid-gas contactor  30  along with a predetermined amount of carbon dioxide gas under pressure from storage source  22  which is moistened in humidifier  24  that is kept supplied with water by means of water pump  26 . The carbon dioxide gas could be pure or mixed with air or other gases, depending on the source. The wetted alkaline ash is deposited on a continuously-moving “through-circulation screen-conveyor belt” in one example of a solid-gas contactor  30 , and pressurized, moistened carbon dioxide gas is introduced into the alkaline ash from below the perforated belt. Other examples of solid-gas contactors include rotary drum, rotary tray, and fluidized bed designs. In all cases, pressurized, moistened carbon dioxide gas is introduced into the solid-gas contactor type with the pre-wetted alkaline ash in appropriate amounts to convert oxides and hydroxides to carbonates. The solid-gas contactors are designed to maximize the contact between alkaline ash and carbon dioxide and to provide for continuous processing. 
     After reacting the alkaline ash with carbon dioxide gas, the carbonated ash is discharged from the solid-gas contactor  30  to a carbonated ash storage bin  32 . 
     DETAILED DESCRIPTION OF THE ALTERNATIVE EMBODIMENT 
     In  FIG. 2 , the alkaline ash and water are mixed exactly as described above. Also, the wetted ash is loaded and discharged from a conditioning bin  18  to a moist ash feeder  20 , and a predetermined amount of humidified carbon dioxide ( 22 ,  24 ,  26 ) is prepared as described above. In  FIG. 2 , however, the humidified carbon dioxide and wetted ash are brought into contact in a packed bed contactor  40 , known as a “batch” contactor. The humidified carbon dioxide is passed through the moist ash in the packed bed contactor from the bottom of the contactor. After carbonation, the packed bed reactor  40  is emptied, by rotating it on its axis, into a carbonated ash storage bin  32 . 
     REFERENCE NUMERALS 
     
         
         
           
               10  dry ash storage bin 
               12  dry ash feeder 
               14  water pump 
               16  mixer 
               18  conditioning bin 
               20  moist ash feeder 
               22  carbon dioxide under positive pressure 
               24  carbon dioxide humidifier 
               26  water pump to supply humidifier 
               30  solid-gas contactor (through-circulation screen-conveyer belt in this example) 
               32  carbonated ash storage bin 
               40  packed bed contactor (“batch” contactor)
 
Operation
 
           
         
       
    
     In operation one mixes alkaline ash with a predetermined amount of water to ensure that all ash particles are covered by a water film. In contact with water, metal oxides are “slaked” to form metal hydroxides (calcium is used as an example of the metal oxides):
 
CaO+H 2 O→Ca(OH) 2  
 
     The wetted ash, now containing metal hydroxides, is metered into a solid-gas contactor along with a predetermined amount of humidified carbon dioxide gas. The carbon dioxide dissolves in the water film surrounding the ash particle producing carbonate ions according to the following reaction.
 
CO 2 +H 2 O           2H + +CO 3   −− 

     As the carbon dioxide gas and ash particles mix in the solid-gas contactor (“continuous carbonation” methodology) or in the packed bed contactor (“batch carbonation” technology), the metal hydroxides are converted to carbonates calcium is used as an example of the metal hydroxides).
 
