Technical Papers and Articles

»Environmental Effects of Lime

Summary

The environmental effects of calcium oxide and calcium hydroxide are minimal when properly used. Calcium oxide (quicklime) reacts promptly with water to form calcium hydroxide (hydrated lime). Recarbonation by air or bicarbonate ion in surface waters converts calcium hydroxide to calcium carbonate, the same mineral in limestone. The high pH (12.45 @ 25°C) is naturally reduced to the 7.8 to 8.5 range. Normal precautions of working with chemicals must be taken to protect workers and the environment, but the reactions of lime moderate its effects. The principal hazards are (1) high temperatures of hydrating calcium oxide, (2) the irritating nature of lime dust to the eyes and lungs, and (3) alkaline chemical burn to mucous membranes and the eyes if splashed lime (dust or slurry) is not promptly washed off.

BACKGROUND

Calcium oxide (CaO, quicklime) is an effective agent for soil stabilization and aggregate modification as well as drying up muddy construction sites and a wide variety of alkaline chemical neutralization applications. Calcium hydroxide (Ca(OH)2 as hydrated lime) is effective as an additive to hot mix asphalt to improve adhesion between the cement and the aggregate, to reduce stripping, rutting, moisture damage and age hardening and to increase the retained tensile strength. This review of the chemistry of quicklime and hydrated lime explores the health and environmental effects.

PRINCIPAL REACTIONS - THE LIME CYCLE

Lime (CaO) is a product of high temperature calcination of pure limestone. It is a white, low density, porous solid which reacts more or less vigorously with water. Hydrated lime (Ca(OH)2) is a fine white powder with limited solubility in water.

1) Calcination of limestone: (Typically ~2000°F for calcite)
CaCO3 = CaO + CO2
Limestone or calcite (calcium carbonate) = Quicklime (calcium oxide) + Carbon Dioxide

2) Hydration of quicklime:
CaO + H2O = Ca(OH)2 + Heat (490 BTU/lb. CaO)
quicklime + water = hydrated lime (calcium hydroxide) + 272.8 cal/gram CaO

3) Recarbonation of calcium hydroxide:
Ca(OH)2 + CO2 = CaCO3 + H2O
calcium hydroxide + carbon dioxide = calcium carbonate + water

In most applications, excess calcium hydroxide more or less promptly reacts with carbon dioxide from the air or bicarbonate ion in water to form calcium carbonate.

Ca(OH)2 + HCO3-1aq = CaCO3 + OH-1aq + H2O
calcium hydroxide + bicarbonate ion = calcium carbonate + hydroxyl ion + water

4) Dissociation of calcium hydroxide
Calcium hydroxide has a very limited solubility in water (~1.5 g/L at 20°C) which decreases at higher temperatures. This controls the alkalinity and pH of water in contact with hydrated lime. The limited solubility restricts the amount of material that can be dissociated in the water, thus limiting the amount of hydroxyl ion available to raise pH.

Ca(OH)2 = Ca+2aq + 2 OH-1aq
Calcium hydroxide = Calcium ion + 2 Hydroxyl ions

LIME IN SOIL STABILIZATOIN

Quicklime is commonly used to stabilize clays through the pozzolanic reactions. Pozzolanic reactions convert the clays the cement-like compounds, initially through ion exchange and subsequently, through fundamental rearrangement of the alumino-silicate mineral structure. These reactions start at pH 12 and above and consume water and lime in the course of the reaction, resulting in a reduction both of moisture content and pH. These reactions have been in use since Roman times in a variety of construction applications. The quicklime can be applied dry or as a slurry, but the pozzolanic reactions all involve hydrated lime.

SAFETY AND HEALTH CONSIDERATIONS

A search of the scientific, health and safety literature on effects of calcium oxide and calcium hydroxide has yielded the following information:

- Exposure to calcium oxide dust is regulated. The OSHA PEL (Permissible Exposure Limit) for 8 hour TWA (Time Weighted Average) is 2 mg/m3. The ACGIH also recommends a TLV (Threshold Limit Value) of 2 mg/m3 as do fifteen nations.

