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1.0 Introduction:

The National Development Corporation (NDC) has the mandate of the Government of Tanzania, to promote and develop the Lake Natron Soda Ash deposit in the Loliondo District, Arusha Region. Soda Ash is a basic raw material used in the processing and chemical industries in Tanzania as well as in neighbouring countries (PTA and SADC region) and overseas.

The Lake Natron Soda Ash deposits have been known to exist for many years. The German colonialists of Deutsche Ost Africa, has indicated interest to develop the deposits at Lake Natron at the beginning of the twentieth century, at the time when British entrepreneurs were developing similar deposits located north of the Lake Natron, self styled as Magadi Soda Ash Company (Kenya) Ltd.

The adjoining maps shows that the territorial boundary separating Kenya and Tanganyika (Mainland Tanzania) passes through the northern top of Lake Natron, a short distance from Lake Magadi found in Kenya. After the 1st World War, the two countries were ruled by Britain and it was concievable that the British Company, which was exploiting Soda Ash in Kenya, would wish to extend its claims to the main Soda Ash deposits in the South, at Lake Natron. However, that extension of soda ash exploitation did not take place.

The red ellipse shows location of Lake Natron Soda Ash

After independence from Britain in 1961, the Tanzania Government sought to develop the deposits. Some preliminary studies were carried out by various consulting firms, prominent among included the Toyo Menka and Toyo Soda Company Consultants of Japan and MDPA Inginerie of France. Local organisations which were involved in the studies on Soda Ash included the NDC, State Mining Corporation (STAMICO) and the National Chemical Industries (NCI).

2.0 Geology and Environmental Aspects on Lake Natron Soda Ash Deposits:

Numerous salt lakes, saline springs and some areas of internal drainage covered by salt crust during several months of the year are found in the Central Rift Valley System of Tanganyika. Several of these have been investigated with a view of assessing their potential, as producers of common salt and other salts.

Field Investigations carried out on Lakes Natron, Balangida Gidahung, Eyasi, Balangida Lelu, Manyara and Singida have shown that the first three have great potential as commercial sources of or salts.

The Lake Natron brine is found to contain at least 8% per cent Sodium Chloride. It is supplied by hot springs of volcanic origin and by fumerolic emanations from Ol Doinyo Lengai. It is estimated that, at least 2100 Sq. km. of the North Western part of the lake is covered with a layer of mixed salts averaging 1.5 metres in thickness.

The Lake Natron is located in the Northern part of Tanzania, at latitude 2^o20’ South, and longitude 36^o 10’ East. The Northern Shore of the lake touches the territorial boundary line with Kenya.

The shape of Lake Natron is that of an elongated structure stretching roughly from North to South for a length of approximately 50 km and the East-West width of approximately 20 km. It has a total surface area coverage of about 950 sq. km.

The area around the lake belongs to the semi-desert climate, having low precipitation and high extent of dryness. It seems that the speed of evaporation at the lake is extremely high.

The geology of the areas surrounding Lake Natron indicate that the formations, until Paleogene, presented a stable land mass consisting of manly pre-cambrian rocks. Since the Neogene, a rift fault movement began to take place into the Northern direction which accompanied the activation of a number of volcanoes. The fault further developed to form a Great Rift Valley, and it seems that the volcanic activities persisted in the mean time. The Ol Doinyo Lengai, located on the South side of the lake is still as active volcano. It seems that the concave areas were further formed inside the great Rift Valey, and one of these has become the present Natron Basin.

The constituent rocks in the surrounding areas of Lake Natron are mainly pre-Cambrian basement rocks and the Volcanic rocks which were generated after the Neogene. The pre-Cambrian layer experienced severe metamorphism and folding, towards the end of the pre-Cambrian era and beginning of Palaeozoic Period (620 to 485 million years ago). Consequently, the composition of the rocks is represented by Quartzite, Gneiss, Schists, etc, and the volcanic rocks created after Neogene era Basalt, Trachyte, Nepheline, Tuff, Agglomerate, etc. However, much of the areas surrounding Lake Natron consist of the volcanic rocks.

The volcanic rocks in the surrounding areas of the lake, generally present high alkalinity. The younger the age of the volcanic rock is, the higher its alkalinity. In Ol Doinyo Lengai, which is considered to be the youngest volcano, is presenting eruption of carbonates consisting of Sodium Carbonate (Na² Co³) and Calcium Carbonate (Ca CO³).

