OPTIMIZATION OF SALT CRYSTALLIZATION PROCESS BY SOLAR ENERGY WITH THE USE OF MIRROR REFLECTION , CASE OF CHOTT MEROUANE EL-OUED ( SOUTH EAST OF ALGERIA )

Purpose. This paper aims to improve the harvesting conditions of the crystallized salt layer of the Salins Merouane El Meghaier (SME) – South East of Algeria, by creating favorable conditions for means of harvesting (harvesters), thanks to the acceleration of evaporation-crystallization process of salt by using an installation of flat mirrors, which reflect solar radiation towards the evaporating surface. Methods. To achieve the objectives, a stall installation contains pans equipped with different mirror surfaces. Compared with other designs, this test unit is installed near the Chott during the months of December and January. Findings. The optimization rate of salt evaporation-crystallization process depends on the surface of the reflection mirror used, which allows obtaining a layer of soft salt easy to harvest during the winter months. Originality. The use of mirrors reflecting solar radiation in salt pans of the unit in Salins Merouane El Meghaier enables to improve the salt exploitation conditions in quantitative, qualitative and economic terms, and to minimize the occupation of agriculture area. Practical implications. The exploitation of solar energy for salt production at the unit in Salins Merouane El Meghaier represents a free source, which is inexhaustible and produces no harmful impact on the environment.


INTRODUCTION
Solar energy, which is characterized by a lack of pollution and its availability, has attracted much attention in recent years.Operating systems that utilize this form of energy require little maintenance and have a good operating reliability, increasing autonomy, extreme resistance to natural conditions (temperature, humidity, wind, corrosion, etc.), and therefore a long service life.It therefore appears that solar energy can provide real solutions to the various problems that currently arise with regard to climate change, energy crises (Yettou, Malek, Haddadi, & Gama1, 2009).Kasedde, Lwanyaga, Kirabira, & Bäbler, (2015) conducted analysis of literature describing experimental studies and modeling oriented towards the use of solar energy to improve evaporation of brine.Abdel-Aal & Al-Naafa (1993) studied the increased evaporation of salt water in multi-purpose solar desalination units using flat-plate solar collectors.Their results showed that the contribution of concentrated solar energy was estimated to be 3.5 times that of the direct solar flux allowing the separation of soluble inorganic salts such as NaCl, MgCl 2 and the others.Zhang, Ge, Li, & Li (1993) presented a simulation model for the evaporation of brine by solar radiation for salt production where the various parameters involved in the behavior of the salt table were analyzed.Huang, Shi, & Ge (1999) studied the effect of a black insulating sheet on the rate of evaporation from a shallow salting table .The results proved that the rate of saline table evaporation increased by 10%.In another attempt, Tamimi & Rawajfeh (2007) modeled the thermal performance of solar evaporation basins loaded with the Dead Sea water.The model demonstrated that the efficiency of a solar evaporator is limited by the optical absorption capacity of saline water as the upper limit.Zeng et al. (2011) develop ed a strategy for improving solar evaporation using magnetic particles absorbing floating light.In their survey, evaporation was improved by a factor of 2.3 in solar evaporation of 3.5% water.Diaz, Stewart, & Brownson (2012) provide an analysis using concentrated solar thermal energy systems to assess the increased efficiency of sea salt production in southern Spain.The conclude that the improved system could evaporate water six to ten times faster than the natural evaporation process.More recently, Horri, Nan, Chen, & Wang (2014) modeled the floating solar evaporation process assisted by light absorbing materials.The model shows that the evaporation rate can be improved by approximate factors of 2.3, 2 and 1.8 when using 0.045, 0.023, and 0.015 g of light absorbing material, respectively.
The objective of the present study is to optimize the crystallization process at the lake Merouane unit (National company) in El Meghaier, in order to adjust the quantitative and qualitative plan according to the needs of the market.It is based on the acceleration of evaporation speed using solar energy harnessed in the installation of planar mirrors which is designed to capture the solar radiation and reflect it towards the surface of the brine (Bounouala, Remli, & Talhi, 2015).