- Exposure to calcium hydroxide dust is regulated. The OSHA PEL (Permissible Exposure Limit) for 8 hour TWA (Time Weighted Average) is 5 mg/m3. The ACGIH also recommends a TLV (Threshold Limit Value) of 5 mg/m3 as do fifteen nations. For comparison, the TLV or TWA for nuisance dusts or particulates not otherwise classified (PNOC) is 10 mg/m3, only two times the hydrated lime standard.

- Quicklime is not a hazardous substance under federal highway, rail, water-borne transportation regulations. It is covered by DOT regulations as an ORM-B compound for air transportation. This limits the shipment to 25 lbs or less unless special provisions are taken.

- Hydrated lime is not a hazardous substance under federal highway, rail, water-borne or air transportation regulations.

- Neither quicklime nor hydrated lime is a hazardous waste according to the RCRA 40 CFR 261 criteria.

- California regulations also regard materials under pH 12.5 as non-hazardous. The Corrosivity Criteria, Section 66708 in the California Administrative Code(CAC) , states:
“A waste, or a material, is corrosive and hazardous if ...its mixture with an equivalent weight of water produces a solution having a pH less than or equal to 2 or greater than or equal to 12.5.”

- Both quicklime and hydrated lime are hazardous substances as determined by the CAL/OSHA Director’s List of Hazardous Substances.

- Calcium hydroxide (as 100% dry powder) is classed as a “severe irritant or corrosive” to rabbit eye tissue based on the Draize test.

- Neither calcium oxide nor calcium hydroxide is corrosive according to federally specified tests. Specifically, “No sign of dermal corrosion was noted at any of the treated sites (rabbit skin). Based on these results, the test material (concentrated calcium hydroxide slurry) is considered non-corrosive to the skin when applied as received.”

- Both calcium oxide and calcium hydroxide are on the Food and Drug Administration’s (FDA) list of chemicals that are Generally Recognized As Safe (GRAS) for human consumption in foodstuffs.

- Quicklime and hydrated lime are not carcinogenic, teratogenic or mutagenic according to federal government tests.

ENVIRONMENTAL CONSIDERATIONS

Calcium oxide reacts with water or water vapor to form hydrated lime. Hydrated lime reacts with carbon dioxide or carbonate ions, forming sparingly soluble calcium carbonate (calcite). Any excess hydrated lime in the environment is naturally converted to harmless minerals. The normal pH of lime slurry (pH = 12.4 @ 25°C) is reduced as recarbonation converts the hydrated lime to calcite (pH ~ 7.8 - 8.3). The excess lime concentration and the access to carbon dioxide determine how rapidly the pH drops. Good exposure to air achieves near complete recarbonation in a few days.

AIR EMISSIONS

Air emissions of quicklime and hydrated lime can occur during dry transfers.

Transfer Operation: Unloading quicklime or hydrated lime from a bulk truck to a storage silo

Dust Control Measure: Baghouse on silo; Good housekeeping and unloading procedures by trucker.

Transfer Operation: Transferring quicklime or hydrated lime to the slurry mixer

Dust Control Measure: Covered screws or belts; enclosed mixer; water spray to knock down and entrain any dust.

Transfer Operation: Dust generated from aggregate handling of cured aggregate stockpile material

Dust Control Measure: Generally none except 1) Most of the lime has reacted or is tightly adherent as a partially recarbonated film; 2) Residual moisture on the aggregate; 3) Minimizing handling and drop heights.

SOIL CONTAMINATION EFFECTS

Handling lime treated aggregate inevitably generates some mineral fines through abrasion and crushing. This material will commonly contain calcium carbonate and calcium hydroxide. The calcium carbonate behaves like fine-grained limestone, a natural material that buffers groundwater pH and has a low solubility, causing water hardness. Calcium hydroxide has a higher solubility and pH. However, natural reactions in soils consume this alkaline material.

