Acid Leaching for Metallic Impurities Evaluation of Some Mineral Ores in Nigeria
*R.L. Tyohemba1& S. K. Emgba2
 
Abstract
There is every need to access the impurity ratio of mineral ores in order to furnish investors and industrialists with information required to weigh the gains of venturing into their exploration. Metallic ores including; Zinc ore (Sphalerite), Iron ore (Magnetite), Iron ore (Heamatite), Copper ore (chalcocite), Lead ore (Galena) and gypsum were obtained from the National Geological Survey Kaduna. The ore samples were grounded and digested with aqua regia solution to leach their metallic constituents using standard method. The main metallic components of the ores viz: Zn, Fe, Cu, Pb and Ca were determined by the flame atomic absorption spectrophotometer (Biotech FAAS Phoenix 986) alongside their associated impurities. Galena (Lead ore) recorded the highest metallic impurity content of about 28.64 % and was followed by Sphalerite with about 6.31 % metallic impurities. The other ores recorded Zn>Mg>Ni>Pb. The impurity content of heamatite and magnetite were of the same trend in the order Mn>Cu>Zn>Ni>Pb. However, heamatite recorded higher Fe content than magnetite. Galena contained 534.50 mg/Kg (71.36 %) lead content with its associated impurities present in the order Zn>Mn>Cu>Cd. Sphalerite contained 8362.22 mg/Kg (93.69 %) as zinc while its associated impurities were present in the order of Cu>Fe>Ca>Mn>Pb. In gypsum, its Calcium content was (97.15 %) with its associated metallic impurities in the order Mg>Fe>K>Cr>Pb. The studied ores had a very high level of their major metallic constituents with only Galena which had a high level of impurities. Thus these ores could be good feedstock to mine their principal components.
1.1 INTRODUCTION
Nigeria is richly endowed with a variety of mineral resources, fossil fuelsand solid minerals ranging from precious metals, various stones to industrial minerals such as Lead(Pb), Feldspar(xAl(Al,Si)3O8), ron magnetite(Fe3O4), iron hematite(Fe2O3), gypsum(CaSO4.2H2O),copper(Cu), Kaolinite(Al2O5(OH)4Si), Zinc(Zn), Limestone(caco3) e. t. c. Most of these are yet to be exploited. Statistically, the level of exploitation of these minerals is very low in relation to the extent of deposits found in the country. The presence of impurities in ores is inevitable due the nature of their occurrence. Impurities in minerals may be caused by simple admixtures or by crystal chemical substitutions [1]. Impurities are often responsible for colour changes. Natural impurities are common in minerals, as is the propensity for one element to slip into the crystalline structure in place of some other element. Exchanges of these forms do not cause a modification in mineral name as long as the replacements make up only a small proportion of the entire structure.

