Upgrading mining waste to develop added value final products

V. Aggelatou and E.I.P. Drosos

Department of Mineral Processing, Institute of Geology and Mineral Exploration

Th. Zampetakis and A. Meidani

Grecian Magnesite Research & Development Centre

S. Dambitzias and N. Arvanitidis

Peripheral branch of Central Macedonia, nstitute of Geology and Mineral Exploration ABSTRACT The mean mineralogical composition of all dunite/harzburgites in the western Chalkidiki ophiolite complex is 55-60% olivine, 30-35% serpentine and 9-12% pyroxenes. Dunite/harzburgite formation in Yerakini area is the waste material produced from the beneficiation process of magnesite (MgCO3). It has been proven that after beneficiation of this waste material by appropriate methods, a material rich in olivine (75%) with a low serpentine content (below 10%) and a correspondingly low LOI value (1.5-2.0%) could be provided. Thus, in the frame of an on-going EU program (ProMine), process routes such as dense media separation, shaking table and magnetic separation have been tested. The final product will be evaluated for the production of nano-silica for cement and paper applications.

1. INTRODUCTION

The targets of EU-funded ProMine project (ProMine website) are the development and marketing of new, high added value products from waste raw materials produced by the extractive industry and the development of better exploitation and production practices on sites using a holistic approach, which includes all possible resources, and a more environmentally friendly responsible management. Under this scope, Grecian Magnesite (GM) with the aid of Institute of Geological and Mineral Exploration (IGME), have directed their efforts towards alternative

management of GMs dunite/harzburgite waste stream, promoting its valorisation as raw material for nanosilica production.

In the southernmost part of the ophiolite complex in western Chalkidiki Peninsula, the Gerakini area, magnesite (MgCO3) deposits are exploited by GM producing magnesia (MgO) as final product. The mineable depth of the deposits exceeds 70 m. Dunites and harzburgites, which are serpentinized to different degrees, are the hosts to the magnesite veins. Ore is extracted in open pits and the process to transform the extracted RoM to final products, consists of four different stages: i) pre-beneficiation, ii) main beneficiation, iii) calcination/sintering and iv) end-processing. The capacity of the main processing line is 4500 tonnes of RoM per 8-hour shift. In 2008, the annual gangue material produced by this process was around one million tonnes.

The beneficiation process produces waste types that are inert, non-hazardous and non-leachable under normal environmental conditions. Waste rocks are used to rehabilitate exhausted pits, as gravel and construction materials.

It has been reported by several studies (Dambitzias et al. 2003 & 2009, IGME 2005) that a considerable amount of this waste contains a rather high percentage of olivine. From this material, using appropriate beneficiation methods, it is possible to produce a product with a content of ~75% in olivine, which is tested for various possible uses including the production of nano-silica using sulphuric acid (Lazaro 2010, Jonckbloedt 1998), that can be used as an additive in the production

of high performance concrete (Qing et al. 2007, Brouwers 2006) or in paper coating applications using acids and CO2 (Koljonen et al. 2004, US patent No.2004/0213705).

Research has been undertaken in order to establish a flow-sheet of removing undesired constituents and developing an appropriate processing scheme for the olivine enrichment taking in consideration the economics of the processes. Towards this direction the mineralogy of the ore needs to be considered as this dictates the grade of liberation required and the process that can facilitate this. In order to fulfil the nanosilica production process demands and the final products properties, certain specifications are required as far as the feed material is concerned; i.e. low loss of ignition (a low serpentine content ), absence of carbonates, and high olivine content (>70%).

The objective of this preliminary study is to investigate and develop the procedures which can be employed so as to increase the olivine content in waste material coming from mining and processing of magnesite ores by removing minerals containing water of crystallisation such as serpentine and eliminating, in the highest possible degree, associated pyroxenes.

2. MATERIALS AND METHODS 2.1 Samples preparation procedure quality characterization At first, three types of waste materials that could be supplied by the quarry were identified. Representative sampling and sample preparation together with grain size analysis, mineralogical study and chemical analysis were performed for the three indicative Gerakini dunite waste rocks and their size fractions. These materials were then stockpiled for further use.

Each feed material (initial grain size of

The evaluation results show that sample GM1 has the highest olivine content and does not.

Figure 2: Thin section of sample GM1: serpentine

contain high amounts of calcium carbonate minerals (calcite or dolomite) or free silica. Thus, sample GM1 was selected as the most appropriate to be further tested by various beneficiation methods. 2.2 Grain size analysis The entire quantity of feed sample marked as GM1 having a grain size of 5-100 mm was initially crushed by means of a jaw-crusher (Bronto

Jaw Crusher) to a grain size of +1.70

mm. After that, the products were further processed by a role crusher having cylinders of 250 mm diameter and 150 mm length in order to produce a product with grain size of -1.70 mm. 2827.49 gr of sample GM1 were screened into six different size fractions named GM1/1 to GM1/6 and submitted to chemical analysis. The results of these analyses are presented in Tables 3a-b.

As it is noted from the abovementioned results there is no differentiation in elemental distribution in all size fractions except for the finest size fraction. In GM1/6, the percentage of SiO2 and MgO are reduced and the LOI is almost doubled due to higher serpentine content.iefly sample GM1 consists mainly of olivine (55-60 %), pyroxenes (~12 %) and 20% serpentine. Other constituents include talc, actinolite, chlorite, magnesite and dolimite (each less than 3%).

