Fiziksel ve kimyasal ön işlemlerin lateritik nikel cevherlerinin atmosferik liçine etkisi
The effect of physical and chemical pre-treatments on atmospheric leaching of a lateritic ore
Yazar: HÜSEYİN BAŞTÜRKCÜ
Danışman: PROF. DR. NESET ACARKAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Cevher Hazırlama Mühendisliği Ana Bilim Dalı / Cevher Hazırlama Bilim Dalı
Derece: Doktora, 2016
Konu: Maden Mühendisliği ve Madencilik = Mining Engineering and Mining

Özet:
Teze konu olan cevher Manisa-Turgutlu'nun Çaldağ yöresinden temin edilmiştir. Yapılan kimyasal analizler sonucunda numunenin %1,2 Ni, %24,77 Fe ve %0,062 Co içerdiği saptanmıştır. Modal mineralojik analiz sonuçlarına göre ise, numunede nikel ve kobalt taşıyan fazlar olarak; limonit grubu, Fe-Mn Hidroksitler, Fe-Mg-Mn hidrosilikatlar, Fe-Mg hidrosilikatlar, Fe-Cr-Al hidrosilikatlar, Mg'lu killer,  Fe-Mg'lu killer, yüksek düzeyde silis içeren killer; gang mineralleri olarak hematit, manyetit, kromit grubu mineralleri, kuvars, feldspat, kil mineralleri (klorit, montmorillonit, smektit, halloysit), kalsit, dolomit ve muskovit tespit edilmiştir. 
Kimyasal zenginleştirme deneyleri öncesinde dağıtma + boyuta göre sınıflandırma ve yüksek alan şiddetli yaş manyetik ayırma (Jones ayırıcısı) deneyleri yapılmıştır. Her iki ön zenginleştirme deneyleri sonucunda da bir ön konsantre eldesi ya da temiz bir artık atma olanağı bulunamamıştır.
Ön zenginleştirme deneylerinin ardından kimyasal zenginleştirme deneylerine geçilmiş ve doğrudan çözündürmeyi kapsayan proses (DÇP), asitle pişirmeyi kapsayan proses (APP) ve mekanik aktivasyonu kapsayan proses (MAP) olmak üzere 3 farklı proses incelenmiştir. 
DÇP'de yapılan deneyler sonucu, 74 µm altındaki numune ile 80oC pülp sıcaklığında, %25 pülpte katı oranında, 250 g/L H2SO4 konsantrasyonunda ve 8 saat sürede yapılan çözündürme deneyinde %75 Ni, %60 Fe ve %85 Co çözünme verimlerine ulaşılmıştır. Liç sonunda elde edilen yüklü çözelti, nötralizasyon sonrasında Fe çöktürme deneylerine tabi utulmuştur. 80oC sıcaklık, pH 3 ve 45 dk reaksiyon süresinde gerçekleşen deney sonunda %99,8 Fe çöktürme verimi elde edilirken, %1,2 Ni ve %1,5 Co kaybı gerçekleşmiştir. Demiri uzaklaştırılan çözelti üzerinde Ni,Co kazanımına yönelik birlikte çöktürme deneyi yapılmış (pH 8 ve 20 dk reaksiyon süresi) ve sonucunda %30,42 Ni ve %1,75 Co içerikli çökelek elde edilmiştir.
APP deneylerinde ise, liç öncesinde 74 µm altındaki numune, 200oC sıcaklıkta asitle 60 dk boyunca pişirilmiş ve bu ürün 700oC sıcaklıkta 15 dk süre ile kavrulmuştır. Pişirme+kavurma ön hazırlık işlemlerinin ardından kalsine ürüne 30 dk sürede su ile çözündürme uygulanmıştır. Çözündürme sonunda %83,6 Ni, %25,8 Fe ve %91,7 Co çözünme verimlerine ulaşılmıştır. Bu yüklü çözelti üzerinde pH 3'te, 60oC sıcaklıkta ve 30 dk sürede Fe çöktürme deneyi yapılmıştır. Demirin %99,6'sı çöktürülürken, demiri uzaklaştırılan çözelti üzerinde Ni ve Co kazanımına yönelik birlikte çöktürme deneyi uygulanmıştır.  Bu deney sonucunda %33,27 Ni ve %1,9 Co içerikli çökelekler elde edilmiştir. 
