operation oaxaca: welcome to reality.
the economics of AMD, part one. (032)
this is carlos.
you will notice that i would prove that my original thesis of filtering metals for a profit was wrong.
so wrong.
turns out the economics gave me the middle finger.
and it pointed me in the right direction.
the time has come.
i will show you the economics of both routes.
the economics is king.
everything else is second.
proof: i was wrong.
acid mine drainage (AMD) → acidic water from mining areas is exposed to open air & water… along with a rock called pyrite.
they can be found in gold, silver, and coal mines.
pyrite is the rock people call “fool’s gold”.
pyrite is composed of: FeS2, iron sulfide.
two sulfur molecules, one iron molecule.
the pyrite rock can be found in the gold, silver or coal mines.
and that iron sulfide converts to sulfuric acid via this reaction (for iron in this case):
and the acid mine drainage color is related to this, along with the pH of the water.
but why the red-orange color of acid water?
because at low pH values like one or two, there’s no color…
because the metal ions/minerals are dissolved within the water.
but because some minerals precipitate (aka pH goes up)… the water begins to turn color.
that color is the ions being removed/precipitated from the water and going to the river’s bottom.
and when pH goes up, H+ ions decrease and OH- ions increase.
H+ → hydrogen ions.
OH- → hydroxide ions.
hydroxide ions can form insoluble metal hydroxides through a process known as precipitation.
and the pH varies from element to element.
for iron, it’s pH 3 when we see iron becoming precipitated.
as a result, the pH of acid mine drainage water is about pH 4 or lower.
and this is no bueno economically and for the environment.
this was a warning back in 2013.
about >40 hardrock mines that will generate an estimated 17–27 billion gallons of polluted water yearly.
this includes:
- perpetuity
- costly water treatment.
so AMD must be neutralized… aka to raise the pH from three to seven with an alkaline material.
some common ones include caustic soda, quick lime, hydrated lime, and limestone dust.
all of them contain calcium.
and as AMD neutralizes to pH 7, an iron-rich floc is formed.
floc → a thick, floating clump of “flocculent material” interacting with wastewater.
flocculent material → a small mass formed by gathering fine suspended particles in a liquid.
this floc is collected & begins forming a sludge.
it is stored in a clarifier or a sedimentation pond.
ok… but now what?
what do we do with this iron-and-calcium-rich sludge?
iron and aluminum in sludge are results of the pH neutralization process of acid mine drainage water when both are dissolved metals & become the precipitate.
the 1st thing to do is dewater it.
that can be done via geotubes.
because even if we decide to transport the sludge to a new location…
removing excess water lowers costs.
when AMD has its water removed from it…
the solid part of the sludge will be 20%–30% of the original content.
it will be composed of a mixture of calcium, iron, & aluminum hydroxides.
side tangent:
this is done via active treatment water systems.
active systems work by neutralizing the AMD with an alkaline agent such as caustic soda, hydrated line, quick lime, or limestone dust.
the aluminum comes from limestone dust, & some silica. (chemical composition)
but who needs calcium, aluminum, and a shitload of iron oxides & hydroxides?
turns out, there are three people:
- ppl who need minerals.
- ppl who need cement.
- ppl who need paint.
the most profitable one?
cement.
specifically, portland cement.
this is an ideal portland cement’s composition (in crystalline phases + chemicals):
cement by weight → CBW
tricalcium silicate (C3S) would be 50% of CBW [3CaO x SiO2]
dicalcium silicate (C2S) would be 25% of CBW [2CaO x SiO2]
tricalcium aluminate (C3A) would be 12% of CBW [3CaO x Al2O3]
tetracalcium aluminate (C4AF) would be 08% of CBW [4CaO x Al2O3 x Fe2O3]
gypsum (CSH2) would be 03.50% of CBW [CaSO4 x H2O]
portland cement contains about max 06% of ferric oxide (Fe2O3), ferric iron is Fe³⁺.
and ferric iron is present in the AMD sludge.
so what if we wanted to add part of the sludge into the cement?
we could, in theory…
the amount we should add before firing in cement slurry can be calculated with the bogue equation.
firing → supply of heat to a kiln by burning fuel. the fuel can be either oil, natural gas, or propane.
bogue equation.
the bogue equation is used to:
- determine the approximate proportions of the ‘four main clinker materials’ in portland cement.