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Oxides nodules of Ni, Co and Cu have been found to occur in lattices of iron and manganese [2]. Associated impurities of zinc(II) such as lead, iron, Copper, Silver etc., contained in are said to be found present in Nigerian sphalerite mineral[3]. The high manganese content of the sphaleritesleave them incongruous for processing by conventional smeltingtechniques, facilitating the inevitability to design and construct an onsiterefinery that is specifically suited for Gamsberg ores and concentrates [4]. Naturally occurringsphalerite commonly incorporates variable concentrations ofimpurities (e.g. Fe, Cd, Mn, Cu, Co, Ni, Ge, In) that substitute zincin lattice sites. For example, FeS concentrations can range up to25 mol. %, while MnSseldom reaches up to 14 mol.%. Consequently,the high extent of stoichiometric inconsistency observedhas a marked effect on the processing, as high levels of theseimpurities in some zinc concentrates, i.e. Gamsberg, renders themunsuitable for processing by traditional operations.Copper is associated with basemetals such as nickel and cobalt.
Studies have also been carried out on representative samples of Küre (Turkey) where massive rich copper ore were leached in acidic ferric sulfate solutions in order to recover copper and its associated metals (Zn, Co, Ni) present in the ore[5].The minerals of lead and zinc are naturally associated with eachOther.In many parts of the world, significant deposits of these mixedsulphide–oxide lead and zinc ores are found with the main sulphideand/or oxidised ore bodies. Because of their complex mineralogy,these ores are very difficult to deal with from a mineral processingpoint of view and they are very often left untreated as a result of themetallurgical difficulties encountered in extracting lead and zincfrom them. Although the primary sources of the metals are leadand zinc concentrates from sulphide and oxidised ores, as suppliesof these deplete, the processing of mixed ores must be considered[6]. Also, the lead-zinc ore from a mine is featured by complicated copper-lead-zinc intergrowth and uneven dissemination size. Besides, the minerals containing copper activate by nature the mineral containing zinc, causing difficulty of separation of copper, lead and zinc minerals [7].There also published data on the presence of metallic impurities including; Fe, Se, Mg, Al, Si, Cu, Zn, Pb, Cr, La, Ce, Nd and Y which were removed from desulpurized gypsum [8].
Hayward and Quincy [9] developed a method for the treatment of iron ores containing impurities such as aluminium, silicon, chromium, nickel and cobalt from iron ores of the type which contain nickel in excess of 0.25% and chromium in excess of 0.5%, such as for example those ores found in Cuba known as Mayari ores. The impurities which are present in all iron ores such as sulphur, phosphorus, manganese and silicon appear in iron and steel made there from. Such impurities are generally undesirable in iron alloys, but it has not been possible to remove them completely, and only high grade iron ores are used at present in the production of iron. For this reason, various low grade iron ores have been wholly rejected, either because of their high contents of the impurities mentioned or because they contain varying amounts of base metals [10]. These bulks of impurities mentioned in the essay have obvious unattractive characteristics that will increase the cost of mineral processing.
There exist speculations that mineral ores in Nigeria are characterized by high levels of impurities. It then becomes imperative to access these mineral ores and their impurity ratios to furnish investors and industrialists with information that will help to estimate cost of production as well as to find suitable methods applicable for their purification.
2.1 METHODOLOGY
2.1.1 Sample Collection and Preparation
Mineral ores including; Zinc ore (Sphalerite – ZnS), Iron ore (Magnetite – Fe3O4), Iron ore (Heamatite – (Fe2O3)), Copper ore (chalcocite – Cu2S)), and Lead (Galena-PbS) were obtained from the National Geological Survey Kaduna. The samples were stored in the laboratory for the study. The ore samples were grounded using a porcelain mortar and pistol sieved and digested to leach their metallic constituents using the procedure described below.
2.1.2 Acid leaching of ore samples for metallic content determination.
This was carried out as described by the ISO (1998)procedure [11]. 1g of the grounded air dried ore sample was transferred into a 250 ml reaction vessel (Teflon digestion bomb). 10ml of the mixture of nitric acid and hydrochloric acid in a ration of 1:3 (aqua regia) was added unto the reaction vessel containing 1g of dried sample and heated using hot plate inside a fume hood until white fume was observed and allowed to cool.
The reaction vessel was allowed to stand so that most of any insoluble residue settles out of suspension. The relatively sediment-free extract was decanted carefully onto a filter paper, collecting the filtrate in a 100 ml volumetric flask. All the initial filtrate was allowed to pass through the filter paper, and the insoluble residue was washed onto the filter paper with a minimum amount of nitric acid (0.5 mol/l). The filtrate so collected was collected alongside the initial filtrate and the volume of the flask was made up to mark with deionized water. The extract thus prepared was ready for the determination of the studied metals, by an atomic absorption spectrometer (Biotech FAAS Phoenix 986).
3.1 RESULTS AND DISCUSSION
3.1.1 Metallic Content of Copper Ore (Chalcocite).
The result of metallic content of copper ore is presented in table 1. The concentration of copper in the ore was found to be 10341.73 (mgkg-1) representing about 97.7% of its metallic content covered by this work. Other metallic constituents which are considered to be impurities were also found present. Hence, Ca (99.65 (mgkg-1), Mg (63.89 mgkg-1), Ni (9.38 mgkg-1), Pb (1.99 mgkg-1) and Zn (73.26 mgkg-1) in the order Ca>Zn>Mg>Ni>Pb. These metallic ions exists in their +2 oxidation states as copper and could replace the Cu2+ in its lattice sites. In a similar work by [5], massive rich copper ore was leached in acidic ferric sulphate solutions and was able to recover copper and its associated metals (Zn, Co and Ni).
Table 1 Metallic content of Copper ore (mg/Kg)

S/No.

Metallic content

Concentration

% of main metallic content

1

Cu (Main element)

10341.73

97.7

2

Ca

99.65

0.9

3

Mg

63.89

0.6

4

Ni

9.38

0.1

5

Pb

1.99

0.02

6

Zn

73.26

0.7

3.1. 2. Metallic content of the studied Iron ores (Heamatite and Magnetite)
Presented in table 2 are the available metallic contents of the studied iron ores. The mount of iron in the heamatite ore was found to be 6488.19 mgkg-1, representing (97.3) % of the total metallic content evaluated in the study. The iron content of magnetite was 5571.81 mgkg-1, representing 96.99%. However, an interesting trend was observed in the variation in the amount of metallic impurities which were found present in these iron ores. Both heamatite and magnetite varied in their level of impurities in the order Mn>Cu>Zn>Ni>Pb. Oxides nodules of Ni, Co and Cu have been found elsewhere to occur in lattices of iron and manganese ores [2]. Hayward and Quincy [9] had already identified impurities such as Aluminium, Silicon, Chromium, Nickel and cobalt from iron ores of the type which contained nickel in excess of 0.25%. The values reported in this study for both iron ores are lower in nickel content than those recorded above (0.21 and 0.18) % nickel for heamatite and magnetite respectively.
As earlier stated in literature, iron alloys are generally undesirable but it has not been possible to remove them completely, and only high grade iron ores are used at present in the production of iron. For this reason, various low grade iron ores have been wholly rejected, either because of their high contents of the impurities mentioned or because they contain varying amounts of base metals [10]. From the results presented in this study, Nigerian iron ores are of high iron content with very minimal impurity content in the ratio of (1:35 and 1:32) iron to metallic impurities content of heamatite and magnetite.
Table 2 Metallic content of iron ores (Heamatite&Magnetite) (mg/Kg)

S/No.