MAP deneylerinde, kimyasal zenginleştirme öncesinde cevherin ekzantrik titreşimli değirmen ile mekanik aktivasyonunu kapsayan ön hazırlık işlemi uygulanmıştır. Optimum mekanik aktivasyon süresi (2 saat) tespit edildikten sonra kimyasal çözündürme deneylerine geçilmiş ve 30µm altındaki numune ile 85oC pülp sıcaklığında, %30 pülpte katı oranında, 300 g/L H2SO4 konsantrasyonunda ve 2 saat sürede yapılan çözündürme deneyi sonunda %80,6 Ni, %72,2 Fe ve %84,5 Co çözünme verimlerine ulaşılmıştır.Yüklü çözelti nötralizasyonunun ardından pH 3'te 90oC sıcaklıkta ve 45 dk sürede Fe çöktürmesi uygulanmış, sonucunda demirin %99,7'si çöktürülmüştür. Ni-Co yüklü çözelti ise, diğer prosesteki uygulamalardan farklı olarak, solvent ekstraksiyona tabi tutulmuş, bu işlem sonucunda Ni ve Co yüklü 2 ayrı çözelti elde edilmiştir. Bu çözeltilerin elektroliz ünitesine gönderilerek daha yüksek kazanç ve katma değer sağlayacak metal ürünlerin eldesi mümkündür.

Abstract:
There is a growing interest in improving the processing technology of lateritic nickel from huge reserves representing nearly 70% of the world nickel resources due to the declining nickel sulphide reserves. Although it is possible to process nickel sulphide ores using several methods such as flotation, the extraction of nickel from lateritic ores requires either pyrometallurgical or hydrometallurgical methods with high energy and reagent costs. While the cobalt content in the lateritic ores provides an economic advantage, the processing costs strongly depend on the mineralogy of the ore in which nickel can be hosted in a variety of different minerals.
Goethite is a major nickel-bearing mineral in many laterites, and nickel occurs in three modes: associated with amorphous or poorly crystalline goethite, weakly adsorbed by the crystalline goethite surface, and as a substituent in the goethite structure. There are four different regions of the laterite profile: limonite, nontronite, serpentine, and garnierite. Since the nickel content of all these regions varies, the extraction methods also change.
Today, basically two processing methods are used in the industry: pyrometallurgical and hydrometallurgical methods. Pyrometallurgical processes involve energy intensive techniques such as drying, calcination, and smelting. On the other hand, atmospheric leaching (AL), high pressure acid leaching (HPAL), and the Caron process are classified as hydrometallurgical processes. While the Caron process requires drying, calcination, and ammoniacal leaching, high pressure and temperature within autoclaves are necessary for HPAL. In comparison, AL is undertaken using the agitation or heap leaching methods. When hydrometallurgical methods are compared to each other, AL has considerable advantages over HPAL such as lower CAPEX and energy consumption, simpler process control, and lower maintenance costs. However, higher acid consumptions, more voluminous residue in tank leaching, slower extraction kinetics, ensuring heap permeability in heap leaching and lower nickel/iron ratios are disadvantages of AL .
Various methods have been tried to improve nickel extraction kinetics and selectivity over iron to reduce acid consumption in AL. These comprise increasing temperature or leaching time, controlling redox potential, adding various chemicals and pre-treatment. However, all these tests performed appear to produce no desired improvement.
In order to improve leaching of nickel from laterites, mechanical activation can be used as an alternative method. Mechanical activation, which is not just fine grinding, creates fresh surfaces leading to the significant changes in physicochemical properties of the material. However, the aim of fine grinding is just size reduction and it has an intermediate position between coarse grinding and mechanical activation. 
The mechanical activation process represents intense dislocation flows and point defects occured in the crystalline structure and provides an increase in the fresh and exposed surfaces. Also, phase transformations and various chemical reactions including oxidation-reduction and decomposition etc. can be observed.
A lateritic ore sample obtained from Caldag-Manisa (Turkey) was used in the tests. The ore sample contained 1.2% Ni, 24.8% Fe, and 620 ppm Co. According to the modal analysis, some determinations were made. Since the ore was formed as a resılt of weathering process, the nickel and cobalt bearing phases are in amophous structure and contain clays. These nickel and cobalt bearing phases were determined as limonite group, Fe-Mg hydrosilicate, Fe-Cr-Al hydrosilicate, Mg clay, Fe-Mg clay, high Si clay, Fe oxy/hydroxide, chromite-spinel (CrFeAlMg), and Fe-Al clay. On the other side, hematite, magnetite, chromite group minerals, quartz, feldspar, clay minerals (chlorite, montmorillonite, smectite, halloysite), calcite, dolomite, and muscovite were observed as the gangue minerals. Also, it should be mentioned that nearly 20% of the ore was composed of quartz, which was majorly formed of opal and chalcedony.
