waste stabilization

Fly Ash For Use in Waste Stabilization

Questions Regarding Project ( Courtesy of Robert L. Smith of Resource Materials Testing in Murphy, NC USA):

1.What kind of additive is available to mix with the industrial waste? A Class F fly ash is a good bulking agent that does not harden by itself. Most Class F fly ashes have a pH < 11 which may prevent ammonia release. However you might need a considerable amount of this ash (and therefore a volume increase) to provide load-bearing strengths. Does the ash contain carbon which could reduce the leaching of organic compounds? Is hydrated lime or a lime waste available to react with the Class F ash to produce a pozzolanic reaction. That would provide strength.

On the other hand, many Class C fly ashes have a higher pH and are self-cementing. The pH of the final mixed product is generally below 11.5, which is sufficient to prevent the solubilization of amphoteric metals. There will be a temperature rise and potential volume increase due to ettringite formation.

Hydrated lime will react with Class F fly ash providing long term strength without temperature rise. But the pH increases to 12.5 until the lime is consumed lowering the pH. Quicklime produces more temperature rise. There will be a volume increase.

Portland cement provides lime to react with fly ash. This kind of solidification provides good strength gain at a relatively low addition rate and minimizes the volume increase while moderating the pH increase and temperature rise. One can achieve low permeability. This combination is effective in the production of the grouts used in the disposal of radioactive wastes.

FBC or dry scrubber ashes (FGD) are good in solidification in that these high surface area materials contain quicklime and sulfur. One can achieve a rapid strength gain due to ettringite formation at a relatively low addition rate. The pH of these materials generally decrease over time as the lime is consumed.

Cement or lime kiln dusts are finely-divided materials that have available lime (a good indicator of solidification potential) . These dusts will provide good strength gain at a relatively low dose rate and correspondingly low volume increase. These materials are highly variable, lead to temperature increase, higher pH and are of limited availability.

2. Is the industrial waste safe to handle by laboratory personnel? How hazardous is it? What are the hazardous constituents? What precautions must be taken?

3. Where is the site? What is available nearby? Only a few additives are economical to deliver to a given area. A small pit may require only a few tons of additive to solidify. The cost of the additive would be small compared to other project costs such as mobilization. On larger projects the cost of the additives predominates. Will the additive be available when it is needed?

4. What are the solidification objectives? What remedial action has been recommended? Must the waste be taken to a hazardous waste landfill? Then a low level of treatment is required. The solidified material must pass the Paint Filter Test before admission to the landfill.

In-place closure is an option for most non-harardous wastes and perhaps some harardous wastes. A typical site is solidified in situ and covered with a clay cap. Thus the solidified material must have a load-bearing capacity. In-place closure might be allowed only if certain metals or organic compounds are rendered to some level of insolubility (fixation). The permeability, strength, and leachability of the solidified mass are important factors. What is the rate of leaching for a given constituent and its impact on the environment?

5. Which additive performs best with the waste? A small volume of waste (100 cc) is used in each trial in the laboratory. What early unconfined strength increase is required? Graphs of C 109 cube strengths are informative. Many jobs require that the equipment be on top of the solidified mass in a few days.

What is the volume increase? Volume increase provides an indication of how much expansion could be expected during the solidification process. Will the solified waste be trucked offsite where space is at a premium?

For permeability tests, a larger amount of waste is required. The mixtures of additive and waste are mixed in a Hobat mixer and compacted in a Proctor mold with a falling head permeability attachment.

A number of leachate tests are available including the EPA EP Toxicity Test, TCLP, the California Waste Extraction Test, the Texas 7-day distilled water leachate test, etc.

6. What is the effect of the pH of the leachate after extraction? It is known that the solubility of metals is a function of pH. In order to pass the EPA Ep Test, the sludges must contain enough basicity to neutralize the acid extracting fluid so that the pH of the leachate after extraction is that of the minimum solubility of the metal hydroxide. If there is not enough basicity, then the metal concentration in the acidic leachate is too high. However if the extracting fluid is rainwater or groundwater, the resulting pH of the leachate may be above the value of minimum solubility. The concentration of the metal goes back up. Thus these metal hydroxides have formed the soluble hydroxyl compounds.

Determine the pH of the TCLP extract over time (0-100 days) using Class F fly ash, Class C fly ash, CKD, LKD, Portland cement, lime, etc.

PROCEDURES: Determine density (lbs/ft3) of the sludge using a 6 oz. (198 cc) styrofoam cup. Multiply the g/cc by 62.4.

Weigh a sample of sludge equivalent to 100 cc in a tared plastic 400 cc beaker. Weigh the additive in a tared styrofoam cup. Typical amounts of additive are 11.4, 22.8, and 34.2 g for cement or quicklime. This corresponds to .1/1, .2/1 and .3/1 tons/yd3. Add the additive to the sludge in the 400 cc beaker and mix thoroughly using a blade spatula. Transfer all the mixture back to the styrofoam cup, scraping the plastic cup thoroughly to get as much of the material as possible. Tap the sides of the styrofoam cup lightly to remove air voids or if more solid, use a tamping rod to compact the material slightly using three lifts to achieve good consolidation. Place a tight fitting lid on the styrofoam cup to prevent drying of the solidified sludge. Place a small amount of silicone grease in the small hole of the lid to prevent evaporation.

After 24 hours and 7 days of curing measure the unconfined compressive strength (using a soil pocket penetrometer; read to the nearest 0.1 tons/ft2 and multiply by 13.9 to obtain psi) and determine the volume increase and density.

For volume increase, remove the lid from the cup containing the solidified sludge and place the cup on a balance which has been tared with an empty cup. Fill the cup as full as possible and weigh. Vol. Increase,% = 98 +weight sludge + weight of additive-(wt. of solidified sludge+water).

Density in g/cc= (wt. sludge + wt.additive)/100 + vol. increase percent). Multiply by 62.4 to obtain lb/ft3.

See Ash Development Association of Australia-Literature. Solidification and Stabilisation of Wastes Using Fly Ash