Calcium hydroxide reacts with a wide variety of finely divided siliceous materials (pozzolans) to form cement compounds. These “pozzolanic reactions” are the basis for soil stabilization construction practices that have been in use at least since Roman times. Pozzolanic materials include volcanic ash, clays, some glasses, fly ash, and many pulverized siliceous rocks.

Mixing a lime material with a soil can start the pozzolanic reactions. This will convert the clays (and other pozzolanic materials) in the soil to a cement type compound. These reactions consume water just as hydrating and curing a portland cement concrete does. The typical product is a poorly crystalline zeolite-type mineral found in cured concrete, Tobermorite, Ca5H2(Si3O9)2•4H2O. As the calcium hydroxide is consumed, the pH drops as the hydrous calcium aluminosilicate minerals form. The stable pH is comparable to concrete until the outer portion has developed an impermeable silicate/carbonate layer.

In the absence of pozzolanic materials, the calcium hydroxide undergoes recarbonation reactions as discussed above. This lowers the pH from the high 12.4 range of hydrated lime to the 7.8 to 8.4 range of limestone. This reaction is generally limited by the availability of carbon dioxide. This mitigates the long term effects of applying excess calcium hydroxide to a soil or water exposed to air.

WATER POLLUTION CONCERNS

Naturally, every effort should be made to prevent contact with water and control runoff from the limed aggregate stockpiles. However, the reactions involving lime, water, air and soil materials ameliorate the effects of hydrated lime contamination.

Water that is saturated with calcium hydroxide will have a pH of 12.4. However, the low solubility means that rain water falling on a treated aggregate pile are unlikely to dissolve enough lime to significantly raise the pH of commingled run-on/runoff water from adjacent sites. Each ten-fold dilution by water brings a decrease of one pH unit for the mixed waters.

Calcium hydroxide in runoff water will initiate pozzolanic reactions with clays or muds in suspension or on the stream bed. This consumes the calcium hydroxide and lowers the pH. Carbonate and bicarbonate ions in rainwater and surface waters will also consume calcium hydroxide through standard water softening reactions:

Ca(HCO3)2,diss + Ca(OH)2,aq = 2 CaCO3,ppt + H2O
Calcium bicarbonate + Calcium hydroxide = Calcium carbonate + Water

SAFETY CONCERNS

Refer to a current MSDS for legally mandated information.

Principal hazards associated with lime slurry are:

- Personnel Safety: Dust exposure to the eyes or respiratory tract.

- Personnel Safety: Splashes of lime slurry to the eyes.

- Site Hazards: Slick surfaces from wet lime or lime slurry spills.

- Site Hazards: Lime dust from vehicles driving through dry spills.

- Transportation Hazards: Spills resulting from traffic or loading accidents.

- Environment Protection: Discharge to streams or bodies of water.

PERSONAL SAFETY

The principal hazards associated with quicklime, hydrated lime and lime slurry are:

- Thermal burns from contact with hydrating quicklime.

- Thermal burns from spills or splashes of hot lime slurry when made from quicklime.

- Chemical burns through extended contact with lime dust or concentrated lime slurry.

- Dust irritation from quicklime or hydrated lime dust exposure to eyes, mucous membranes or sweaty skin.

- Dust irritation from disturbed dried deposits of lime slurry.

Thermal burns can be minimized by proper handling. Avoid contacting hydrating quicklime or equipment in which the hydration reaction occurs. Avoid getting quicklime on your clothes or body. Lime slurry produced from quicklime is in the 210° - 190°F (100° - 88°C) temperature range. The temperature remains high in large tank storage, but drops with time and as it is handled in pipes, pumps and smaller vessels. Lime slurry produced from hydrated lime remains at the water temperature.

First Aid: If a person is exposed to lime dust, remove them from that situation, remove the dusty clothes and wash off the lime with copious amounts of water. Follow the provisions on the MSDS.

