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All rights reserved. This article appeared in Volume 8, Number 1 of ATF.
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New Mill Design Grinds More Grain for Less Money
Grinding grain into flour is one of the most labor-intensive tasks performed by women in rural areas of developing countries. It takes about half an hour to grind a kilogram of flour by hand, but only about one minute when using a motor-driven grinding mill. It is no surprise that mills are among the most frequently requested items by women's groups in developing countries. Motorized mills would save most of these women the back-breaking chore of pounding or grinding grain for their households. However, existing milling technologies such as the conventional hammermill often fall short of the women's expectations. Several components must be imported and require frequent replacement. This raises the operating and maintenance costs of the mill, and ultimately the cost of ground flour.
A new technology, the screenless hammermill, has recently been developed at the Massachusetts Institute of Technology in collaboration with Washington, DC-based Appropriate Technology International to address these problems and make a mill which is more appropriate for use in rural areas of developing countries. Compared to conventional mills, the simpler and more energy efficient "Amtech" mill has significantly fewer components and cuts the cost of milling by over 40 percent.
The Way Mills Work
There are three commonly used types of mill, known by their mechanisms of operation: roller mills, plate mills, and hammermills. Roller mills compress and shear the grain between two rollers, with the distance between the rollers determining the fineness of flour. There may be several sets of rollers which must be precisely machined and aligned, making roller mills expensive. Plate mills grind the grain between two abrasive grinding plates, one rotating and one stationary. As in the roller mill, the plate distance is used to adjust the fineness of the flour. In some cases, the grain must be passed through the mill a second time to achieve the desired fineness. Plate mills are relatively easy to maintain, but the grinding plates wear out and must be imported at significant cost.
Conventional hammermills have rotating blades which grind the grain in a grinding chamber and pass it through a screen which separates the flour from the larger, unground particles. The operation of a hammermill is shown in Figure 2. As the blades rotate, the flour is carried through the screen by the airflow through the mill. Larger grain particles remain inside the chamber. The screens are the most fragile component of the mill; they cannot be produced locally and are expensive to replace. Furthermore, the screens often get clogged and the mill must be stopped for the screen to be cleaned out.
Developing The New Mill
In 1990, Carl Bielenberg of Appropriate Technology International (ATI) first began to work on a hammermill version which eliminates the expensive screen. He produced a prototype of the screenless hammermill which separated the flour from the grits through an opening in the circumference of the grinding chamber. As the blade rotates, it blows air outwards through the opening. The flour particles are small enough to be carried out by the air flow; the opposite direction from the rotation. The larger particles remain inside the chamber. The results were promising: the mill was able to achieve separation of the flour from the grits, but the flour quality was poor and the mill clogged easily. Encouraged by these early results and hoping to improve upon the original design, Bielenberg brought the project to MIT where a group of students took over. The group's initial design introduced a secondary chamber to separate the flour. While this eliminated the clogging problem, the throughput and collection efficiency (as measured by weight of flour versus weight of grain input) were very low.
The new model of the screenless hammermill, named the Amtech mill, has been simplified by removing the collection chamber and placing the flour outlet on the faceplate of the mill instead of on the circumference. The design takes advantage of aerodynamic separation by placing the flour outlet in an optimal location. The fine particles are carried out through the outlet by the airflow generated by a sheetmetal fan attached to the hammer, as shown in Figure 2. The larger particles remain in the mill, as the velocity of the airstream at the position of the flour outlet is not sufficient to divert their motion and carry them out. There is a discharge at the bottom of the front plate which can be opened periodically to discharge the larger particles, or grits, without stopping the mill. This makes the mill capable of producing both flour and grits, and it is not necessary to sift the product again as is normally done.
Evaluating the Amtech Mill
A prototype of the Amtech mill was developed at MIT then built and tested at the Sodefitex Cereal Transformation Facility in Tambacounda, Senegal with the help of the ATI staff. Materials lists and costs of the conventional and Amtech hammermills are compared in Table 1. After adding labor costs—estimated at 40% of material costs—the new mill can be manufactured at about 25% of the cost of the conventional mill.
In reaching the final design for the mill, the team tested many factors, the most important being the position of the flour outlet. Testing was conducted with a mill fitted with a transparent Lexan faceplate so the inner workings of the mill could be examined while it was operating. The best results were obtained with the flour outlet at about two-thirds of the radius from the center of rotation at an angular position of about 40 degrees past vertical. Samples of wet corn, dry corn, sorghum, and millet were milled and the flour quality was found to be at least as good as the conventional mill product. Women from a local village were invited to evaluate the quality of flour produced, and found it to be of very high quality. A comparison of the grain size distribution of the Amtech mill and conventional hammermill is shown in Figure 5.
The new mill operates without a screen and includes several other improvements. A single hammer blade is mounted directly to the motor shaft, allowing the mill to be run by a smaller motor without a transmission system. This decreases the cost of the mill and reduces the amount of energy it consumes from about 180 watt-hours/kg for conventional hammermill to 30 watt-hours/kg for the Amtech mill. A grits discharge allows continuous operation of the mill and the fine flour is automatically separated from the coarser grits, eliminating the need to sift the final product. These improvements allow the mill to be produced locally, decreasing the energy consumption by about seventy percent while producing a superior product. These design improvements mean that over the regular life of a mill, the cost of grinding grain with the Amtech mill is reduced from 24.8 CFA/kg to an estimated 14.7 CFA/kg.
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