Lime is an important and versatile chemical used in a wide range of industrial and other applications. The term lime, which strictly refers to calcium oxide (CaO), is applied to a range of products arising from the grinding, calcination and hydration of limestone and dolomite. Many less developed countries do not have adequate lime production and this leads to problems associated with under-utilisation of lime. In particular, insufficient application of agricultural lime can lead to soil acidification, with associated aluminium / manganese toxicity and poor crop yields.
As part of the UK Government’s commitment to provide technical assistance to developing countries, a project was initiated to help solve this problem. The BGS/DFID project “Local development of affordable lime in Southern Africa” (R6492) carried out under the DFID Knowledge and Research (formerly the Technology Development and Research) programme aimed to encourage the development of low-cost lime for agriculture and water treatment. As part of the project, carbonate resources were matched with appropriate lime production technology to provide a methodology for the establishment of local production units.
The project was undertaken in collaboration with the Zambian Geological Survey Department (GSD), who carried out field sampling and laboratory evaluation of carbonates, and Intermediate Technology Zimbabwe (ITZ), who reviewed existing small-scale lime production practices in Zimbabwe. The project focused on agricultural lime production in Zambia, following the recommendations of a recent survey of the lime industry of the Southern African Development Community (SADC) region (AUSTROPLAN, 1990). The project was sub-divided into three main activities:
1) Lime evaluation programme
2) Small-scale lime production research
3) Dissemination
The aim of the lime evaluation programme was to identify carbonate resources suitable for use as lime close to the point of need. A ‘market survey’ was carried out to determine the demand for lime, its quality, quantity and where it is most needed. The findings indicated that Zambia has a suppressed demand for agricultural lime, especially in the Northern provinces and those provinces more heavily cultivated. Agricultural lime consumption is held back by its poor availability and relatively high cost.
A review of the carbonate resources of Zambia identified numerous dolomitic carbonates that are potentially suitable for the production of agricultural lime. Most of the carbonates occur in the Basement Complex and the Katanga Supergroup, along the line of rail from Livingstone up to the Copperbelt and North-Western provinces, as well as smaller deposits in Luapula, Northern and Eastern provinces.
Appropriate laboratory evaluation test methods were identified to determine the properties that are critical for the effective use of limestone and dolomite as agricultural lime, as follows:
i) Plant nutrient content, especially calcium & magnesium oxide content
ii) Neutralising ability, important for amelioration of soil acidity
iii) Particle-size distribution, a principal factor in controlling the effectiveness of agricultural lime
iv) Agronomic effectiveness, the rate at which agricultural lime reacts to neutralise soil acidity
v) Ease of pulverisation (or grindability), an important factor in the production of ground limestone & dolomite
A carbonate-sampling programme resulted in the collection of 35 samples from approximately 20 sites across Zambia. All the samples were evaluated, firstly for their mineralogy and major element chemistry at the BGS, and secondly for their plant nutrient content, neutralising ability, agronomic effectiveness and ease of pulverisation at the GSD. The test results were used to “screen” the samples, those with Neutralisation Values higher than 80% calcium carbonate equivalent (CCE) and more than 6% magnesium oxide were considered to be suitable for use as agricultural lime.
The aim of the small-scale lime production research was to identify a low-cost method for the small-scale production of lime. A review of small-scale lime production practices in Zimbabwe was carried out by ITZ. The most appropriate small-scale method for the production of aglime would probably involve contract extraction, manual crushing and dressing, mechanical milling and manual bagging. The cost of producing aglime using a small-scale method would be approximately US$29 per tonne (1997 prices). Also, the Technology Development and Advisory Unit (TDAU) at the University of Zambia (UNZA) carried out a small-scale lime production trial. A bulk sample of dolomitic carbonate from Mkushi was milled using the TD hammer mill and the resulting product was sufficiently fine grained to be used as agricultural lime.
As part of the dissemination process, a workshop was held at the Pamodzi Hotel, Lusaka in February 1998. At this meeting representatives of Government Ministries & research institutes, as well as UNZA and the farming community, met to learn of the project findings and also to discuss the ‘way forward’ for small-scale lime production and use.
Recommendations for the evaluation and production of lime are summarised below:
i) A survey of the lime market and a review of the carbonate resources is recommended in order to identify appropriate local production sites.
ii) Identification of suitable laboratory test procedures and a lime evaluation programme is recommended in order to identify those limestone & dolomite samples that are suitable for use as lime.
iii) Identification and testing of production methods is recommended in order to develop the most appropriate procedure for the small-scale production of lime