GEOGRAPHICAL SITUATION AND HYDRO-GEOLOGY OF THE CHOTT MEROUANE
Chott Melghair (34° 15'N, 06° 17'E) and Chott Merouane (34 10.63'N, 6° 17.32'E) are the largest salt lakes in Algeria.These Chotts are located in the northeast of the northern Sahara and are part of the Melghair Chott basin in south-eastern Algeria covering large area of 551500 ha to 337700 ha.The altitude of these Chotts is considered to be the lowest in northern Africa, and in some places is 31 m below sea level.The Melgair Merouane Chotts constitute a vast strip that extends from southern Tunisia to the Atlas Mountain in northern Algeria (Mahowald, Bryant, del Corral, & Steinberger, 2003).
Evaporation of the Chotts, especially during the dry season, gives rise to salt crystals composed mainly of sodium chloride, from 0 to 10 cm big, which makes the zone an important source of salt mineral.Other minerals identified on the surface of the Chotts include gypsum, calcite, and clays (Hacini, Kherici, & Oelkers, 2008).
Due to the combined effect of evaporation and incoming water, the Merouane Chott experiences annual cycles of lake filling and complete evaporation.It is fed by three main water sources, namely the Oued Righ Canal, which also drains local urban water, groundwater from the terminal complex aquifer and precipitation (Fig. 1) (Hacini, Kherici, & Oelkers, 2008).The 150 km long canal drains the waters up to Chott Merouane.The total amount of water drained through the Oued Righ Canal was estimated at 131.5×10 6 m 3 in 1994 (Hacini, Kherici, & Oelkers, 2008).It is fed, according to Ballais (2010), by collector water from baths and oases.
The annual share of the groundwater supplying Chott Merouane was estimated at 62×10 6 m 3 (Unesco, 1972).While the contribution of precipitation, according to Hacini, Kherici, & Oelkers (2008), to the water supply of the Chott was estimated by the meteorological station of Touggourt at 4.9×10 6 m 3 .

Studies of Chott Merouane climatic parameters
Through a series of observations of the climatic parameters of Chott Merouane (1975Merouane ( -2010)), we can establish the following patterns of natural physiochemical conditions impact (Fig. 2): -average monthly evaporation (Fig. 2a) occurs according to a simple pattern, showing decrease towards the winter season and increase approaching the summer season; this fluctuation of evaporation is governed by the temperature variation (Enasel, 2011); -average monthly precipitation (Fig. 2b) shows that January is the most watered month with 19.14 mm and July is the driest month with 0.87 mm; -average monthly temperature (Fig. 2c) shows that July is the hottest month, with recorded 33.50°C, and January is the coldest month with average temperature of 10.30°C.The influence of temperature on the brine chemical quality is felt during the summer period, when we observed concentration of elements resulting from evaporation; -wind speed (Fig. 2d) is stable at about 3 m/s, except for two months (March, July), when it increased from 3.31 to 4.13 m/s; -the humidity of the air (marked by two distinct periods -Figure 2e).Thus, the period from October to February is relatively humid, with more than 50% humidity, growing from 51.69% to a maximum of 65.85% in December.The period from March to September is dry, with humidity decreasing to a minimum of 34% in August; -the duration of insolation (the region receives a very high amount of sunlight during all months of the year (Fig. 2f), with a maximum of about 358 hours in July and a minimum of 229 hours in January (Enasel, 2011).

The solar radiation of El Oued region
El Oued region is characterized by a high level of solar potential.As shown in Figure 3, this region receives an average amount of annual solar radiation at 4.85 kWh/m 2 with a total insolation period of 3900 h/year.
It can be seen that in winter the region obtains less solar energy with the average daily solar radiation varying between 3.17 and 3.77 kW h/m 2 .Solar energy amount becomes very high between April and September, when the average daily solar radiation varies from 4.93 to 7.54 kWh/m 2 (Hadj Ammar, Benhaouaet, & Balghouthi, 2015).