- clinker bulk analysis (chemical composition)
- figure out ‘free lime’ material to calculate mineral proportions (i.e. calcium, iron, aluminum, other minerals present)
clinker → limestone + other minerals to create the primary materials for cement production… heated to about 1400°C — 1500°C in a rotary kiln.
the bogue calculation utilizes four clinker materials:
- C3S, tricalcium silicate
- C2S, dicalcium silicate
- C3A, tricalcium aluminate
- C4AF, tetracalcium aluminate
these clinker materials will be mixed with small amounts of gypsum to make the cement we utilize daily.
the gypsum is what allows us to control how fast cement hardens to provide mechanical/compressive strength.
when gypsum is added to cement, it will react with C3A molecules to form ettringite.
ettringite is a calcium sulfoaluminate hydrate.
it forms a protective coating on the C3A molecules… which slows down the hydration of these molecules and delays their hardening time.
and this is what happens if we do not add any gypsum:
the C3A reacts with H2O immediately and becomes the following chemical:
2(3CaO x AlCO3) + 21H2O → [4CaO x Al2O3 x 13H2O] + [2CaO x Al2O3 x 8H2O]
[4CaO x Al2O3 x 13H2O]: calcium sulfoaluminate hydrate.
[2CaO x Al2O3 x 8H2O]: calcium aluminate hydrate.
water hardens the cement. just get a kilo of cement powder, sand, and water, and you’ll understand.
i’ll come back to this point in a minute… i’ll need to explain u this 1st.
and these are the bogue calculations for each of the four clinker materials.
these calculations also include the gypsum required for commercially viable portland cement.
but why are subtraction symbols in the equations?
the coefficients/numbers represent the proportional relationship between the oxide contents & the formation of the clinker phases for each material (C3S, C2S, C3A, C4AF).
if there is a minus symbol in front of a coefficient, it means this:
let’s used C3S equation as the example.
this coefficient “-7.600 x SiO2%” means that for every 01% increase in SiO2 cement, the C3S content decreases by 7.60%.
aka: it’s a decrease in the amount of the corresponding oxide (SiO2, in this case), that goes into forming the C3S equation.
this explanation applies to any of the equations.
likewise, it applies for positive coefficients as well.
using C3S equation…
the coefficient of “4.07 CaO” means that for every 01% increase in CaO content, the C3S content increases by 4.07%.
it’s a proportional relationship between oxide contents and forming different parts of the clinker.
those parts are the equations that give us C3S, C2S, C3A, C4AF, with the gypsum included.
and notice how there is sulfate in the cement.
we need it.
it comes from gypsum in the form of the chemical:
gypsum: CaSO4·2H2O
but too much sulfate is no bueno.
why?
it causes the sulfate to expand.
remember, sulfate causes the C3A molecule + gypsum to create ettringite.
the formation of ettringite requires a source of sulfate, calcium, and aluminum from the cement mixed with water.
too much sulfate causes the cement to expand, leading to poorer compressive strength and mechanical properties.
ettringite/calcium sulfoaluminate: 3CaO·Al2O3·3CaSO4·32H2O
^this is found in the majority of portland cement.
commercially produced portland cement in the usa contains less than 05% sulfate.
if someone adds too much gypsum, we can simply wash AMD sludge with deionized water… it’ll balance off the amount we added for gypsum to harden the cement.
speaking of water, we cannot have excess water in our AMD wet sludge concrete.
we must dewater it.
it lowers the cost of transporting sludge and saves us extra energy from having to fire up the kiln to manufacture the cement.
ideally, we want the kiln to go between ~1100°C — 1400°C with maximum four hours.
before we go in for more, let’s explain transportation costs, with two examples.
sludge & trucks.
case study #1: toledo, ohio.
it’s about 50 miles south of detroit, michigan.
the wastewater treatment plant in toledo serves about 500,000 people.
dewatered sludge is typically handed over for farmland applications.
the company that handles the sludge is “soil enrichment materials corporation (semco)”.
it has contracted about 867 acres of farmland in wood county, ohio, but so far, it has about 415 acres of received sludge… at no cost to the farmers in that county.
