TREATMENT OF APATITE NEPHELINE ORE
WASTEENRICHMENT WASTE
Abstract
The study demonstrated the efficiency of using the cascade reaction method
applied to polymineral systems treatment, including technogenic waste of apatite
nepheline ore. The conditions for minerals decomposition by sulphuric acid, content,
and properties of forming solid phases have been researched. The sequence for
carrying out production operations has been defined; the parameters and their
operation modes were optimized. The paper demonstrates that cascade reaction
method based on serial-parallel reactions allows the products if one reaction to
participate as active reagents in the second reaction. This method provides complete
use of initial mineral wastes and conversion of components in the form of wide range
of useful products.
Keywords: mineral waste, nepheline, apatite, titanite, cascade reaction method.
1. Introduction
The necessity for the rational use of mineral resources is caused by their
scarcity and non-reproducibility. Under the conditions of highly intensive field
exploitation, minor part of minable raw material is used. In recent years Mining,
Minerals and Sustainable Development project has gained world support (MMSD
Project, 2002). In respect to the use of mineral resources, this conception provides
disposal of mining waste since under exploitation of natural resources is source of a
considerable waste amount having adversely effect on the environment (European
Parliament and the Council of the EU, 2002). Waste deposits not only deprive useful
area from economic circulation, but also represent the pollution sources of air, soil,
underground, and surface water (Branwart, and Malmstrom, 2001, Broadhurst, and
Petrie, 2010, Broadhurst, et al., 2007, Moncur, et al., 2005, Tadessa, 1994). In this
regard, the researches on waste disposal followed by obtaining the new types of
inorganic materials are of current interest. As a rule, flow charts of waste treatment
are based on concentrating or chemical methods deriving only one or several
components, thus not contributing to the complete waste disposal and slightly
decreasing the environmental load.
The offered waste treatment processing method is based on the cascade
reactions principle, consisting in the fact that a share of products resulting from
serial-parallel reactions is a raw material for the subsequent technologic stages and
resource for getting new products. It is significant that as far as all or the majority of
mineral components being the result of chemical transformations form end products,
non-waste and low-waste technologies can be considered. The cascade method can be
widely applied for polymineral waste treatment. However, regarding the
implementation, it is necessary to conduct a particular complex of physical and
chemical researches that includes the following stages:
- Determination of amount for each mineral component composing the test
material;
- Definition of function for each mineral component;
- Selection of chemical reactions leading to end product formation;
- Study of kinetics and mechanism of end products formation;
- Selection of optimum components ratio for targeted synthesis of products
with the set properties.
The possibility of cascade method application is shown in this work by the
example of apatite nepheline ore wastes treatment (Kola peninsula, Russia). This
waste is formed during ore flotation concentration with apatite and nepheline
concentrate production. They are scarcely treated and, in the form of aqueous
suspension, are sent to waste facilities. The new flow scheme of wastes disposal will
allow producing a whole series of cheap products used in paint and construction
materials industry, and sewage and drinking water purification of suspensions and
pernicious contraries, including radioactive nuclides.
2. Material and method
Nepheline wastes (fraction 5-150 μm) with composition (weight %): nepheline
–30, apatite –5, titanite –25, aegirine –35, feldspar – 5 were used in this study. The
bulk of the fine fraction (up to 50 m) consists of apatite and nepheline with traces of
titanite, aegirine, and feldspar. The latter is concentrated in the coarser fraction. 1040% sulphuric acid was used in decomposition processes.
The results of preliminary studies of individual concentrates were used to
prepare a synthetic concentrate mixture similar to industrial wastes. The experiment
was carried out in a reactor equipped with a stirrer and a thermometer, under reflux.
In the reactor, the calculated amount of sulphuric acid was initially placed and
stepwise, under stirring, the corresponding amount of concentrate was added. The
obtained suspension was heated (if necessary) and stirred; the mixture was kept for 28h. The liquid phase was filtrated and both solid and liquid products were analysed.
