ORIGINAL ARTICLE

Utilization of milk of lime (MOL) originated from carbide limewaste and operating parameters optimization study for potentialprecipitated calcium carbonate (PCC) production

Onimisi A. Jimoh1,2 • Norsuria Mahmed4 • P. U. Okoye3 • Kamar Shah Ariffin1

Received: 28 October 2015 /Accepted: 6 September 2016

� Springer-Verlag Berlin Heidelberg 2016

Abstract Vast amounts of carbide lime waste generated as

a by-product of acetylene gas production require urgent

utilization to avert handling and disposal difficulties. The

carbide lime waste is often rich in calcium hydroxide

(Ca(OH)2), rendering it an alternative potential precursor

for precipitated calcium carbonate (PCC) production. The

study demonstrated that suspension of carbide lime can be

utilized to synthesize PCC at favorable conditions. The

characteristics and crystal morphology of the lime and as-

synthesized PCC were determined using X-ray fluores-

cence and scanning electron microscope. The influencing

reaction parameters based on temperature, flow rate, total

dissolved solid and carbide lime concentration were

investigated. Under specific reaction conditions of 2 M

carbide lime concentration, final pH of 6.98, 90 min, and

452.30 mL/min CO2 flowrate, high purity of 99 % PCC

was attained. The produced PCC from carbide lime meets

end user requirement on a par with conventional PCC

products.

Keywords Carbide lime � Precipitated calcium carbonate �Total dissolved solid � Waste utilization

List of symbols

K Molar conductivity (Sm2/kmol)

k Conductivity (conductance per meter S/m)

C Concentration (kmol/m3 or mol/L)

M Molarity (mol/L)

Introduction

As a result of the fast and alarming depletion of mineral

resources, attention has been redirected to the identification

of more eco-friendly and renewable alternative sources.

Waste from acetylene gas production, which is rich in

calcium, is an alternative high-quality hydrated lime source

(Kenny and Oates 2000). Usually, the carbide sludge is

washed into settling ponds, creating a lime mud that poses

difficulties in utilization. Although there is no approved

method for carbide sludge disposal, the large lime ponds

remain a reservoir for soil liming material that is compa-

rable in quality to basic hydrated lime. These large ponds

for settling lime mud adversely impact the environment

(Ayeche and Hamdaoui 2012). Furthermore, the charac-

teristic of carbide lime, such as high alkalinity (pH 12.5),

unpleasant odor, irritation to skin and throat poses severe

difficulty in handling (Armour 2003). In addition, the lime

manufacturing process is energy intensive and generates

undesired CO2 (Ma et al. 2015; Watkins et al. 2010) as an

unavoidable by-product. However, the unique mineral

compositions of carbide lime waste such as high calcium

hydroxide (Ca(OH)2) and other minor minerals of carbon,

ferrosilicon, silica, traces of inert mineral confers on it a

suitable potential material for many applications. Particu-

larly, the high calcium hydroxide content of carbide lime

can be utilized in hydroxyapatite production, catalysis, and

precipitated calcium carbonate (PCC) production.

& Kamar Shah Ariffin

kamarsha@usm.my

1 School of Materials and Mineral Resources Engineering,

Universiti Sains Malaysia Engineering Campus,

14300 Nibong Tebal, Malaysia

2 Department of Geology, Federal University Lokoja,

Lokoja, Nigeria

3 School of Chemical Engineering, Universiti Sains Malaysia,

14300 Nibong Tebal, Malaysia

4 School of Materials, Universiti Malaysia Perlis, 02600, Arau,

Perlis, Malaysia

123

Environ Earth Sci (2016) 75:1251

DOI 10.1007/s12665-016-6053-z

Recent advances in the utilization of carbide lime have

focused on PCC production because of its humongous

applications. As long as PCC meets certain purity

requirement, it can be utilized as an artificial pigment in

paper, plastics, drug carriers in pharmaceuticals, sealants,

food industries, paint manufacturing and fillers in adhe-

sives (Nasser et al. 2015; Thenepalli et al. 2015). As a filler

material, it provides high tensile reinforcement due to its

unique particle size and morphology (Sae-oui et al. 2009).

