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Gold has been known to the ancients and highly valued since
prehistoric times; it has been used in ornamentations and rituals. Processing
of gold ore requires first the ore to meet certain head grade, known as a
cut-off grade. The cut-off grade is the grade below which the mineral within an
orebody does not contain a sufficient value to be recovered at a profit. The
set cut-off grade varies from one gold mine to another. Usually, gold ores
mined from different orebodies have different characteristics such as hardness
and grade. The fluctuations in the ore grade have a significant impact on the
final product. In order to minimize the grade fluctuations for ores mined from
different sources, the gold-bearing ores are usually blended. In the Mineral
Processing field, the process of mixing together ores coming from different
sources to obtain one product with certain grade and other characteristics such
as hardness is known as ore blending.

The Geita Gold Mine (GGM) is one of the large-scale gold mines
operating in Tanzania. The mine is located in Geita district of the Geita
region.

The Geita Gold Mine conduct its mining
operations at about five sites namely, Kokuruma, Geita Hill East, Geita Hill
West, Nyankanga and Star-and-Comet. Kokuruma is currently an open pit site, as
well as the Geita Hill East and West; Nyankanga and Star-and-Comet are both
open pit sites but with underground mining still in development. Mineralogy
variations are often exhibited in the run-of-mine gold-bearing ores mined from
different sources and sometimes in the ores mined from the same orebody. The nature
of gold ores mined from the mentioned mining sites is summarized in Table 1.1.

Table 1.1:
Sources of gold ore at Geita Gold Mine

No.

Source of gold ore

Nature of the ore

1.

Kokuruma open pit

Carbonaceous ore

2.

Geita Hill East open pit

Oxide ore

3.

Geita Hill West open pit

Oxide ore

4.

Nyankanga open pit and underground

Oxide – sulfide ore

5.

Star-and-Comet open pit and
underground

Oxide – sulfide ore

The mineralogy of the gold ore at Geita Hill East
(and West) has an oxide facies banded iron formations (BIF), and gold is
disseminated in the hematite matrix. The gold ore mined from Nyankanga is
oxidized near the surface, but with mineralogy variations to arsenic-rich
sulfide (that is, arsenopyrite) when going deep down the Earth surface.
Moreover, the gold ore mined at the Kokuruma open pit is a carbonaceous gold
ore, that is, it contains significant amount of carbon materials that tend to
adsorb gold-cyanide complex during gold leaching and thus lead to gold losses. At the moment, the gold ore mined from
Kokuruma is not being processed.

The gold-bearing
ores mined from different sources are placed in “fingers” at the run-of-mine
(ROM) pad, where they can be blended to minimize variations in the ore
mineralogy and obtain one product with certain quality of feed (mainly the head
grade and hardness) prior to ore processing.

Normally, at the Geita Process Plant,
the gold-bearing ore mined from Star-and-Comet and Nyankanga are blended at
various ratios (but most commonly 25:75 in percent). The desired optimal grade
of the gold ore can be calculated using Equation 1.1. The Mine-Geo Engineers
determine the blending ratio of the gold ore mined from two different sites
before they are subjected to mineral processing techniques. Blending of the
gold-bearing ore is usually accomplished by mixing a high-grade (at least 5g/t)
gold ore with a low-grade (about 1g/t) gold ore.

 

(1.1)

Although the
desired optimal grade of the gold-ore blend is attained, the targeted recovery
value of about 91.5% is not achieved upon processing of the ore blend. It is
common that gold recoveries usually in the range of 88% up to 90% are obtained.
This is mainly attributed to the presence of significant sulfide minerals in
gold ore mined from Star-and-Comet, which tend to consume cyanide and oxygen during
gold leaching in the carbon-in-leach (CIL) circuit.

 

 

1.1.      PROBLEM STATEMENT

Currently, the mine rely upon
processing the gold ore from Nyankanga and Star-and Comet. The gold ore mined
from Nyankanga is normally a low-grade ore at a grade of as low as 1.4g/t as it
was determined by Société Générale de Surveillance (SGS)
laboratory in August 2017. On the other hand, the gold ore from Star-and-Comet
is a medium to high-grade ore at a grade of about at least 4.42g/t, but with
significant cyanide and oxygen consumers that tend to consume significantly the
cyanide and oxygen during gold leaching and thus adversely affect the process.
The gold ores from the two sources are usually blended at the ROM pad to obtain
a feed to the process plant that upon leaching, usual gold recoveries of about
88% up to 90% are obtained. The optimal blending ratio is currently not known
and the ores are blended at various ratios. Thus, in order to meet the goals of
production set by the Geita Gold Mine, the targeted leaching recovery value of
at least 91.5% must be achieved by leaching the gold-ore feed determined at an
optimal blending ratio.

 

1.2.      OBJECTIVES

1.2.1.      MAIN OBJECTIVE

This project aims at determining an
optimal blending ratio of the gold-bearing ore from Star-and-Comet and
Nyankanga that will yield leaching recovery of at least 91.5%.

 

1.2.2.      SPECIFIC OBJECTIVES

       
i.           
To
determine the statistical influence of the blending ratio, sodium cyanide
concentration and hydrogen peroxide concentration and their interactions on the
leaching recovery of gold,

      
ii.           
To
develop a regression model that will relate the percentage leaching recovery of
gold to percentage blending ratio, concentration of sodium cyanide and hydrogen
peroxide,

    
iii.           
To determine
the factors settings (that is, levels of blending ratio, sodium cyanide and
hydrogen peroxide) that will give acceptable higher gold recoveries of at least
88%, and

To conduct a cost analysis associated with
blending of gold ore from Star-and-Comet and Nyankanga, as well as that of the
leaching reagents (that is, sodium cyanide

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