ENGLISH ABSTRACT: Leaching of nickel laterite was conducted with a solution of ammonia and ammonium carbonate in a closed vessel. The vessel used in this study was designed to leach and perform solid-liquid separation at the same time. For solid-liquid separation, stainless steel sintered metal filter media were used. The sintered metal filter medium was selected for its high strength to withstand pressure, chemical resistance to caustic solution and back flushing properties.
Optimum leaching conditions were determined by varying temperature, ammonia concentration, ammonium carbonate concentration and oxygen pressure. After leaching and filtration, the pH of the leach liquor was measured and samples were analyzed for dissolved metals (Ni, Fe and Co) using atomic absorption spectrophotometry.
The most significant variable effect on leaching of nickel was the ammonia concentration. The maximum dissolution of nickel from the unroasted ore was 11.90% at 4 M NH3, 100oC, 2 M (NH4)2CO3 and 2 bar O2 pressure. Optimization from the leaching data was done using response profiling and desirability in Statistica software. Optimum leaching conditions were determined to be 3 M NH3, 2 M (NH4)2CO2, 100oC and 2 bar O2 pressure. The mineralogy of the ore before and after leaching was studied to understand why nickel extraction from unroasted ore was poor. XRF analysis of solids after leaching showed that iron, silicon, and magnesium remained the same. The only metal which showed significant decrease from solids was nickel. XRD analysis of solids after and before leaching showed that most mineral phases present in the ore are not affected by the leaching solution. SEM with EDS detection was used to determine nickel distribution within the ore. The results showed that nickel is mostly associated with iron. The iron is surrounded by magnesium and silicon. Silicate minerals do not react with ammonia and ammonium carbonate solution.
From filtration experiments, the filtration differential pressure had no significant effect on the filtration rate. An average filtration rate of 0.29±0.07 ml/min.cm2 was obtained. The filtration rate from these experiments was very low. The main reason was due to quick pore clogging of sintered metals. Pore clogging was found to be mainly on the surface of the filter medium. Laterites have been found to have low permeability due a lot of clay present in the ore. Rheological studies on this ore showed that the ore has shear thickening behavior. However, a very clear filtrate was obtained. After each leach and filtration experiment, the sintered metals was unblocked by back flushing with water and air. Back flushing was successful because all 18 experiments were carried out using the same sintered filter medium.
The effect of roasting the ore prior to leaching was investigated using optimum conditions obtained when leaching the unroasted ore. There was a slight improvement in nickel extraction when the ore was roasted. The average percentage extraction of nickel from 3 experimental runs was 19.25%±0.19 at 100oC, 3M NH3, 2M (NH4)2CO3, and 5 bar oxygen pressure. Some part of nickel in the ore was unextractable due to association of nickel with recrystallized silicate minerals in the reduced ore. Roasting improved permeability of the ore. The filtration rate improved significantly after roasting the ore. The average filtration rate was 2.60±0.05 ml/min.cm2.
Dissolution kinetics of the unroasted and roasted saprolitic laterite were investigated with regard to the effects of temperature, ammonia concentration, ammonium carbonate concentration, and oxygen pressure. For the unroasted ore, it was found that dissolution rate and degree of nickel extraction increases with increasing temperature. Increase in ammonia concentration improves the degree of nickel extraction. Nevertheless, nickel extraction does not depend entirely on ammonia concentration because even when ammonia concentration is high and ammonium carbonate concentration is zero nickel extraction is low. An increase in ammonium carbonate concentration also increases the degree of nickel extraction. Ammonium carbonate is critical for the extraction, since ammonium ions in the solution prevent hydrolysis of the nickel ammine complex. Oxygen did not have a significant effect on the degree of nickel extraction. The leaching of nickel laterite was found to be a two stage leaching process. In the first stage, the dissolution of nickel is faster but after 15 minutes, the reaction rate is reduced. The reaction rate is reduced by inert minerals which host nickel. These minerals contain iron magnesium and silicon. The fast dissolution of nickel in the first stage represents leaching of free nickel in the ore. The data for the second stage of leaching was analyzed by the shrinking core model, and the results suggested that the dissolution rate is controlled by mixture kinetics (ash layer diffusion and surface reaction control). The activation energy for the dissolution reaction was calculated as 56.5 KJ/mol. The reaction order with respect to ammonia and ammonium carbonate were determined to be 0.3 and 0.26 respectively. For the roasted ore, the highest degree of nickel extraction was obtained at 60oC, 3M NH3, 2M (NH4)2CO3, and 5 bar oxygen pressure. The percentage extraction under these conditions was 28.7%. Temperature did not have a significant effect on the leaching rate. An increase in NH3 and (NH4)2CO3 increased the final extraction of nickel but did not have any effect on leaching rate in the first stage of leaching. In the absence of ammonium carbonate, nickel extraction is almost zero. The experimental data did not give linear fit to the shrinking core models investigated for the unroasted ore. The reason for this could be due to the sampling time interval which was too far apart, or the leaching behavior of roasted nickel is complicated and cannot explained by shrinking core model alone.