PCC is usually produced via three routes, namely the cal-

cium hydroxide–sodium hydroxide, calcium chloride–

sodium carbonate double salt decomposition process, and

carbonization process (Onimisi et al. 2016). All these

routes utilize milk of lime usually generated from lime-

stone. The limestone is first calcined (*1000 �C) to form

CaO. Thereafter, the obtained lime is screened to remove

impurities in the limestone and the lime is dispersed in

water to form milk of lime. Finally, CO-2 gas is passed in

the milk of lime resulting in calcium carbonate precipita-

tion. Therefore, the CO2 produced during the calcium

carbide production can as well be channeled to produce

PCC. This will minimize or compensate for the high

energy input during calcium carbide production. The car-

bonation process enables PCC of a given specification to be

produced in a dedicated plant, irrespective of the local

geology. The fineness of the particles, as well as the crystal

morphology (e.g., aragonite, calcite), is controlled by

temperature, concentration of reactants and time. The

process is environment-friendly and does not emanate toxic

pollutants (Suwanthai et al. 2015).

As a continuous effort to utilize waste materials gener-

ated from industries, researchers have produced PCC using

varying wastes from gypsum and steel slags (Mattila et al.

2012). Ciullo (1996), reported that high purity PCC

([95 %) can be produced using pseudo-catalytic lixiviant

to selectively extract calcium from slag material before

being dissolved as PCC. Also, ammonium salts have been

used to selectively extract calcium from steel slag, result-

ing in high purity PCC product and reduction in CO2

emission (Adams 2005; De Crom et al. 2015). They

reported that the smallest solid to liquid ratio 5 g/L resulted

in the maximum calcium extraction efficiency (73 %),

while the reverse using 100 g/L produce the lowest

extraction efficiency of 6 %. Liu et al. (2016), investigated

the performance of two organic acids (succinic and acetic

acid) for the possible extraction of calcium from steel-

making slag for PCC production. They observed that the

carbonation of succinic acid leachate did not result in the

production of PCC, while the carbonation of acetic acid

leachate resulted in the synthesis of PCC. Furthermore,

studies by Suwanthai et al. (2015) hinted that high purity

PCC (mainly calcite) can be synthesized from gypsum

waste by using an acid gas (H2S) to improve the aqueous

dissolution of the poorly soluble CaS. Huang et al. (2007)

produced high purity PCC (mainly amorphous) from

medium and low-grade limestone using strongly acidic

cation exchange resin. This improvement was due to the

reaction of HCO3- in aqueous solution during the slaking

reaction process. Valuable information from their study

indicated that impurities were eliminated during the car-

bonation reaction.

The use of carbide lime waste for the synthesis of PCC

has not been adequately reported. Therefore, the appro-

priate techniques, operating conditions and physical prop-

erties of carbide lime sludge in PCC production were

envisaged for the first time in this study. Also, the influ-

encing parameters that control the precipitation process,

products morphology and particle size were thoroughly

investigated.

Experimental methodology

Material

In order to carry out this experiment, calcium carbide

produced by MCB industries Sdn. Bhd (Malaysian Carbide

Berhad), Kemunting, Taiping was used. The typical

chemical composition [by X-ray fluorescence (XRF)] of

the CaC2 is tabulated in Table 1. Pure CO2 gas was sup-

plied by Merck. Conductivity meter-Istek 455C model for

pH, reaction temperature, conductivity and total dissolved

solid (TDS) online monitoring.

Hydrated milk of lime precipitation

Hundred grams calcium carbide was dissolved in 1 L

double distilled water, resulting in acetylene gas liberation

and precipitation of hydrated lime (Ca(OH)2). When the

reaction is complete, observed from high pH [12, the

resultant hydrated lime is then dried in an oven at 105 �Cfor 8 h, and subsequently screened to obtain \125 lmpowder. In this investigation, various concentration of milk

of lime was prepared from the carbide lime powder and

experimented in accordance with operating variables as

designed in Table 2. Milk of lime suspension produced by

dissolution of dried carbide lime powder was screened to

remove any coarse grits that may affect PCC particle

morphology.

Precipitated calcium carbonate production

The precipitation of calcium carbonate production using

CO2–Ca(OH)2 was carried out in an agitated reaction

vessel (a 1000 mL rounded glass with a multi-socket

reaction vessel). CO2 gas was bubbled through the batch

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reactor containing Ca(OH)2 generated from the carbide

lime sludge. The pH, temperature and conductivity changes

taking place during the course of the precipitation reaction

were monitored by the conductivity meter. A high pH ([9)

generally indicated the presence of free lime and a corre-

sponding high surface potential. Effects of the milk of lime

concentration, CO2 gas flow rates, reaction time on particle

morphology and size were investigated. Finally, the syn-

thesized PCC was screened below 45 lm, dewatered, dried

and then characterized.