The pH of solutions was measured with Orion pH meter 611. Concentrations of metal
ions in solutions were determined using AAS-300 atomic absorption spectrometer.
Powder X-ray diffraction studies of samples were performed using a Siemens D 5000
diffractometer with a monochromatic Cu Kα radiation (λ = 1.5418 Å). Scanning rate
of 2.4º min-1 was used for a 2θ diffraction angle range of 10-60 degrees.
3. Results and discussion
The process of polymineral waste separation with synthetic and mineral
products production is based on the different solubility of particular minerals in
sulphuric medium. Thereby, gradual leaching of components from minerals into
liquid state is taking place with their subsequent conversion into compounds, in
which they individually or together with other compounds form end products. The
alternation of chemical (solid phase interreacting with sulphuric acid) and physical
(resulting suspension separation) reactions allows effecting process with preparation
of intermediate and end product according to low waste technology.
Principal chemical reactions involved in the method are as follows:
(NaK)2Al2O32SiO2 +4H2SO4 = (NaK)2SO4 +Al2(SO4)3 +H2SiO3 + 3H2O
(1)
nepheline
Ca5(PO4)3F + 5H2SO4 = 3H3PO4 +HF + 5CaSO4 ↓
(2)
apatite
Al2(SO4)3 + 2H3PO4 = 2AlPO4 ↓ + 3H2SO4
(3)
CaSiTiO5 +2H2SO4 = TiOSO4 + CaSO4 ↓ + SiO2 ↓ + 2H2O
(4)
titanite
TiOSO4 + H3PO4 + H2O = Ti(OН)2(HPO4)↓ + H2SO4
(5)
Complexity and instability of system composition under study are the reason
for particular challenges when developing technology. That is why an amount of each
mineral component that composes waste under study was initially determined.
Further interreacting kinetics of each mineral with selected sulphuric acid solution
was studied, as well as kinetics and mechanism of mono- or polyphase residues
formation based on primary reaction products, their optimum ratio for streamlined
synthesis to produce tailor-made end products were defined.
3.1 Nepheline decomposition
It has been found that both the type and structure of minerals incorporated in
the wastes determine the order and rate of their dissolution. Therefore, nepheline, as
the weakest mineral, was leached out at the first stage (Tole, et al., 1986, Hamilton, et
al., 2001).
When studying the first stage of the process (reaction 1) sulphuric acid solution
with a concentration of 50-150 g/l H2SO4 was used. Sulphuric acid consumption made
up 50-85% of theoretical necessary for interaction with acid-soluble components of
nepheline. Nepheline decomposition reaction is followed by significant amount of
heat release, which allows carrying out the process without external power supply.
Nepheline adding in sulphurous solution was carried out for 15-20 minutes with the
following agitation of reaction suspension for 1 hour. When using sulphuric acid with
a concentration of less than 100 g/l H2SO4 leaching of main components proves to be
incomplete and their content in liquid phase does not exceed 20 g/l in aluminium and
silicium when converted into oxides. Acid strengthening to 150 g/l is accompanied by
nearly complete leaching of mineral components. The crystallization process of
aluminium sulphates and potassium alum takes place simultaneously. Under these
conditions, silicium is prone to hard filterable colloid systems formation. At the
consumption of H2SO4, less than 85% of its stereometric amount the degree of
nepheline decomposition dips down and suspension tendency to polymerization with
producing of colloids increases. Increase in acid consumption more than 85% of
stoichiometry contributes to increase in transition degree of main components into
solution to 90%, but resultant acid solutions are unusable, for example, as coagulum
flocculants, since low рН of such solutions may lead to acidification of purifying
water. According to findings, optimum conditions for nepheline decomposition are as
follows: H2SO4 concentration - 100 g/l at the rate of 85% of stoichiometric rate
(Table 1).