If a person is exposed to large amounts of hot lime slurry, remove the wet clothing and wipe or wash off the slurry. Be especially careful to remove and wash slurry from the eyes first. Treat any thermal burns using normal burn treatment. Wash lime from any burn area. Obtain medical care.

Chemical burns are rarely associated with lime slurry. Quicklime (calcium oxide) is more commonly involved in chemical lime burns than calcium hydroxide or lime slurry. Lime slurry has a pH of 12.4 and is not a hazardous waste by the RCRA Corrosivity Criteria. Lime slurry is not corrosive to intact skin, but should be washed off as a regular prevention.

First Aid: Lime slurry splashes on the skin should be washed off. A skin lotion may be applied to counteract the drying tendency of calcium hydroxide. Wash lime out of any cut or laceration.

Lime dust is an irritant to the eyes and mucous membranes. Control of lime dust during handling of dry lime products should be an integral part of any plant operations involving lime.

Lime dust is not generated in normal slurry handling operations. However, if a lime slurry spill is permitted to dry, calcium hydroxide cake forms. Traffic and handling over dried slurry solids may generate calcium hydroxide dust. This can be an irritant if breathed or if it contacts the eyes. The OSHA PEL is 5 mg/cu.m. Spills should be cleaned up promptly.

First Aid: Wash off all lime material. Flush the eyes with clean water or eye wash solution. Obtain prompt medical attention.

SAFETY EQUIPMENT

Standard safety equipment for each person working in the slurry area should include:

- Safety glasses with side shields or safety goggles,

- Dry lined gloves for those handling slurry pipes or hoses.

- Rubber boots with safety tread for work areas where slurry might be spilled.

- Dust masks for those working in quicklime or dry hydrate handling areas and on clean up duty. Dust masks of OSHA category, N95, are appropriate, unless the dust concentration is very high.

- Install eye wash stations convenient to the lime slurry handling and operations areas. A safety shower is recommended, but not required.

SPILL PREVENTION AND CONTROL

The best control is spill prevention. Training and qualifying trucking firms and drivers minimizes accident and spill opportunities. Lime products, including lime slurry, are not hazardous materials under US DOT regulations for surface transport and placarding transport containers is not required.
Should an accident or spill occur, follow these guidelines:

- Remove persons from the accident scene to a safe place.

- Render first aid as appropriate.

- Remove any dry lime material in contact with a person by brushing or wiping the material away.

- After removal of most of the lime, wash the body parts exposed to the lime. Especially flood the eyes with water to dilute and remove the calcium hydroxide.

- Notify emergency personnel.

- Reroute or stop traffic from driving through spilled lime slurry. Lime slurry is slick and will reduce traction, possibly causing accidents.

- Contain the spill. Prevent people from contacting the lime. For quicklime, do not dispose large quantities to landfills without first fully slaking the lime (hydrating it to calcium hydroxide). Enough heat can be generated in special conditions to ignite flammable materials. Follow the MSDS provisions.

- Contain the spill. Block storm drains and access to streams using materials at hand, such as dirt, hay bales, plastic, etc. High solids slurries are viscous, assisting leak control.

- Clean up the spilled material with a front-end loader, vacuum truck or shovels, being careful to avoid excessive dust exposure.

- The small amount of solid material remaining at the spill site after clean up may, with permission, be flushed to sewer with sufficient water to reduce the pH below 9.5. or

- The solids may be swept up and disposed of at a landfill. or

- It may be permissible to dry and recarbonate to limestone dust, CaCO3.

FOR QUICKLIME SPILLS

- Do not add water to large quantities of quicklime. Clean up the bulk of the material in a dry form. Do not dispose of quicklime with flammable materials.

- Appropriate disposal of spilled quicklime depends on the local situation. It may be used in applications like soil stabilization, drying up construction sites, or waste acid neutralization. It may be necessary to return it to the production facility or terminal.