PCC characterization

The morphology and particle size of PCC for each sample

were examined by scanning electron microscope (VPSEM,

Carl Zeiss, SUPRA35VP model). Prior to SEM examina-

tion, the various PCC powder samples were dispersed in

methanol, ultrasonically treated to reduce particle aggre-

gation then coated with gold to improve material

conductivity. The chemical composition of the powdered

samples was analyzed using a Rigaku RIX 3000 X-ray

fluorescence spectroscopy. Powder samples were used to

make fusion bead before analyzing elements.

Results and discussion

In this reaction, feed concentration, gas flow rate and

reaction time are the important operating variables to

control PCC properties. The particle formation and growth

process in the precipitation, which depend directly on the

supersaturation of solution, vary with the milk of lime

concentration, time and gas flow rate. The rate of particle

growth and the particle is critically determined by the

mixing of milk of lime suspension and gas phases. Hence,

the stirring efficiency suggests that if Ca(OH)2 dissolution

reaction regime is chemically or diffusional controlled.

Therefore, all the experiments were conducted above

Table 1 Typical chemical

composition of CaC2 and PCC

(by XRF)

Composition (wt%) MCB carbide Other typical carbidea PCC (exp. 8)

SiO2 0.75 0.34–3.40 0.44

SiO2 ? insoluble – 0–2.2 –

Al2O3 ? Fe2O3 0.365 1.2 0.169

Al2O3 0.270 0.06–8.80 0.12

Fe2O3 0.095 0.01–0.11 0.049

CaO 58.6 54–57.40 –

K2O 0.0003 0.01–0.03 –

MgO 0.29 0.098–0.22 –

SO3 0.10 – 0.033

P2O5 0.035 – –

SrO 0.05 – 0.014

CaCO3 – – 99 ??

Free carbon 9.10 – –

CO2 – 2.0 –

LOI 30.43 30.05–44.30

MCB Malaysian carbide Berhada Muntohar et al. (2016) and Othman et al. (2015)

Table 2 Experimental operating variables setting and carbonate content of resultant PCC

Experiment no. Molarity

(M)

Reaction

time (min)

Flow rate

(mL/min)

CaCO3 content

of PCC (wt%)

Final pH

of reaction

1 (PCC 1) 1.0 60 262.60 76.86 7.91

2 (PCC 2) 90 83.55 6.82

3 (PCC 3) 60 452.30 77.98 7.83

4 (PCC 4) 90 85.64 6.92

5 (PCC 5) 2.0 60 262.60 74.25 8.12

6 (PCC 6) 90 88.93 7.0

7 (PCC 7) 60 452.30 81.57 7.71

8 (PCC 8) 90 99.18 6.98

Environ Earth Sci (2016) 75:1251 Page 3 of 7 1251

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[400 rpm to overcome mass transfer barrier and sustain

the PCC production to chemically controlled reaction

regime.

Influence of reactant concentration and reaction

time on PCC production

At different milk of lime molar concentration (1 and 2 M),

varying time (60 and 90 min) and flow rates (262.60 and

452.30 mL/min), PCC of various shapes and purity were

produced. The yield of PCC is presented in Table 2, while

the morphology of the resultant PCC for the 8 experiments

are presented in Figs. 1 and 2, for 1 and 2 M milk of lime

molar concentration, respectively. Observably from Table 2,

1 M milk of lime concentration at 60 min reaction time and

262.60 mL/min CO2 gas flow rate, resulted in a cluster of

short or long structured prismatic scalenohedron shapes

(Fig. 1a, PCC-1) with estimated individual particle size less

than 0.5 lm. The scalenohedron shapes formation is likely

as a result of Ca(OH)2 pH close to 13. However, the indi-

vidual particle size appears coarser, double or triple of PCC-

1 size, at prolonged reaction time (90 min) (Fig. 1b, PCC-

2). Mattila et al. (2012), reported in their study of PCC

production using steel slag that coarse particle sizes evolve

as a result of longer reaction time. Increasing the Ca(OH)2

concentration to 2 M at 262.60 mL/min CO2 flow rate and

60 min reaction time, displayed a well-defined rosette

scalenohedron calcite (Fig. 2e, PCC-5). However, increase

in bubbling time of CO2 from 60 to 90 min tends to produce

even more distinct rosette scalenohedron calcite crystal

(Fig. 2f, PCC-6) with more marked spindle-like feature

rather than prismatic look (Feng et al. 2007). This can be

explained by increased mass transfer of CO32- into the

solution and consequent increase in ionic strength of the

milk of lime. The mass transport of these carbonate ions into

the solution is a function of the stirring efficiency and can be

tailored by appropriate selection of stirring speed. Hence, the

optimum purity and desirable scalenohedron calcite PCC

morphology was obtained at 2 M milk of lime concentra-

tion, 90 min reaction time and 452.30 mL/min CO2 gas

flowrate (Fig. 2h, PCC-8).