Table 1. Nepheline decomposition by sulphuric acid
H2SO4,
concentration, g·l-1
50
100
150
H2SO4,
consumption, %
of stoichiometric
value
50
85
100
50
85
100
50
85
100
Content of components
in suspension, %
Leaching to solution, %
Al2O3
SiO2
Al2O3
SiO2
11.8
21.2
50.2
51.1
10.9
18.8
79.0
76.8
10.0
17.2
84.3
82.5
29.5
50.3
62.3
60.5
24.9
44.2
89.7
90.4
21.8
39.4
92.2
94.7
52.0
88.2
73.4
70.7
33.3
56.3
79.8
76.6
Polymerization in the process of preparation
Solution stability,
h
48
48
6
3
18
8
1
2
Тhe pH of the resulting solution was 1.9-2.0. These conditions ensure only the
nepheline decomposition and release of the elements into the solution (Reaction 1).
The mixture was stirred for one hour, and the pH was increased up to 2.1-2.3. When
the reaction was completed, the solid products were isolated by filtration to obtain
stable colloid products (Goto, et al., 1992, Otterstedt, et al., 1987). The
concentrations of all elements in the resulting liquid phase (in terms of oxides) are gl1
: Al2O3 –25, SiO2 –44, Na2O –15, K2O –5, Fe2O3–1.5. The degree of nepheline
decomposition, calculated from the quantity of aluminium and silicate oxides
extracted to liquid phase, is about 90%. Because colloidal hydrosilicic acid
coagulates in aqueous solutions, it can be used as a coagulating-flocculating agent
(Zakharov, et al., 1996). The colloidal silica particles of a polymeric structure have a
high surface area and facilitate the sorbent of dissolved impurities, increasing the
degree of wastewater purification.
3.2 Apatite decomposition
The second stage is interaction of apatite with sulphuric acid solution. The
process of sulphuric decomposition of apatite results in phosphoric acid with a
concentration of 28-50% in P2O5 and in calcium sulphate in the form of gypsum
(Evenchik and Brodskii, 1987, Pozin, 1974).
Sulphuric acid concentration impact and rate have been studied, as well as
process time impact on apatite decomposition degree and structural features of
calcareous phase. For this purpose when agitating concentrated sulphuric acid in
amount of 110% of its stoichiometric amount necessary for calcium banding in the
form of gypsum was slowly added (30 min) into aqueous suspension of apatite.
Resulting concentration of H2SO4 in suspension liquid phase is 15-40%. Reaction
mixture was heated to 80оС, and held when agitating for 2-6 hours. Calcium sulphate
structure and fluorine recovery, which is evolved in the form of hydrofluosilicic acid
with gas-vapour phase (SiO2+6HFH2SiF6+2H2O) and decreases medium
aggressivity, depends on hold-up time and temperature. Resulting gypsum is
separated by filtrating and washed with water at S:L=1:3. Apatite decomposition data
is shown in Table 2.
Table 2. The effect of the time of apatite decomposition and H2SO4
consumption on the liquid phase composition
Decomposing conditions
H2SO4, %
, h
15
2
40
2
40
4
15
4
15
6
Liquid phase composition, g·l-1
P 2O 5
SO3
FSO2
The filtrates are unstable, precipitating
immediately after filtration.
205.4
3.14
0.95
0.51
162.6
3.87
0.83
0.48
226.7
2.28
0.45
0.30
Solid phase composition, %
P2O5
SO3
SiO2
-
-
-
1.89
1.87
1.97
1.56
40.8
41.5
42.7
35.0
1.58
0.64
0.47
0.25
Sulphuric suspension holding for 2 hours at a temperature of 80оС regardless of
H2SO4 concentration is insufficient for complete solid phase precipitation. With
increase of reacting mass holding time to 4 h the system is close to equilibrium and
calcium precipitates almost completely. The higher sulphuric acid concentration, the
more phosphorus content in the liquid phase. In the case of process time of 6 h,
2
ultimate recovery of phosphorus from apatite is reached (Fig. 1).