and land that has received sludge was used for the production of corn, soybeans, and hay.
drive time from the plant to ‘semco’ is a round-trip of 45.2 miles and 2.5 hours.
the annual sludge treatment and handling costs are estimated to be about 40% — 50% of the wastewater plant operating costs.
the annual costs of trucks, equipment, and vacuum filters are about USD 500,000.
labor, chemicals, and power for the vacuum filters is about USD 730,000.
semco takes care of hauling and spreading the sludge onto the farmland, not the city of toledo.
but still…
sludge hauling at (USD 2.92/wet ton) is about USD 237,000.
sludge spreading at (USD 3.24/wet ton) is about USD 263,000.
so the annual total cost of the treatment plant (including administrative overhead) is about USD 1,903,000.
and 16,250 dry tons were produced this year.
divide the total cost by tons… and we got USD 117/ton.
the top three heavy metals were:
- aluminum (2,200–53,500 ppb) | 32 ppb per dry ton
- calcium (60,000–135,000 ppb) | 204 ppb per dry ton
- iron (22,000–101,000 ppb) | 142 ppb per dry ton
case study #2: belleville, illinois.
population is about 41,700 people.
it’s located ~15 miles southeast of st. louis, missouri.
liquid sludge is also applied for farmland uses.
farmers who are between 5–10 miles from the wastewater treatment plant and want to use the sludge on their land can ask for it and they’ll get their sludge.
total sludge generated in one day is ~33,000 gallons/day.
sludge that leaves the treatment plant is ~15,000 gallons/day… or 2.4 dry tons of sludge/day.
the transport vehicles purchased were two 2,000-gallon trucks.
vehicle costs were ~USD 18,600.00
total operations budget annually is ~USD 325,549.00
in this town, the salaries and wages were >65% of the plant’s cost.
at the time, they had 22 full-time employees for the city’s water treatment plant.
but this plant did air-dried sludge & liquid sludge.
the total cost for air-dried sludge disposal annually was USD 17,130.00
the total cost for liquid sludge disposal annually was USD 12,060.00
air-dried sludge produced 500 dry tons.
so did liquid sludge.
so the average cost for air-dried sludge was USD 34/dry ton.
so the average cost of liquid sludge was USD 24/dry ton.
and the top three heavy metals present (on a dry-weight basis) were:
- iron (33,510 ppm)
- aluminum (5,380 ppm)
- calcium (20,660 ppm)
one big town, one small town. you see why dewatering sludge can help?
ok, now, onto the last piece for today.
this dewatered sludge can be mixed with the portland cement powder.
but the AMD dry sludge + powder must be homogeneous as possible so they can mix.
homogeneous → two components must be mixed & distributed evenly so they can obtain the desired properties.
this is what a typical portland cement can consist of:
p.s. — we are not including gypsum, this will be discussed in another article.
we want these chunks to be crushed into smaller pieces before preparation of sintering.
sinter → forming a new solid mass of material by pressure or heat without melting it to the point of liquefication.
and here are the tests by ASTM that portland cement must pass:
ASTM C109 → compressive strength of hydraulic cement mortars.
ASTM C150 → specification of chemical & physical requirements for various types of portland cement. (depending on application)
ASTM C204 → measurements of fineness of portland cement by determining its specific surface area.
ASTM C114 → chemical analysis tests performed on cement to determine its oxide composition and other properties.
ASTM C452 → tests an expansion of mortar bars from a mixture of portland cement + gypsum to see if sulfate content is below 7.0% in mass (values are in SI units).
ASTM C1565 → mechanical force needed to overcome consolidation of portland cements (values are in SI units).
- consolidation → reducing or eliminating the air pockets for cement and entrapped air from freshly placed concrete.
these tests will need to be considered if we will insert some dry AMD sludge (rich in ferric iron and calcium, aluminum, and a pinch of magnesium) into portland cement.
but how much of the sintered AMD sludge should go into cement?
1%?, 3%, 5%, 10%? what’s the max i can go without a loss in cement properties, like compressive strength?
this is a good idea.
but one word determines if this can be a genuine solution:
economics.
sources.
© 2024–2100 by Carlos Manuel Jarquín Sánchez. All Rights Reserved.