Metallic content

Concentration

% of main metallic content

H

M

H

M

1

Fe

6488.19

5571.81

97.3

96.99

2

Cu

53.11

49.82

0.8

0.87

3

Mn

70.32

72.09

1.1

1.25

4

Ni

13.73

10.21

0.21

0.18

5

Pb

11.04

9.94

0.17

0.17

6

Zn

35.02

30.99

0.5

0.54

**H: Heamatite **M: Magnetite
3.1. 3 Metallic Content of Lead Ore (Galena)
The recorded amount of Pb in the galena ore was 534.50 mgkg-1, representing (71.36) % Pb content of the studied metallic components. Also, other metallic components considered to be impurities including; Cadmium, Copper, Manganese and Zinc were found present in the ore as presented in table 4 in the order Zn>Mn>Cu>Cd. The metallic impurities of Zn and Mn were present in relatively large amounts. (i.e. 14.21 and 12.85) %. The minerals of Lead and zinc are naturally associated with each other. In many parts of the world, significant deposits of these mixed sulphide-oxide lead and zinc ores are found with the main sulphide and/or oxidized ore bodies. Because of their complex mineralogy, these are very difficult to deal with from mineral processing point of view and they are very often left untreated as a result of the metallurgical difficulties encountered in extracting lead and zinc from them [6].
Table 4. Metallic content of Lead ore (mg/Kg)

S/No.

Metallic content

Concentration

% of metallic content

1

Pb (Main element)

534.50

71.36

2

Cd

1.24

0.17

3

Cu

10.62

1.42

4

Mn

96.21

12.85

5

Zn

106.43

14.21

3.1.4 Metallic Content of Zinc ore(Sphalerite)
The results are found in table 5. Zinc content was determined to be 8362.22 mgkg-1 (93.69) % of the ore’s metallic content studied. Other metallic components which are considered to be common impurities associated with zinc ores were also found present in the other Cu>Fe>Ca>Mn>Pb. The occurrence of zinc with such impurities has earlier been reported in the previous session.
Table 5. Metallic content of Zinc ore (mg/Kg)

S/No.

Metallic content

Concentration

% of metallic content

1

(Zn) Main element

8362.22

93.69

2

Ca

103.81

1.16

3

Cu

220.31

2.47

4

Fe

116.90

1.31

5

Mn

98.32

1.10

6

Pb

23.98

0.27

3.1.5 Metallic content of gypsum
Heiska (2011) have reported the presence of metallic impurities including; Fe, Se, Mg, Al, Si, Cu, Zn, Pb, Cr, La, Ce, Nd and Y in gypsum. As presented in table 6, the main metallic component of gypsum which is calcium was evaluated to be 9921.03 mgkg-1, representing 97.15 % of the total metallic content of the studied metals. Other metals such as Cr, Fe, Mg, K and Pb were found present in the acid leached mineral in the order Mg>Fe>K>Cr>Pb. The dominance of Mg as an impurity in this ore is much expected as the metal ion (Mg2+) is known to occur in areas where there calcium deposits exchanging at its lattice and together causing water hardness.
Table 6. Metallic content of Gypsum ore (mg/Kg)

S/No.

Metallic content

Concentration

% of metallic content

1

Ca(main element)

9921.03

97.15

2

Cr

1.56

0.02

3

Fe

40.91

0.40

4

Mg

212.82

2.08

5

K

32.65

0.32

6

Pb

2.92

0.03

3.1.6 Main metal component and impurities ratios of studied ores
In table 7 and fig. 2, the summary of these ratios are presented. Lead ore had the highest level of total metallic impurities recording about 28.69 % as impurities in its ore. Others had low metallic impurities in them. Zinc ore recorded less than 10 % as metallic impurities. On the other hand, metallic impurities in copper, heamatite, magnetite and gypsum were less than 5 %.
Table 7.Metal/metallic impurity ratios of studied ores.

S/No.

sample

% of major metallic content

% of metallic impurities

1

Copper Ore

97.66

2.34

2

Iron(heamatite) Ore

97.25

2.75

3

Iron (magnetite) Ore

96.99

3.01

4

Lead Ore

71.36

28.64

5

Zinc Ore

93.69

6.31

6

Gypsum ore

97.15

2.85

Fig.2 Metal/metallic impurity ratios of studied ores
4.1. Conclusion
The analysis of metallic content of some of the Nigerian mineral ores presented from the result indicates that the studied metallic ores are rich in their principal metallic constituent. The only exception observed is the high impurity content of galena (Lead ore). All the other mineral ores had a little percentage of metallic impurities in them. It is gain saying that these raw materials could serve as very rich industrial feedstock that will require little processing and thus serve cost. The high Zinc and Manganese content of the galena is disadvantageous in terms of the metallurgical process that will be required to recover the major metallic content as well as the appreciable contents of the other metals which are equally of industrial importance.
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