Experimentation
In the present study, the test was conducted during December and January of 2016 -2017.This period represents the first filling of the Chott by the brine.The test started on 12.12.2016and lasted to 01.02.2017.The five plastic pans under observation (Fig. 4) were buried in the ground to avoid heating of their external walls by solar radiation.
They were installed near the Chott Merouane and filled by brine with a thickness of 120 mm each.Two pans P1 and P2 were equipped with flat simple mirrors (31.49% of the brine surface for SM and 77% of the brine surface for GM1), while the third was a control pan P0, with installed mirrors capturing the sun rays and reflecting them towards the surface of the brine.From 09:00 in the morning until 16:00 in the evening, the mirrors followed the movement of the sun by manual adjustment of their angles to the sun position (azimuth and elevation) Figure 5.
During the test period, we conducted daily measurements of wind speed, humidity and especially evaporation in mm, which was done using the ruler fixed in the wall of each pan (Evaporation = the thickness of the brine yesterday -the thickness of today's brine).In this work, we present the results for the control pan P0, and those with large and small mirrors -P1 and P2.The other results for B3 and B4 pans will be studied later (Table 1).Chemical analysis of the brine taken from Chott Merouane showed that the concentration of the halite is important when it reaches 313.304 g/l (Table 3).

Figure 6. Layer thickness curves (salt + brine) of P0, P1 and P2
The spacing of these curves in comparison with the brine and crystallized salt curve of the control pan increases with time until the final crystallization of salt (formation of a crystallized layer 40 mm thick).We also observed an increase in thicknesses of 2 mm during the day 11.01.2017caused by precipitation of rain (2.5 mm).
From the measurements taken during the test, the average wind speed is 1.13 m/s and the average humidity is 36%.The excess temperature generated by the reflection of the solar rays thanks to the plane mirrors directed towards the brine surface in the two pans appeared in the curves (Fig. 7) due to the increase in the evaporation rate of about 1 mm/day compared to that of the control pan.
The amount of water in the brine of P1 and P2 is sufficient to stop evaporation on 21.01.2017 and 23.01.2017, on the other hand, evaporation in the control pan P0 continues until 01.02.2017.
Thus, formation of a crystallized salt layer suitable for harvesting with a thickness of 40 mm in each pan took 52 days for P0 and 41, 43 days for P1, P2 respectively.Therefore, we received a gain of 9 days thanks to the SM, and 11 days thanks to the GM1, hence a rate of the crystallization process optimization was about 17% for the P2 and 21% for the P1.
During this test, we conducted hourly monitoring of the air and brine temperatures from 09:00 to 16:00 on 28.12.2016.The results are shown in Table 5.The curves of the brine temperatures in P1, P2 in comparison with the brine in P0 (Fig. 8) suggest that there is a gap symmetrically from 1 to 3 C° caused by the influence of the solar rays reflection thanks to the plane mirror.Their increase is based on the elevation of the air temperature curve.

Chemical analysis of crystallized salt
The crystallized salt samples obtained from the test pans were analyzed at the laboratory of unit SME and recorded in the following summary Table 6.It is found that almost all chemical analyzes of salt in the pans are identical where the halite content is between 95.80% and 95.97%.So it can be said that solar radiation has no influence on the quality of the salt.

CONCLUSIONS
With low humidity and favorable wind speed, the test conducted on Chott Merouane suggests that there is an influence of excess temperature generated by the concentration of solar rays on the surface of brine in the pans, accelerated evaporation rate due to the reduced length of exposure compared to the control pan.Then, optimization of salt crystallization by solar energy comprises: -17%, if the mirror surface represents 31.49% of the brine surface in the pan; -21%, if the mirror surface represents 77% of the brine surface in the pan.
This optimization was carried out during December and January.It means that in winter, the coldest season in Algeria, the sky at times is covered with clouds which hinders the performance of mirrors in comparison with the spring and summer seasons.

Figure 4 .
Figure 4. Photograph of the experimental brine pans on Chott Merouane-El Oued

Figure 5 .
Figure 5. Projection of solar radiation towards the pan

Figure 8 .
Figure 8. Temperature curves for brines and air