Before the chemical extraction processes, scrubbing + classification according to size fractions and high intenstity wet magnetic separation were tested. Both enrichment methods could not provide satisfactory results, in order to obtain a pre-concentrate.  
In terms of chemical treatment, three different extraction processes were used for the laterite sample: Direct Extraction Process (DEP), Acid Baking Process (ABP), Mechanical Activation Process (MAP).
In the DEP experiments, under the conditions of -74 µm particle size, 80oC pulp temperature, at 25% solids rato by weight, 250 g/L H2SO4 concentration, and 8 shours leaching time, 75% Ni, 60% Fe, and 85% Co extractions were achieved. After leacihng, iron precipitation was performed on the pregnant leach solution under the conditions of 80oC temperature, pH 3, and 45 min reaction time. This resulted in 99.8% Fe precipitation with 1.2% Ni and 1.5% Co  losses. The remaining solution was subjected to Ni-Co co-precipitation tests (pH 8 and 20 min reaction time). Finally a concentrate containing 30.42% Ni and 1.75% Co was obtained.  
In the ABP, digestion with sulfuric acid, roasting, leaching with water and precipitation of iron in the presence of Na2SO4 was applied on the lateritic nickel ore sample. The -74 µm sample was digested with 40% sulfuric acid at 200oC for 60 min and then roasted at 700oC for 15 min using Na2SO4. This product was leached for 30 min using just water and 83.6% Ni, 25.8% Fe and 91.7% Co extractions were obtained. Later, iron was precipitated at pH 3 and 60oC for 30 min. The remaining solution was subjected to Ni-Co co-precipitation tests (pH 8 and 20 min reaction time). Finally a precipitate was produced containing 33.27% Ni and 1.9% Co was obtained.  
Although the energy costs of sulfation and roasting, the total process time (nearly 6 hours) created advantage to the process with 0.02 Fe/Ni ratio. However, agitation leaching process takes about 15 hours considering the leaching, thickening and precipitation processes. When the total duration and acid consumption are evaluated, ABP process has the edge over agitation leaching process examples in these issues.
Also, in this study, the effect of mechanical activation of laterite ore in an eccentric vibratory mill on its atmospheric leaching was examined. In this scope, the mechanical activation time and leaching conditions were optimized. Subsequently, the whole process of lateritic nickel ore leaching was completed by applying the iron precipitation and solvent extraction (SX) steps, in order to obtain pure nickel and cobalt pregnant solutions.
After mechanical activation, it was determined that the amount of nickel and cobalt bearing clayey and amorphous Fe-Mg hydrosilicate phases decreased. But, formation of clay like compounds was observed. In this case, the mineralogical structure, which was composed of Fe-Mg hydrosilicates and determined as serpentine in XRD analysis, underwent a phase transformation. It is thought that a type of clay (Fe-Mg clay – talc) was formed as a result of this phase transformation.
The process including mechanical activation provided 80.6% Ni, 69.6% Fe, and 84.5% Co extractions under the conditions: 2 h leaching time, 1/2.5 S/L ratio, 85oC, and 300 g/L H2SO4. It should be noted that this result could be achieved under the same conditions after 8 hours without mechanical activation. This result shows that mechanical activation accelerated the leaching kinetics of the laterite sample 4 times. Another advantage of mechanical activation comes up in terms of the sulfuric acid consumptions, which were obtained as 667 kg/t and 620 kg/t H2SO4 for the non-activated and activated samples, respectively. 
The iron precipitation was carried out at 90oC using Ca(OH)2 for 60 min. At the end, 99.9% iron precipitation was achieved, however with 12% Ni and 10.8% Co losses, as it was attributed to the general adsorption phenomena. After the removal of iron, the solution contained 3.65 g/L Ni and 0.21 g/L Co. In order to separate nickel and cobalt, the solvent extractant Cyanex 272 was preferred. According to the results, 95.8% of the cobalt was extracted from the solution selectively at pH 6 in a single stage. This pH value was determined as the optimum, since 95.3% of the nickel left in the solution. Cobalt stripping tests were also performed using 200 g/L H2SO4. It was found that 98% of the cobalt could be stripped in two stages with an A/O ratio of 1/2.5. The final Co content of the solution increased to 0.50 g/L.