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A State of the Art Paper on Improving Salt Extraction from Lake Katwe Raw Materials In Uganda
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Mechanical Metallurgy.
Mechanical Engineering, Makerere University, Uganda.
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Energy Processes.ORCID iD: 0000-0001-7995-3151
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.ORCID iD: 0000-0003-1919-9964
Show others and affiliations
2012 (English)Report (Other academic)
Abstract [en]

The characteristics of Katwe salt lake are briefly discussed. The lake is the largest of the eight saline lakes in the Katwe-Kikorongo volcanic field and is a major source of salt production in Uganda. Today, salt production at the lake is carried out using traditional and artisanal mining methods. Attempts to mechanize the production of domestic and commercial grade salt at the lake were unsuccessful due to the use of a wrong technology. In this paper, the most common available technologies for salt extraction from brine are described. These are divided into four broad categories, namely thermal, membrane, chemical and hybrid processes. A review of the state of the art, previous research and developments in these technologies is presented. A detailed analysis of the processes used was done based on studies reported in the literature. From the analysis, it was observed that thermal salt production processes, especially distillation and solar evaporation have the highest share in installed capacities worldwide. Membrane technologies such as Electro-dialysis, Reverse Osmosis and chemical technologies have not found wide application in the commercial salt industry. Electro-dialysis and Reverse Osmosis have been used mainly as pre-concentration processes for subsequent thermal processes. Prospects for application of hybrid systems for salt production through integration of thermal desalting processes should be investigated for better performance efficiencies and recoveries at the salt lake.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2012. , 28 p.
Keyword [en]
Lake Katwe, salt, salt recovery and purification, separation processes
National Category
Chemical Process Engineering
Identifiers
URN: urn:nbn:se:kth:diva-103348OAI: oai:DiVA.org:kth-103348DiVA: diva2:559865
Projects
Improving Salt Extraction at Lake Katwe, Uganda
Note

QC 20121010

Available from: 2012-10-10 Created: 2012-10-10 Last updated: 2015-12-17Bibliographically approved
In thesis
1. Characterization of Raw Materials for Salt Extraction from Lake Katwe, Uganda
Open this publication in new window or tab >>Characterization of Raw Materials for Salt Extraction from Lake Katwe, Uganda
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Uganda is well endowed with economic quantities of salt evident in the interstitial brines and evaporite deposits of Lake Katwe, a closed saline lake located in the western branch of the great East African rift valley. Currently, rudimentally methods of salt mining based on solar evaporation of brine continue to be used for salt extraction at the lake. These have proved to be hazardous and unsustainable to the salt miners and the environment. In this work, literature concerning the occurrence of salt and the most common available technologies for salt extraction is documented. Field studies were undertaken to characterize the salt lake deposit and to devise strategies of improving salt mining and extraction from the salt lake raw materials. The mineral salt raw materials (brines and evaporites) were characterized to determine their physical, chemical, mineralogical, and morphological composition through field and laboratory analyses. In addition, laboratory extraction techniques were undertaken to evaluate possibilities of future sustainable salt extraction from the lake deposit. Also, PHREEQC simulations using Pitzer models were carried out to determine the present saturation state of the lake brine and to estimate which salts and the order in which they precipitate from the brine upon concentration by evaporation.

Results reveal that the raw materials from the salt lake contain substantial amounts of salt which can be commercialized for optimum production. The brines are highly alkaline and rich in Na+, K+, Cl-, SO42-, CO32-, and HCO3-. Moreover, they contain trace amounts of Mg2+, Ca2+, Br-, and F-. The lake is hydro-chemically of a carbonate type with the brines showing an intermediate transition between Na-Cl and Na-HCO3 water types. The evaporites are composed of halite mixed with other salts such as hanksite, burkeite, trona etc, with their composition varying considerably within the same grades. The laboratory extraction experiments indicate that various types of economic salts such as thenardite, anhydrite, mirabilite, burkeite, hanksite, gypsum, trona, halite, nahcolite, soda ash, and thermonatrite precipitate from the brine of Lake Katwe. The salts crystallize in the order following the sequence starting with sulfates, followed by chlorides and carbonates, respectively. Moreover, thermodynamic modeling in PHREEQC accurately predicted the solubility and sequence of the salt precipitation from the lake brine. Understanding the sequence of salt precipitation from the brine helps to control its evolution during concentration and hence, will lead to an improved operating design scheme of the current extraction processes. The work providesinformation towards future mineral salt exploitation from the salt lake.

