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Section Five SOLIDIFICATION, FOUNDRY AND COMBUSTION RESIDUES 36 THE USE OF FLY ASH AND PORTLAND CEMENT TO CHEMICALLY FIX METAL MINE DRAINAGE TREATMENT SLUDGES Craig C. Hustwit, Civil Engineer U. S. Bureau of Mines Pittsburgh Research Center Pittsburgh, Pennsylvania 15236 INTRODUCTION The conventional treatment of metal mine drainages produces a sludge consisting principally of metal hydroxides and oxides. The volume of these sludges can be substantial, commonly rang- ■ ing from 5% to 10% of the volume of mine drainage treated. Sludge volumes of 33% of the treated water volume have been reported.' In addition, mine drainage treatment sludges are pH- sensitive. Exposure to even mildly acidic environments can cause the solid-phase metal hydroxides and oxides to dissolve and mobilize metals. Figure 1 illustrates the pH-phase relationship of several metals commonly found in metal mine drainages. High volumes and chemical instability make mine drainage sludge disposal a challenging engineering problem and an expensive component of mine drainage treatment. Sludge generated during the treatment of mine drainage typically has a high water content and low density. Sludge density is normally expressed in % solids. Sludges separated from treated water in settling ponds have 0.5% to 4.5% solids.1 Clarifiers produce sludges with 1% to 7% solids. A dewatering operation is often used to decrease the sludge volume prior to disposal. Lagoons can serve as sludge settling units, dewatering units, and permanent disposal sites.1 They are relatively inexpensive where land values are low. Sludges dewatered in lagoons have between 0.5% and 20.0% solids.1 This wide density range reflects differences in lagoon design parameters, initial mine drainage quality, and the neutralizing reagent used. Sludge disposal in lagoons is problematic since mine drainage treatment sludges possess thixotropic properties and can liquify if subjected to vibrations.1 Sand filters with underdrains can produce a sludge cake with densities ranging from 15% to 25% solids.' However, removing the sludge cake from sand filters with earth moving equipment can be difficult. Vacuum filters produce sludge cakes with solids between 8.8% and 30.9%.' All sludge dewatering methods add to the total treatment costs. The high-density sludge (HDS) process was developed by Bethlehem Mines Corporation. In this process, a split of sludge is recirculated, often by mixing it with the lime slurry prior to the neutralization step. HDS treatment systems generate sludges having between 15% and 40% solids.'-2 The probability that metals will leach from a mine drainage sludge in a landfill is principally dependent on the ambient pH and the metals present in the sludge. If the environment becomes acidic but the sludge is well-buffered, then the onset of leaching may be delayed or even prevented. There is evidence that some metal hydroxides and oxides become less pH-sensitive as they age.3 Some reagents, such as hydrated lime, do not completely dissolve in water. When this occurs, the undissolved fraction setties with the sludge and buffers the sludge at a high pH. There are no commonly used treatment technologies or methods to convert the metal hydroxides and 50th Purdue Industrial Waste Conference Proceedings, 1995, Ann Arbor Press, Inc., Chelsea, Michigan 48118. Printed in U.S.A. 329
Object Description
Purdue Identification Number | ETRIWC199536 |
Title | Use of fly ash and Portland Cement to chemically fix metal mine drainage treatment sludges |
Author | Hustwit, Craig C. |
Date of Original | 1995 |
Conference Title | Proceedings of the 50th Industrial Waste Conference |
Conference Front Matter (copy and paste) | http://e-archives.lib.purdue.edu/u?/engext,45474 |
Extent of Original | p. 329-338 |
Collection Title | Engineering Technical Reports Collection, Purdue University |
Repository | Purdue University Libraries |
Rights Statement | Digital object copyright Purdue University. All rights reserved. |
Language | eng |
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Description
Title | page 329 |
Collection Title | Engineering Technical Reports Collection, Purdue University |
Repository | Purdue University Libraries |
Rights Statement | Digital copyright Purdue University. All rights reserved. |
Language | eng |
Type (DCMI) | text |
Format | JP2 |
Capture Device | Fujitsu fi-5650C |
Capture Details | ScandAll 21 |
Transcript | Section Five SOLIDIFICATION, FOUNDRY AND COMBUSTION RESIDUES 36 THE USE OF FLY ASH AND PORTLAND CEMENT TO CHEMICALLY FIX METAL MINE DRAINAGE TREATMENT SLUDGES Craig C. Hustwit, Civil Engineer U. S. Bureau of Mines Pittsburgh Research Center Pittsburgh, Pennsylvania 15236 INTRODUCTION The conventional treatment of metal mine drainages produces a sludge consisting principally of metal hydroxides and oxides. The volume of these sludges can be substantial, commonly rang- ■ ing from 5% to 10% of the volume of mine drainage treated. Sludge volumes of 33% of the treated water volume have been reported.' In addition, mine drainage treatment sludges are pH- sensitive. Exposure to even mildly acidic environments can cause the solid-phase metal hydroxides and oxides to dissolve and mobilize metals. Figure 1 illustrates the pH-phase relationship of several metals commonly found in metal mine drainages. High volumes and chemical instability make mine drainage sludge disposal a challenging engineering problem and an expensive component of mine drainage treatment. Sludge generated during the treatment of mine drainage typically has a high water content and low density. Sludge density is normally expressed in % solids. Sludges separated from treated water in settling ponds have 0.5% to 4.5% solids.1 Clarifiers produce sludges with 1% to 7% solids. A dewatering operation is often used to decrease the sludge volume prior to disposal. Lagoons can serve as sludge settling units, dewatering units, and permanent disposal sites.1 They are relatively inexpensive where land values are low. Sludges dewatered in lagoons have between 0.5% and 20.0% solids.1 This wide density range reflects differences in lagoon design parameters, initial mine drainage quality, and the neutralizing reagent used. Sludge disposal in lagoons is problematic since mine drainage treatment sludges possess thixotropic properties and can liquify if subjected to vibrations.1 Sand filters with underdrains can produce a sludge cake with densities ranging from 15% to 25% solids.' However, removing the sludge cake from sand filters with earth moving equipment can be difficult. Vacuum filters produce sludge cakes with solids between 8.8% and 30.9%.' All sludge dewatering methods add to the total treatment costs. The high-density sludge (HDS) process was developed by Bethlehem Mines Corporation. In this process, a split of sludge is recirculated, often by mixing it with the lime slurry prior to the neutralization step. HDS treatment systems generate sludges having between 15% and 40% solids.'-2 The probability that metals will leach from a mine drainage sludge in a landfill is principally dependent on the ambient pH and the metals present in the sludge. If the environment becomes acidic but the sludge is well-buffered, then the onset of leaching may be delayed or even prevented. There is evidence that some metal hydroxides and oxides become less pH-sensitive as they age.3 Some reagents, such as hydrated lime, do not completely dissolve in water. When this occurs, the undissolved fraction setties with the sludge and buffers the sludge at a high pH. There are no commonly used treatment technologies or methods to convert the metal hydroxides and 50th Purdue Industrial Waste Conference Proceedings, 1995, Ann Arbor Press, Inc., Chelsea, Michigan 48118. Printed in U.S.A. 329 |
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