S
P2O5, gl-1
200
150
100
50
0
1
2
, h
3
4
5
Figure 1. Phosphorus (V) recovery from apatite during its decomposition by H2SO4,
%(▲-10,●-20, ■-40)
X-ray fluorescence analysis shows that main phase in all samples is gypsum CaSO42H2O, according to crystal optical analysis its particles is of needle prismatic
form from 4-28 µm to 5-64 µm, are prone to aggregation (Fig. 6c). Impurity phase is
anhydrite, the amount of which increases with increase in sulphuric acid
concentration.
The studies has proved that sulphuric acid concentration increase for more than 20%
in H2SO4 is unfeasible since during real wastes processing titanite will be composed
together with apatite (Fig. 2). For that reason the optimum conditions for apatite
decomposition are as follows: sulphuric acid concentration 15-20%, temperature 80о
С; time 5-6 h. Gypsum precipitation is well aggregated, low-level in relation to the
mother solution.
TiO2, gl-1
80
60
40
20
0
1
2
3
4
h
5
6
7
Figure 2. Titanium (IV) release from titanite during its decomposition (Tboil) by
H2SO4, %(■-15,●-40)
3.3 Titanite decomposition
Choosing conditions for titanite decomposition is based on the necessity of
titanium (IV) conversion into liquid phase. Previously it had been established that this
process proceeds under rigid conditions: sulphuric acid high concentration – 10001700 g/l Н2SO4, elevated temperature – 140-200оС (Goryachev, and Dvegubskii,
1990). At a low concentration of sulphuric acid to 300 g/l, H2SO4 titanium (IV) in
liquid is unstable and precipitates in the form of titanium hydroxide. Herewith,
mineral decomposition degree does not exceed 50% (Motov and Maksimova, 1983).
For the research, sulphuric acid with a concentration of 400-600 g/l H2SO4 was
chosen. Such choice provides high stability of titanium (IV) in liquid and leads to
preparation of all-purpose titaniferous precursor (as solution), which acts as a basis
for subsequent syntheses of various titaniferous products (Lazareva, et al., 2006).
Fine-grained titanium (TiO2 – 35-37%) was used as a test object.
Decomposition process was carried out at a boiling temperature (108-115оС), mass
ratio of titanite and acid - Т:VL= 1:3. Resultant suspension was held when boiling
and agitated for 7 hours. At that point leaching of sphene components takes place into
liquid sulphuric phase with the later formation of reaction solid products – calcium
sulphate and amorphous silica. Owing to high solubility, titanium (IV) is
concentrated in liquid. Since decomposition process belongs to heterogeneous and
proceeds at the interface of liquid (sulphuric acid) and solid phases (mineral
particles), its speed depends on not only sulphuric acid concentration and
temperature, but also on particles’ dispersion degree.
Fig. 3 shows titanium (IV) content change in liquid suspension in the process
of heating at the chosen concentration of sulphuric acid. The first stage of the process
is a chemical reaction that proceeds at the surface of mineral particles with
components leaching in liquid phase and early producing reaction solid products .
-1
TiO2, gl
80
60
40
20
0
0
200
400
600
800
1000
1200
1400
1600
min
Figure 3. Titanium (IV) release from titanite during its decomposition (Tboil) by
H2SO4, г/л (▲-400,●-500, ■-600)
Temperature rise and concentration of H2SO4 have a positive effect on reaction
speed. This stage time is about 60 minutes. According to X-ray phase analysis, there
is mainly titanite in solid phase and anhydrite in fewer amounts. With increase in
reaction products in reaction mass, the process slows down. According to SEM
reaction, products are formed in the form of individual phases, and forms porous shell
at the surface of undecomposed sphene particles as well (Fig. 4).
Figure 4. SEM image of titanite particle.
Further decomposition process reaches the second stage related to reagent
diffusion through that shell. The reaction speed at this stage is much lower than at the
first one. Approximately, in 5 h decomposition speed is so low that one can consider
pseudoequilibrium state of the system that can be tilted by external influence only.