Ca ++ +CO 3   −−             CaCO 3  
 
H + +OH −           H 2 O

     When alkaline ash is reacted first with water and then with carbon dioxide and the alkaline metals and alkaline earth metals are converted to their carbonate forms, two chemical changes occur: 
     (1) the pH of ash is decreased to 8.7 or lower, and 
     (2) the soluble salt content, as measured by electrical conductivity, is reduced dramatically. 
     The contacting methods included bubbling carbon dioxide through water-saturated mixtures of wood ash (“static” and “once-through bubbling”), slurries of ash and water (“recycled bubbling”), and barely moistened preparations (“thin film”) of wood ash and water. The “thin film” approach was preferred as best results were achieved. With this approach less than 10% water was added by weight. In some tests 95 parts wood ash was mixed with 5% water as determined by weight. 
     As evidence of these two changes, the results obtained with “Pulp Mill Boiler Ash” (wood ash) are reported in the Table below. The wood ash was generated in the hog fuel boiler in the West Fraser Timber Co. Ltd. (formerly Weldwood of Canada) pulp mill in Hinton, Alberta, Canada. The pH and electrical conductivity measurements were conducted in the slurry or filtrate that was generated by thoroughly mixing solid (wood ash or carbonated wood ash) with distilled water at a weight ratio of 1 to 5. The pH and electrical conductivity measurements were done using appropriate meters. 
     The following shows a summary of the results. The pH of the wood ash was about 12.7, close to that of pure calcium hydroxide. When sufficient contact times were provided, the resulting average pH after carbonation was 8.3, with a standard deviation of 0.6. This corresponds closely to the pH of pure calcium carbonate. Therefore, the results support the validity of the postulated carbonation mechanism. 
     The average reduction in electrical conductivity was 8.4 dS/m, with a standard deviation of 3.4 mS/cm. This is statistically significant at a 95% confidence level. Therefore, we contend that the proof-of-concept has been successfully demonstrated. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 
               
             
             
               
                   
               
               
                 Summary of carbonation results 
               
             
          
           
               
                   
                 Initial** 
                 After Carbonation*** 
                 Reduction 
               
             
          
           
               
                   
                 pH 
                 EC 
                 pH 
                 EC 
                   
                 pH 
                 EC 
               
               
                 Contacting Method* 
                 — 
                 dS/m 
                 — 
                 dS/m 
                 Note 
                 — 
                 dS/m 
               
               
                   
               
             
          
           
               
                 1. 
                 Static Exposure 
                 12.7 
                 20.9 
                 8.5 
                 6.3 
                 A 
                 4.2 
                 12.9 
               
               
                   
                 Static Exposure 
                   
                   
                 10.0 
                 7.4 
                 A 
                 2.7 
                 11.8 
               
               
                 2. 
                 Once-through Bubbling(i) 
                   
                   
                 10.1 
                 10.7 
                 A 
                 2.6 
                 8.5 
               
               
                   
                 Once-through Bubbling(ii) 
                   
                   
                 9.4 
                 9.7 
                 A 
                 3.3 
                 9.5 
               
               
                   
                 Once-through Bubbling(iii) 
                   
                   
                 9.7 
                 7.9 
                 A 
                 3.0 
                 11.3 
               
               
                 3. 
                 Recycled Bubbling 
                 12.7 
                 17.5 
                 7.4 
                 15.8 
                 A 
                 5.3 
                 3.4 
               
               
                   
                 Recycled Bubbling 
                 12.7 
                 18.6 
                 8.9 
                 14.1 
                 A 
                 3.8 
                 5.1 
               
               
                   
                 Recycled Bubbling(iv) 
                 12.9 
                 19.7 
                 8.5 
                 10.6 
                 A 
                 4.2 
                 8.6 
               
               
                 4. 
                 Thin film (ground)(v) 
                 12.0 
                 16.5 
                 8.2 
                 12.0 
                 A 
                 3.8 
                 4.5 
               
               
                   
                 Thin film (unground)(vi) 
                 11.6 
                 18.6 
                 8.0 
                 11.7 
                 A 
                 3.6 
                 6.9 
               
               
                   
                 Thin film (ground)(vii) 
                 12.0 
                 16.5 
                 8.3 
                 12.1 
                 B 
                 3.6 
                 4.4 
               
               
                   
                 Thin film (unground)(vii) 
                 12.0 
                 16.5 
                 8.1 
                 11.5 
                 B 
                 3.9 
                 5.1 
               
               
                   
               
               
                 *Contact times: i) 5 min, ii) 15 min, iii) 20 min, iv) 1 hr, v) 2 hr, vi) 3 hr, vii) 8 hr. 
               