Influence of CO2 gas flow rate on PCC yield

The CO2 inlet gas was supplied through a frit of 10–40 lmwhich resulted in a gas bubble of 1.2 mm diameter. Pro-

duction of PCC requires small bubble size to overcome

mass transfer barrier and enhance liquid to gas contact

leading to smaller particle sizes. The purity of obtained

PCC at 1 M Ca(OH)2 molar concentration and fixed

PCC-1 PCC-2

PCC-3 PCC-4

Fig. 1 Images of the synthesized PCC (PCC-1 to PCC-4) produced at a reactant concentration of 1 M

1251 Page 4 of 7 Environ Earth Sci (2016) 75:1251

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reaction times (60 and 90 min) increased marginally

(Table 2) as CO2 flow rate increased (262–452 mL/min).

However, different crystals morphology was observed at

different reaction times which are attributed to increasing

CO2 flow rate. The PCC-3 (Fig. 1c) produced at similar

conditions as PCC-1, displayed a cubic-like rhombohedron

crystal morphology particle. This can be explained by the

presence of lower ionic strength or OH- of the Ca(OH)2suspension with higher a presence of CO2 gas, promoting

the yield of rhombohedron crystals. Similarly, PCC-4

produced at same conditions [1 M Ca(OH)2 and 90 min]

and different CO2 flowrate as PCC-2 presented a closely

packed cluster of dendritic-like (radiated) scalenohedral

PCC particles, prismatic to needle-like crystals (Fig. 1d).

In general, increasing the bubbling rate not only reduced

the particle size but also resulted in a change in crystal

morphology. For the higher reactant concentration (2 M

Ca(OH)2), boosting the rate of CO2 flow triggered gener-

ation of finer PCC powder. In spite of that, PCC-6 (Fig. 2f),

with a good scalenohedral crystal was attained, increasing

the flow rate and bubbling time has significantly reverted

the crystal shape again into cubic-like, rhombohedron PCC

(Fig. 2h, PCC-8).

Temperature–conductivity relationship

Conductivity sensors are usually used to measure the

concentration of total dissolved solids (TDS) during PCC

production. The conductivity of acid or base depends on

both ion concentration and ion mobility. The ion mobility

is promoted by temperature which increases about 2 % for

each �C increase in temperature. The degree at which

temperature affects conductivity depends on the types of

ions involved. The gradual temperature increase, and

decreasing conductivity and TDS at precipitation reaction

regime are presented in Figs. 3, 4 and 5. From Fig. 3, the

conductivity for 8 PCC experiments decreases steadily

with increasing precipitation time. Similarly, the TDS

decreases as the conductivity decreases (Fig. 4). Since the

pH is a measure of hydrogen ion concentration, hence,

lower pH (i.e., higher H? ion concentration) translates to

higher conductivity. It is crucial to end the precipitation

reaction at optimum pH where efficient and effective

conversion of milk of lime to CaCO3 is achieved and

before the CO2 concentration becomes too high (increased

acidity) to initiate dissolution of the suspended CaCO3

precipitate. The initial pH of the milk of lime is greater

PCC-5 PCC-6

PCC-7 PCC-8

Fig. 2 Images of the synthesized PCC (PCC-5 to PCC-8) produced at a reactant concentration of 2 M

Environ Earth Sci (2016) 75:1251 Page 5 of 7 1251

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than 12.5. The final pH monitored presented a variation of

7.0–6.8 for all PCC’s produced at 90 min reaction time.

However, pH of the PCC’s produced at 60 min reaction

time varied in the range of 8.1–7.7 as observed from

Table 2. Consequently, the yield of CaCO3 varied with the

pH, where all PCC’s produced at final pH of 7.0–6.8 dis-

played higher yield of CaCO3 compared to those produced

at pH of 8.1–7.7. The pH around 7 signifies the end of the

precipitation reaction. Hence, the higher pH and conse-

quent lower CaCO3 yield for all PCC’s produced at 60 min

suggests possible incomplete precipitation of CaCO3. The

exponential decrease in the conductivity and TDS is

attributed to a decreasing number of free ions during the

precipitation reactions as the ions react to form solids. The

exponential decrease in TDS depends on the initial milk of

lime concentration and CO2 flow rate. Hence, the con-

centration of the solution at any time can be measured

using the mathematical relation (Eq. 1)

K ¼ k=C ð1Þ

where K (Sm2/kmol), is the conductivity, k correction

factor (typically 0.7) and C is the concentration.