Sulphuric acid optimal concentration should be considered to be 600 g/l H2SO4,
whereby titanium (IV) content in liquid phase is not less than 80g/l in TiO2
4. Description of the flow sheet, target product composition, and properties
Flow chart of mineral wastes treatment with end products obtaining is shown in Fig.
5.
Figure 5. Schematic flow chart of nepheline waste recovery
The process proceeds in a periodic mode. At the first stage of decomposition wastes
composed of minerals mixture (Fig. 6а) are treated with sulphuric acid with a
concentration of 100 g/l at the solid-liquid stage ratio of S:L=1:8.
o – titanite; ∆ – nepheline; ◊ – apatite; х – aegirine; s – СaSO4
Figure 6. XRD powder patterns for (a)-initial mineral waste, (b)-after nepheline
leaching, (c)- gypsum, (d)- after apatite leaching, (e)-titanium adsorbent, (f)- the
residue after third stage
Under the chosen conditions nepheline decomposition degree exceeds 90% . When
рН reaches 2.1-2.3, liquid phase is separated by filtrating or decantation. The filtrate
is a colloid solution aluminium and silicium, which is watered to 10 gl-1 of SiO2 in
order to increase aggregative stability. Efficiency of this colloid solution usage as a
coagulum flocculants (Product I) is shown in Table 3.
Table 3. The efficiency of alumosilicium coagulant for purification of industrial
wastewater, pH-7.2
Impurities
Before purification, mg·l-1
After purification, mg·l-1
Oil
Suspensions
Nitrate
Sulphate
Chloride
Phosphate
Ferrum
0.53
87.60
1.41
69.81
17.20
2.93
3.50
0.27
14.20
1.00
63.62
14.61
0.89
0.99
Purification
efficiency, %
49.0
83.8
29.1
8.9
15.1
69.6
71.7
Represented data allows recommending aluminosilicic coagulum flocculants for
cleaning industrial waste of suspensions and oil (Zouboulis and Tzoupanos, 2009,
Pacheco and Rodriguez, 2000, Tzoupanos, et al., 2008, Morgunov, et al., 2006) At
the second stage of decomposition mineral residue (Fig. 6б) is treated with 15%
H2SO4 at S:L=1:1. The suspension is heated to 80оС and stirred for 6 h. In the process
of heating decomposition of the residual quantity of nepheline and apatite with the
formation of phosphoric solution containing aluminium and silicium takes place.
Calcium sulphate is crystallized in the form of gypsum and transits in residue (Fig.
6с), which is separated from the solution by filtrating and sent to the third stage of
decomposition. Phosphoric acid is added into the solution to precipitate aluminium
and silica gel phosphates. Resultant residues wash off mother solution and heattreated (Product II). Washing degree and heat treatment temperature depend on
requirements to the end product (Table 4) (Kalendova, 2002., Kalendova, 2003,
Latishev, et al., 1997, Tolmachev, 1998).
Table 4.Characteristic features of Product II
Heat treatment, oС
Up to 50
pH of
extract
1.0-1.5
Up to 100
5.5-7.0
450-500
6.0-7.0
aqueous
Components
Areas of application
Acidicaluminiumhydrophosphate,
silica gel, water
Basic aluminiumhydrophosphate, silica
gel
Basic aluminium phosphate,
amorphous silica
Anticorrosion paint
primers, underpaint putty
Water-based paints (dull)
Water- and organic-based
paints (gloss)
After the second stage of decomposition solid residue, consisting of titanite,
aegerite, and calcium sulphate, (Fig. 6d) is treated with sulphuric acid with a
concentration of 600g/l H2SO4 at S:L=1:3. Reaction mass is heated to boiling
temperature and held for 7 h and the suspension is filtered. Sulphuric filtrate with a
concentration of titanium (IV) when converted into TiO2 – 70-80 g/l and 450 gl-1
H2SO4 is used for titaniferous compositions synthesis, in particular hydrated titanium
phosphate (Product III) (Bortun, et al., 1997, Clearfield, 1995, Nilchi, et al., 2004).