The existence of new volcanic rocks and a number of faults in the vicinity of Lake Natron generated numberous hot springs found in the surrounding area of the lake.

3.0 Natural Soda Resources:

The occurance of natural soda reserves, reported from several places in the country can be classified into two categorises, i.e. those occuring in the form of Sodium mineral salts, such as TRONA (Na₂ CO₃ Na HCO₃ 2H₂O), and those occuring in the form of BRINE, a solution containing Carbonate, Sulphate, Sulphite and Sodium salts.

Natural Soda Ash is found, mainly in the USA and Africa. The natural Soda in the USA has become the world’s largest Soda Ash supply source e.g. Green River deposits, Wyoming.

In Africa, Soda Ash deposits has been identified in Lake Magadi, Lake Natron, Lake Balangida, Lake Eyasi, Lake Makgadikgadi of Botswana etc. Generally, the bed of the deposit consists of Brine, which is the body of the lake water, with a superimposed layer of crystalline Trona, formed due to evaporation of the water content from this brine.

The brine flow into lake Natron appears to be considerably high. The Peninj river flows in from the North West side, while the Ngare Nyiro River drains into the lake after flowing across along the border with Kenya. These two rivers have large valley areas, and part of the flow passes through forest giving those rivers a perennial flow of water.

3.1 Quality of Soda Ash:

Soda Ash is produced in the form of white hygroscopic powder or as granule, and it can be used as a final product in that form. Soda Ash with bulk density of less than 0.8% per cent is called light soda ash. Recent trends indicate increase in the demand for the Soda Ash of higher density of approximately 1.2 or more; and also having larger particle sizes.

It is not difficult to obtain high quality soda Ash. The industrially advanced countries consume only grades of more than 99% purity, which do not call for a process to eliminate impurities.

Example of Soda Ash Quality is shown in Table No. 1 below:-

Percentage %

Impurities such as Salt, mud etc. in Soda Ash, may cause corrosion of equipment and also deteriorate the quality and yield of industrial chemicals made out of Soda Ash. It follows, therefore that Soda Ash containing much impurities is unacceptable to the users.

The atmospheric pollution by fluorine, which inflicts damages on human skin and respiratory system is among one of the most serious problems. In the past the presence of fluorine was considered useful in the manufacture of glass products. However, with the emergence of the necessity for conserving environmental conditions, the dispersion of fluorine from plants into the atmosphere has become unacceptable.

3.2 Composition of the Lake Natron Soda Ash:

The Lake Natron Soda Ash deposit is essentially composed of the Crust, Brine, Water insoluble matter (mud, sand etc.).

The major components of the Crust, are Sesqui-Sodium Carbonates (Na₂ CO_3.Na HCO₃. ₂H₂O) and hydride of Sodium Carbonate (Na₂ CO₃.H₂O), which together, comprise approximately 90% by weight of the total components. The average composition of the Crust is as shown in Table II below:-

4.0 BRINE

In most places of the lake, brine exists underneath the crust. In some points close to the shore of the lake, brine sometimes does not exist. It is assumed that the brine existing area corresponds to 80% of the crust area.

Table V

Brine area 400 x mm² x 0.80 = 320mm²

Average thickness 40 cm

Average specific gravity of brine 1.3

Weight of brine 320 km² x 0.4m x 1.3 = 166.4 x 10⁶ tonnes

Weight of Sodium Carbonate 166.4 x 10⁶ x 0.201* = 32.4 x 10⁶ +

Estimates of Sodium Carbonate (Na2CO3), Common salt-Sodium Cloride (Na CI), Sodium Sulphate (Na2 SO4), Sodium Fluoride (Na F) are shown in Table No. VI below:

Table No. VI: Approximate Amount of resources contained in Soda Crust and Brine

(Unit: 10% tonnes) better "millions"

5.0 Characteristic Properties for Lake Natron Soda Ash:

Soda Ash (Sodium Carbonate unhydride) is hygroscopic white powder whose molecular weight is 105.99 and specific gravity 2.533. It has a high melting point at 851o C. and generates heat of solution in water. It has a strong degree of alkalinity. The other properties include:

5.1 Application and Uses:

The Soda Ash, as alkali Carbonates has extensive application in Industry, Households and Agriculture. The major components of the alkali carbonates include:

6.0 DEMAND AND MARKETING

Generally the demand for natural Soda Ash has been higher than the synthetic Soda Ash because of the several advantages contained in the natural Soda which include presence of some rare mineral elements. The trade on Soda Ash has been essentially restricted to the developed industrialised countries with means to produce and foster Chemical and Glass factories for which more than 70% of the Soda Ash demand is applied.