               
                 **Based on average values of un-carbonated ash. 
               
               
                 ***A: 100% CO 2 ; B: 20% CO 2   
               
             
          
         
       
     
     CONCLUSIONS 
     We have demonstrated the use of carbon dioxide gas to reduce the pH and electrical conductivity of wood ash. Carbonated wood ash, with its lower pH and electrical conductivity values, is of more value as a soil amendment and fertilizer. Although carbonation is a recognized process in the scientific and engineering literature, it has been applied to few industrial products. We observe that:
         1. Carbonation reduces the level of soluble salts of wood ash residue by 30%-40%.   2. Carbonation reduces the level of pH of wood ash residue to below 8.7 or less.   3. “Thin film” carbonation, or a method for carbonating mineral matrices at very low moisture contents—less than 10% water as determined by weight gave the best results and was also more environmentally friendly and efficient.       

     One of the current limitations to the spreading of wood ash on land as a liming agent or fertilizer is its excessive content of soluble salts. This carbonation technology reduces soluble salts and would remedy that limitation. The “thin film” carbonation using minimal water eliminates the need to treat or dispose of wastewater after carbonation, or the need to dry wood ash prior to or after pelletization. This carbonation technology, due to the minimization of water in the process, is cheaper and more efficient than other systems of carbonation. Carbonation is a means of sequestering carbon dioxide from the atmosphere into a stable, solid product. Although eventually the carbonate form in the solid product will decompose, upon acidification, to carbon dioxide again, it could be centuries or even millennia before carbon dioxide is re-emitted. If carbonated wood ash is applied to soil, re-emitted carbon dioxide will be taken up by growing plants as a part of photosynthesis and stabilized again. Hence, wood ash carbonation is a means of reducing one of the principal components of climate change. 
     Summary of Findings 
     The carbonation levels required to achieve a successful result are difficult to quantify. They depend upon a number of factors. One factor is the pH and electrical conductivity of the ash prior to treatment. Another factor is the quantity of soil that the ash will be mixed with after treatment. Will it be mixed in a 2000 parts soil to 1 part ash or will it be mixed in a 200 pails soil to 1 part ash. A further factor is the pH and electrical conductivity of the soil with which the ash is to be mixed. Having noted these factors, the objective is to achieve full carbonation, or near to it, in order to get to desired pH and electrical conductivity levels. There may be circumstances in which full carbonation is not practical, whereas 80% carbonation achievable. Any carbonation level of 80% or more should, therefore, be considered to be “substantially” carbonated. There may be circumstances where you have reached both your desired pH level and electrical conductivity level targets and there appears to be little point in continue with further carbonation. It is important that both pH and electrical conductivity targets be reached. An excessively high electrical conductivity level will be harmful to plants in and of itself. One can attain a desirable pH level and still have an excessively high electrical conductivity level. Although it is preferred that one monitor both pH and electrical conductivity, when electrical conductivity targets are reached there is also a reduction in pH. In the tests that were performed the electrical conductivity targets was under 15 dS/m. When the wood ash residue is substantially carbonated, a realistic target is a 30%-40% reduction in electrical conductivity. When wood ash residue is substantially carbonated, a realistic target for pH is 8.7 or less. Carbonation is a matter of exposure to carbon dioxide over time. Virtually any concentration of carbon dioxide can be used, but the time needed to achieve full carbonation increases as the concentration of carbon dioxide is reduced. This is demonstrated by the two graphs appearing in  FIGS. 3 and 4 . The graph in  FIG. 3 , entitled “Dry Dilute Carbonation,” was performed with a carbon dioxide concentration of 20.6%. The graph in  FIG. 4 , entitled “Thin Film Carbonation,” was performed with 100% carbon dioxide. From this a conclusion has been reached that in order to achieve desired carbonation targets within a reasonable time one should use a carbon dioxide concentration of at least 40%. Although carbonation can be performed with carbon dioxide concentrations of less than 40%, it is not viewed as being efficient in view of the time that is required to achieve full carbonation. 
     Advantages:
         the method provides a safe and economical method of utilizing large quantities of wood ash residue as a soil amendment, that is presently disposed of in land fills;   the method can use waste combustion gases rich in carbon dioxide and addresses atmospheric environmental concerns by reducing greenhouse gas emissions.       

     In this patent document, the word “comprising” is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements. 
     It will be apparent to one skilled in the art that modifications may be made to the illustrated embodiment without departing from the spirit and scope of the invention as hereinafter defined in the Claims.