For this purpose the solution was heated up to 70 oС and the corresponding
amount of phosphate agent (molar ratio of TiO2 : P2O5 = 1:0.5-5 ) was slowly added
to it, leading to solid phase formation, titanium phosphate (Reaction 5). Evaporated
filtrate after the second stage of decomposition is used as a phosphoric reagent. The
suspension was stirred at this temperature for 8 h, the solid then filtered and washed
firstly with 5% H3PO4 (to remove the excess of H2SO4) and secondly with water (till
pH 2.8-3.2). The solid products were dried initially on air and then at 60оС.
Precipitation degree of titanium (IV) when producing titanium phosphate is 95-99%.
The final material is amorphous titanium hydroxyphosphate (TiP) with formula
Ti(OH)2·(HPO4) ∙ 2H2O can be used as an ion-exchanger (fig.6e) ((Maslova, et al.,
2008). Resultant adsorbent can be successfully used to clean liquid radioactive wastes
of radionuclides and industrial wastes of heavy nonferrous and toxic metals. This
adsorbent use efficiency is shown in Fig. 7.
160
140
120
A, mg/g
100
80
60
40
20
0
Cs
1
+
Sr
2
2+
Fe
3+
3
Cu
4
2+
Co
5
cations
2+
2+
Ni
Zn
6
7
2+
3+
Cr
8
Figure 7. Sorption capacity (A) of some cations by titanium phosphate
Residue from the third stage of decomposition (Fig. 6f) is divided into two
factions by means of hydraulic classification. Light fraction, collected in catchers,
includes phosphogypsum, calcium sulphate in anhydrite form and amorphous silica.
Upon filtrating, the residue is heat treated at a temperature not more than 3000С.
Resulting Product IV is a white or ecru powder, which can be used for dry
construction mixtures preparation (Olmez, and Erden, 1989., Singh, and Garg. 2000.,
Singh, 1988, Singh, and Garg. 1992). Mineral part mainly consisting of aegerite
(Product V) is dried and used for weatherproof fillers of paint and construction
materials production (Table 5).
Table 5. Characteristics of the products obtained from mineral waste
Product
Product I
Components
Colloidal solution of aluminium and
silicium—a coagulating-flocculating agent
Product II
Acid aluminium phosphate, silica gel
Product III
Titanium hydrophosphate or its composite
with silica gel
Calcium sulphate, amorphous silica,
titanium dioxide
Sphene and aegirine concentrates
Product IV
Product V
Field of use
Thickening of suspensions with a finely divided
solid phase. Water purification from oil,
suspensions, iron, etc.
Anticorrosion compounds on the organic and
water bases
Sorbent purification of toxic liquid effluents
Dry building composites for whitewashing
In the production of pigments, weather-resistant
paints and paint primers
Conclusions
The composition of apatite-nepheline ore wastes has been studied. The
chemical processing stages parameters in the cascade reactions method has been
established. During the kinetic studies, the operation sequence and conditions of
interaction between the waste components and H2SO4 were determined. The
mechanism of the formation of mono- and polyphase residues based on primary
reaction products has been established. Optimal parameters for directed synthesis of
target products with tailored properties and the area of their application have been
determined. The new process of nepheline wastes recovery is technologically
attractive, considering that all steps are inter-connected and all by-products are
utilized. This technology will make it possible to reduce the amount of dumped
wastes by 70-80%.
It has been shown that in order to obtain efficient nepheline removal from the
wastes it is necessary to introduce the process of its interrelation with sulphuric acid
in the mode providing for mineral dissolution rate not exceeding 90%. Moreover,
aggregative non-stable system formation being accompanied by silica gel formation
is excluded.
The correct choice of titanium decomposition is based on the analysis of
kinetic aspects of the process thus providing the high degree of titanium conversion
(IV) into solution – 92% in TiO2; by adding phosphoric solution, the product is
precipitated out of it. This product is a precursor for obtaining titanium-phosphatic
sorbent capable to purify low-medium LRW as well as effluents containing cations of
nonferrous heavy metals.
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