Leaching experiments demonstrate that for a high degree metal extraction and improved reaction kinetics with ammonia and ammonium carbonate, the solution temperature should be high (>100oC) for the unroasted ore. In order to leach at high temperature with ammonia and ammonium carbonate a closed vessel is required to prevent reagent loses. The reaction kinetics showed that the reaction is controlled mostly by ash layer diffusion; this indicates that a low degree of nickel extraction in the unroasted saprolitic laterite is due to inert minerals (ash layer) which host nickel within the ore.
In order to obtain a high degree of nickel extraction, the ore needs to be roasted under reducing conditions. Roasting conditions need to be carefully controlled to ensure high dissolution of nickel. In fact optimum roasting conditions which will give maximum dissolution of nickel, must be determined before working with the bulk of the ore.
AFRIKAANSE OPSOMMING: Logingstoetse van saprolitiese lateriet met 'n oplossing van ammonia en ammonium karbonaat is gedoen in 'n druk houer. Die logingsvat vir hierdie studie is ontwikkel om die loging sowel as die vloeistof – vastestof skeiding te doen. Gesinterde metaal filter medium was gebruik vir die vloeistof – vastestof skeiding aangesien dit die volgende eienskappe vertoon; die vermoë om druk te weerstaan, die chemiese weerstand teen bytsoda oplossing, asook voordelige terugspoel eienskappe.
Optimum loogkondisies is bepaal deur die temperatuur, ammoniak konsentrasie, ammonium karbonaat konsentrasie, en suurstof druk te varieer. Na loging en filtrasie is die pH van die loogvloeistof gemeet en monsters is deur atoom absorpsie spektrofotometrie geanaliseer vir opgeloste metale (Ni, Fe en Co).
Die veranderlike wat die grootste effek op die loging van nikkel gehad het was die ammoniak konsentrasie. Die maksimum herwinning van nikkel van uit ongeroosterde erts was 11.9 % by 4 M NH3, 100 oC, 2 M (NH4)2CO3 en 2 bar O2 druk. Optimisering van die loogdata is gedoen deur die respons profiel te analiseer met Statistica sagteware. Optimum loogkondisies was bepaal as 3 M NH3, 2 M (NH4)2CO2, 100 oC en 2 bar O2 druk.
Die mineralogie van die erts voor en na loging is bestudeer om te bepaal waarom die nikel opbrengs van ongeroosterde erts so laag was. XRF analise van die vastestof na loging het gewys dat yster, silikon en magnesium nie deur loging affekteer is nie. Slegs nikkel het 'n merkwaardige afname getoon. XRD analsiese van die vastestof voor en na loging wys dat die meeste mineraal fases teenwoordig in die erts nie deur die loogoplossing affekteer is nie. SEM met EDS deteksie is gebruik om die nikkel verspreiding in die erts te bepaal. Die resultate wys dat nikkel meestal met yster assosieer. Die yster is omring deur magnesium en silikon. Silikaat minerale reageer nie met ammoniak en ammonium karbonaat oplossing nie. In filtrasie eksperimente is daar gevind dat die filtrasie differensiële druk geen noemenswaardige effek op die filtrasie tempo gehad het nie. Die gemiddelde filtrasietempo was 0.29+0.07 ml/min.cm2. Die filtrasie tempo van hierdie eksperimente was baie laag, hoofsaaklik as gevolg van blokkasie van porieë van die sinter metaal filter medium. Dit is gevind dat blokkasie van porieë hoofsaaklik op die oppervlak van die filter medium plaasvind. Lateriedes toon 'n lae deurlaatbaarheid as gevolg van die erts se hoë klei inhoud. Rheologiese studies op hierdie erts wys dat die erts skuif verdikking (“shear thickening”) gedrag vertoon. 'n Baie helder filtraat is egter verkry. Die gesinterde metale is na elke loog en filtrasie eksperiment skoongemaak deur terugspoeling met water en lug. Hierdie procedure was suksesvol, aangesien al 18 eksperimente met dieselfde filter medium uitgevoer is.