Optic fibres and fibre-glass are among the high value added products made from Soda Ash. Other important users are in Pulp and Paper industries, detergents, and water treatment for which relatively little demand can be located in developing countries such as Tanzania, Kenya, Uganda etc.

In other words, profitable Soda Ash production in the developing countries, such as in Tanzania, has to aim principally for the external market.

7.0 STUDIES OF LAKE NATRON SODA ASH PROJECT

A number of studies have been conducted on Lake Natron Soda Ash deposits since it became known to European commercial interests, in the late nineteenth century.

The Geological Surveys undertaken todate include several exploratory investigations in the area and geological mapping conducted during 1950’s. Detailed studies conducted on Lake Natron Soda Ash include the following:-

1. Study on Soda/Salt Crust of Lake Natron by N.J. Guest - Geologist, Geological Surveys of Tanganyika 1951.
2. Preliminary Survey report on Lake Natron Soda Ash deposits by Toyo Menka and and Toyo Soda Manufacturing Company Ltd. 1973.
3. Evaporation of Volcanogenic Sediments in Playa Environments of the Lake Natron Area and their Implication for Metallogenesis by Dr. Joseph Mushi of the Mineral Resources Division, Dodoma 1985.
4. The Prefeasibility Study Report on Natural Soda Development through Japanese International Corporation Agency (JICA) and carried by M/s Toyo Soda Manufacturing Co. Ltd. and Toyo Menka Keisha Ltd. Tokyo during 1978.

   The prefeasibility study revealed that there were more than about 136 million tonnes of Soda Ash reserves in Lake Natron Soda Crust and that these reserve could increase with annual exploitation of the Soda Ash deposit because several brine rivers continue to flow into the lake, a process which replenishes the reserves.

   Based on the size of the reserve, it was estimated that about 1 to 1.2 million tonnes of Soda Ash Crust could be mined annually. Most of the Soda Ash was to be exported through the Port and Railway facilities at Tanga and Arusha respectively.

5. Feasibility Study on Lake Natron Soda Ash

   Conducted by M/s MDPA Inginerie of France, during 1992 - 1993.

   This Study funded by African Development Bank on request by the National Chemical Industry (NCI). This study revealed the following:-

   That a production capacity of about of 150,000 tonnes of Soda Ash could be sustained from Lake Natron Brines out of which 80,000 tonnes could be supplied to the Internal Market, and the rest for export. It also indicated that a Caustic Soda Plant of the capacity of 50,000 tonnes per year could be constructed at Tanga out of the 70,000 tonnes of Soda Ash meant for export purposes.

   The Investment Costs for this limited capacity project was estimated at US $125 million that would also cover costs of the Road from Lake Natron to Mto wa Mbu and facilities at Site and Tanga. Cost of the Road Infrastructure covering about 118Km. was estimated at US $12.5 million (i.e. 10% of the total cost of the Project).

6. Other Studies

   Various studies had been conducted on a number of Inland Lakes in Tanzania which are known to contain Soda Ash and Salt deposits, such as: Lake Gidahung, Lake Balangida, Lake Eyasi, Lake Singida, Lake Manyara, etc; especially by Geologists of the Mineral Resources Division, Dodoma.

9.0 CONCLUDING REMARKS

Based on the previous studies carried on Lake Natron Soda Ash deposits the following conclusions could be made:-

1. That the prefeasibility and feasibility studies had proved that it is technically and economically feasible to use the Lake Natron Soda Ash for the manufacture of processed Soda Ash of good quality and quantity.
2. The Lake Natron Soda Ash generally contain from 0.4% to 1.2% Sodium Flouride, which could cause some technical problems in some of its applications and use. Extraction of Flourides from the Soa Ash will be necessary in order to enable the commodity meet International Standards of purity and universal usage.

LAKE NATRON SODA ASH DEPOSIT

Status of Soda Crust Distribution Observed in Dec. 1973

The National Development Corporation is intent on conducting a techno-economic study of the Lake Natron Soda Ash deposits, to explore its viability for commercial exploitation. NDC will provide local component of the risk capital for the project study both in cash and kind, while the external (foreign) cost component is being solicited from outside the country, so that the study can be conducted by consultants of World repute.