 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. vi, 35 p.
Keyword
Lake Katwe, salt extraction, brine, evaporites, characterization, PHREEQC
National Category
Chemical Process Engineering
Identifiers
urn:nbn:se:kth:diva-134708 (URN)978-91-7501-767-9 (ISBN)
Presentation
2013-12-13, Sal Kuben, Brinellvägen 23, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Sida - Swedish International Development Cooperation Agency, Phase III Uganda
Note

QC 20131129

Available from: 2013-11-29 Created: 2013-11-27 Last updated: 2013-11-29Bibliographically approved
2. Towards the Improvement of Salt Extraction from Lake Katwe Raw Materials in Uganda
Open this publication in new window or tab >>Towards the Improvement of Salt Extraction from Lake Katwe Raw Materials in Uganda
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Uganda is well endowed with economic quantities of mineral salts present in the interstitial brines and evaporite deposits of Lake Katwe, a closed (endorheic) saline lake located in the western branch of the great East African rift valley. Currently, rudimentally and artisanal methods continue to be used for salt extraction from the lake raw materials. These have proved to be risky and unsustainable to the salt miners and the environment and they have a low productivity and poor product quality. This work involves the investigation of the salt raw materials that naturally occur in the brines and evaporites of Lake Katwe. The purpose is to propose strategies for the extraction of improved salt products for the domestic and commercial industry in Uganda.

The literature concerning the occurrence of salt and the most common available technologies for salt extraction was documented. Also, field investigations were undertaken to characterize the salt lake deposits and to assess the salt processing methods and practices. The mineral salt raw materials (brines and evaporites) were characterized to assess their quality in terms of the physical, chemical, mineralogical, and morphological composition through field and laboratory analyses. An evaluation of the potential of future sustainable salt extraction from the lake deposits was done through field, experimental, and modeling methods. Moreover, the mineral solubilities in the lake brine systems and dissolution kinetics aspects were investigated.

The results reveal that the salt lake raw materials contain substantial amounts of salt, which can be commercialized to enable an optimum production. The brines are highly alkaline and rich in Na+, K+, Cl-, SO42-, CO32-, and HCO3-. Moreover, they contain trace amounts of Mg2+, Ca2+, Br-, and F-. The lake is hydro-chemically of a carbonate type with the brines showing an intermediate transition between Na-Cl and Na-HCO3 water types. Also, the evaporation-crystallization is the main mechanism controlling the lake brine chemistry. These evaporites are composed of halite mixed with other salts such as hanksite, burkeite, trona etc, but with a composition that varies considerably within the same grades. The laboratory isothermal extraction experiments indicate that various types of economic salts such as thenardite, anhydrite, mirabilite, burkeite, hanksite, gypsum, trona, halite, nahcolite, soda ash, and thermonatrite exist in the brine of Lake Katwe. In addition, the salts were found to crystallize in the following the sequence: sulfates, chlorides, and carbonates.

A combination of results from the Pitzer’s ion-interaction model in PHREEQC and experimental data provided a valuable insight into the thermodynamic conditions of the brine and the sequence of salt precipitation during an isothermal evaporation. A good agreement between the theoretical and experimental results of the mineral solubilities in the lake brine systems was observed with an average deviation ranging between 8-28%. The understanding of the mineral solubility and sequence of salt precipitation from the brine helps to control its evolution during concentration. Hence, it will lead to an improved operating design scheme of the current extraction processes. The dissolution rate of the salt raw materials was found to increase with an increased temperature, agitation speed and to decrease with an increased particle size and solid-to-liquid ratio. Moreover, the Avrami model provided the best agreement with the obtained experimental data (R2 = 0.9127-0.9731). In addition, the dissolution process was found to be controlled by a diffusion mechanism, with an activation energy of 33.3 kJ/mol.

Under natural field conditions, the evaporative-crystallization process at Lake Katwe is influenced by in-situ weather conditions. Especially, the depth of the brine layer in the salt pans and the temperature play a significant role on the brine evaporation rates. With the optimal use of solar energy, it was established that the brine evaporation flux can be speeded up in the salt pans, which could increase the production rates. Moreover, recrystallization can be a viable technique to improve the salt product purity. Overall, it is believed that the current work provides useful information on how to exploit the mineral salts from the salt lake resources in the future.

 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. ix, 69 p.
Keyword
Lake Katwe, salt extraction, brine, evaporites, saltpan, characterization, evaporation - crystallization, solubility, Pitzer ’s ion - interaction model, PHREEQC software, dissolution kinetics, Avrami model.
National Category
Chemical Process Engineering
Identifiers
urn:nbn:se:kth:diva-179445 (URN)
Public defence
2016-01-20, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Sida - Swedish International Development Cooperation Agency
Note

QC 20151217

Available from: 2015-12-17 Created: 2015-12-16 Last updated: 2015-12-18Bibliographically approved

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