Table 3a. Grain size distribution of GM1 size fractions

Sample Grain size (mm) %Mass

Distribution GM1/1 -1.70+0.212 65.8 GM1/2 -0.212+0.180 2.53 GM1/3 -0.180+0.125 7.16 GM1/4 -0.125+0.075 7.70 GM1/5 -0.075+0.0380 6.57 GM1/6 -0.038 10.3 TOTAL 100.0 Table 3b. Chemical nalysis of GM1 size fractions

Sample SiO2 Fe2O3 MgO LOI

GM1 44.15 8.50 37.5 6.8 GM1/1 43.70 8.35 37.6 5.50 GM1/2 44.15 8.50 38.5 4.25 GM1/3 43.80 8.40 38.5 4.60 GM1/4 44.15 8.35 38.0 4.70 GM1/5 43.80 8.25 37.3 5.85 GM1/6 42.50 8.30 34.0 10.0 TOTAL 43.63 8.35 37.4 5.84 2.3 Olivine beneficiation testwork Beneficiation tests towards the enhancement of the compositional quality of the rocks in terms of enriching the olivine content were carried out in laboratory scale.

Several series of tests were carried out involving various processing techniques such as DMS, Wilfley shaking table and magnetic separation. Quality and recovery results of products are based on chemical and mineralogical analyses of the initial material and the resulting products. Loss of ignition (LOI) itself, is a quite clear indication of the efficiency and the success of the process followed. 2.3.1. Magnetic separation In our study a Frantz dry magnetic separator of CHAS.W.COOK was used. Two individual size fractions of the selected sample (GM1) were processed namely GM1/1 and GM1/2 having a grain size of -1.70 mm and -0.212+0.180 mm, respectively, at two different field intensities (0.4 and 0.6 A) keeping other parameters stable (slope 20

0). In Tables 4a-4b the data from the

conducted experiments are summarised.

50m

50m

Table 4a. Results of magnetic separation process (Intensity 0,4 A)

Size fraction

Product % w/w

% Olivine

Table 7a. Results of dense media separation tests for

sample GM1 for 2-16 mm grain size

BD=3,10 g/cm3

SINKS FLOATS %LOI 2,15 8,23 %Recovery

1 37,1 62,9

%Serpentine 7,6 18,6 %Carbonates 1,7 8,4 % Olivine

2 74,7 57,0

BD=3,00 g/cm3

SINKS FLOATS %LOI 3,25 7,95 %Recovery

61,0 39,0

%Serpentine 10,0 19,0 %Carbonates 3,0 7,3 % Olivine

71,0 57,7

BD=2,90 g/cm3

SINKS FLOATS %LOI 3,50 10,22 %Recovery

73,9 23,0

%Serpentine 11,0 21,9 %Carbonates 3,3 7,0 % Olivine

69,7 55,1

The total results of this study are presented in Table 7a-b. 2.3.3 Wilfley shaking table

The experimental study of the dunite waste material (GM1) was conducted in lab scale using Wilfley shaking table. The size fraction -1.70 mm was fed to the table and the analyses of the concentrates (N

o1, N

o2) and tailings (N

o3 to

No6) are presented in Table 8. According to the

results, an enrichment in olivine content is achieved in the first fractions-products. The olivine content increases up to ~61% but at 33% w/w of the feed material. In any case it should be noted that these results are referred to grain sizes below 1.70 mm.

3. CONCLUSIONS This study represents only a preliminary experimental work on the possibility of using different mineral beneficiation methods for enriching waste dunite material in olivine. The results from the tests outlined here are encouraging and support the conclusion that waste material can be reclaimed effectively.

Table 7b. Results of dense media separation tests for sample GM1 for 2-5,6 mm grain size

BD=3,10 g/cm3

SINKS FLOATS %LOI 2,01 7,43 %Recovery

1 30,6 69,3

%Serpentine 7,0 19,0 %Carbonates 2,0 6,0 % Olivine

2 75,0 59,0

BD=3,00 g/cm3

SINKS FLOATS %LOI 2,96 9,53 %Recovery

1 57,4 42,6

%Serpentine 9,5 21,4 %Carbonates 2,6 8,3 % Olivine

2 71,9 54,3

BD=2,90 g/cm3

SINKS FLOATS %LOI 4,14 10,58 %Recovery

1 78,3 21,7

%Serpentine 10,1 23,1 %Carbonates

4,0 8,7

% Olivine2

69,9 52,2 1. mass recovery, 2 .minimum value

Table 8. Results of shaking table process

Products %R %LOI %Ol. %S. No1 9,32 3,05 60 12 No2 24,01 4,55 62 12 No3 10,71 5,90 54 21 No4 10,40 6,05 53 27 No5 38,82 6,3 52 16 No6 6,74 7,25 51 16

TOTAL 100,00 R: recovery, Ol: olivine, S: serpentine

The metallurgical results show that it is possible by using a feed-waste material with just 55% olivine content, to obtain a product with a higher percentage of olivine (~75%), by means of economical and environmentally friendly methods.

Depending on the required quality of olivine product for each specific application (grain size, purity, etc) and in relation to the beneficiation process cost-benefit analysis, an assemblage of proper processing routes can be applied to achieve the desired result.

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