On the other hand, the recarbonation process which is

thermodynamically exothermic resulted in minimal heat

emission during the precipitation of CaCO3 (10–16 �C).The temperature increase also depends on the concentra-

tion of initial milk of lime. As mentioned earlier an insight

into the effects of temperature–conductivity–TDS rela-

tionship suggests that as solution temperature increases up

to 40 �C (Fig. 5), the conductivity and TDS decreases

exponentially because of decreasing number of mobile ions

in the solution. This indicates that precipitation has

occurred.

X-ray fluorescence (XRF) studies

The XRF of the carbide waste and PCC-8 obtained at best

synthesis conditions are shown in Table 1. Evidently,

calcium oxide is the main mineral element of the carbide

waste with 54.96–57.40 and 58.6 % for computed and

obtained Malaysian carbide waste (Muntohar et al. 2016;

Othman et al. 2015). The slightly higher content of cal-

cium oxide in this work (Malaysian calcium oxide) sug-

gests greater potential for its utilization in PCC

production. Hence, chemical compositions of the calcium

carbide (Table 1) revealed no CaCO3 content in the raw

material, while synthesized PCC-8 contains 99 wt%

CaCO3. The other components (SiO2, Al2O3, Fe2O3, MgO

and P2O5) combined are ranged below 1 wt%, repre-

senting typical contents of minor importance. The effect

of impurities associated with the synthesized PCC seems

to be very insignificant as they are less than 1 % when

combined. The data clearly fall within the limiting values

known for pure PCC. Based on this XRF results, the

synthesized PCC from the carbide lime can be classified

as a high purity PCC.

Fig. 3 Conductivity declining during the course of precipitation of

PCC at various rates

Fig. 4 TDS declining in tandem with the progress of precipitation of

PCC at various rates

Fig. 5 Temperature of reactant increases as the precipitation pro-

ceeds as much as 16 �C

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PCC purity

The best PCC purity obtained at favorable precipitation

conditions are presented in Table 2. Evidently, PCC-8 is

outstanding with 99 % CaCO3 content and other minor

chemical elements under conditions of 2 M concentration

of milk of lime, 452.30 mL/min CO2 flow rate and 90 min

reaction time. The 99 % PCC produced from carbide lime

waste was estimated from the XRF studies and can be

compared with the commercial PCC. As discussed earlier,

the difference in morphology is a consequence of variable

reactant concentration, temperature, measure of final pH,

CO2 flow rate and reaction time, which advertently influ-

ences the particle size of resulting PCC. It should be noted

that agglomeration during the crystallization of PCC was

unavoidable, and the degree of agglomeration have a sig-

nificant effect in particle analysis. Dry PCC is very difficult

to disperse properly and have a tendency to form larger

crystal clusters. Particle size analysis by sedimentation

method frequently failed to provide satisfy results.

Depending on the preset operating variables and conditions

of the process, the scanning electron micrographs, SEM

(Figs. 1, 2) showed that most of the synthesized PCC are in

the range of 0.1–0.5 lm.

Conclusion

Conclusively, carbide lime sludge generated from acet-

ylene gas production can be successfully utilized to pro-

duce PCC. Operating parameters, such as CO2 gas flow

rate, temperature, reaction time, final pH and initial

hydrated lime concentration, influence the resulting pur-

ity, morphology and particle size of PCC. Increasing CO2

flow rate (from 262.60 and 452.30 mL/min) showed a

negligible increase in the obtained PCC purity; however,

the resulting crystal morphology changed significantly.

Also, the conductivity and TDS of the lime solution

decreased as the temperature increases during the pre-

cipitation reaction which can be attributed to in situ

decreasing mobile ions. Hence, at 2 M hydrated carbide

lime concentration, CO2 flow rate of 452.30 mL/min, and

pH of 6.98, 99 % pure PCC was achieved in 90 min

reaction time. Finally, regular, uniform crystalline of

scalenohedral, prismatic and blocky rhombohedron with a

definite particle size of 0.1–0.5 lm was obtained in all the

experiment.

Acknowledgments The Authors sincerely wish to thank people

whose assistance has made this effort became a reality, especially to

technical staff of the School of Materials and Mineral Resources

Engineering and Ministry of Science, Technology and Innovation

(MOSTI) Malaysia under e-science fund research Grant (603316).

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