Die effek van erts roostering voor loging is ondersoek by die optimum kondisies wat verkry was vir die loging van ongeroosterde erts. Nikkel ekstraksie het effens verbeter met geroosterde erts. Die gemiddelde persentasie ekstraksie van nikkel van drie eksperimentele lopies was 19.25 % + 0.19 by 100 oC, 3 M NH3, 2 M (NH4)2CO3, en 5 bar suurstofdruk. 'n Gedeelte van die nikkel in die erts was onherwinbaar as gevolg van die assosiasie van nikkel met her-gekristaliseerde sillikaat-minerale in die gereduseerde erts. Die porositeit van die erts is verbeter deur dit te rooster. Die filtrasie tempo het merkwaardig verbeter nadat die erts gerooster is. Die gemiddelde filtrasie tempo was 2.6+0.05 ml/min.cm2.
Kinetika vir die oplossing van ongeroosterde en geroosterde saprolitiese lateriet is ondersoek, met in ag geneem die effekte van temperatuur, ammonia konsentrasie, ammonium karbonaat konsentrasie en suurstofdruk. Vir ongeroosterde erts is gevind dat die oplossingstempo en graad van nikkel ekstraksie toeneem met toenemende temperatuur. Toename in ammoniak konsentrasie lei tot 'n toename in nikkel ekstraksie, maar nikkel ekstraksie is nie alleenlik afhanklik van ammoniak nie. 'n Toename in ammonium karbonaat konsentrasie lei ook tot 'n toename in nikkel ekstraksie. Ammonium karbonaat is krities vir die ekstraksie, aangesien ammonium ione in die oplossing die hidrolise van die nikkel-amien kompleks verhoed. Suurstof het nie 'n merkwaardige effek op die totale nikkel ekstraksie gehad nie. Vir die bepaling van reaksie kinetika is 100˚C gebruik as die logingstemperatuur. Die loging van saprolitiese nikkel lateriet vind in twee stadia plaas. In die eerste fase is die oplossing van nikkel vinnig, maar na 15 minute neem die reaksietempo af. Die reaksietempo word verlaag deur inerte minerale wat teenwoordig is in die nikkel erts. Hierdie minerale bevat yster, magnesium en silikon. Die vinnige oplossing van nikkel in die eerste fase verteenwoordig die loging van vry nikkel in die erts. Die data vir die tweede stadium is geanaliseer deur die krimpende kern model, en die resultate dui aan dat die oplossingstempo deur 'n gemengde meganisme beheer word (as laag diffusie en oppervlak reaksie beheer). Die aktiveringsengergie vir die oplossingsreaksie was bereken as 56.5 kJ/mol. Die reaksieorde ten opsigte van ammoniak en ammonium karbonaat is onderskeidelik bepaal as 0.3 en 0.26.
Die hoogste graad van nikkel ekstraksie vir die geroosterde erts is verkry by 60oC, 3 M NH3, 2 M (NH4)2CO3, en 5 bar O2 druk. Die persentasie ekstraksie by hierdie kondisies was 28.7 %. Temperatuur het nie 'n merkwaardige effek op loogtempo gehad nie. 'n Toename in NH3 en (NH4)2CO3 het die graad van nikkel ekstraksie laat toeneem, maar het nie enige effek op die loogtempo gehad nie. In die afwesigheid van ammonium karbonaat het byna geen nikkel ekstraksie plaasgevind nie. Die eksperimentele data het nie 'n lineêre passing vir die krimpende kern model soos vir die ongeroosterde erts ondersoek gegee nie. Die rede hiervoor is dat die monsternemings interval te groot was, of dat die logings karakteristiek van geroosterde nikel gekompliseerd is en nie alleen deur die krimpende kern model voorspel kan word nie.
Logings eksperimente wys dat die temperatuur hoog moet wees (> 100 oC) om 'n hoë graad van nikkel ekstraksie te verkry met die ongeroosterde erts. 'n Geslote reaktor word benodig om by 'n hoë temperatuur met ammoniak en ammonium karbonaat te loog om reagens verliese te verhoed. Die reaksie kinetika word grootliks deur aslaag diffusie beheer. Hieruit kan gesien word dat 'n lae graad van nikkel ekstraksie uit die ongeroosterde saprolitiese lateriet die gevolg is van nie-reaktiewe minerale (aslaag) waarin die nikkel binne die erts bevat word.
Om 'n hoë graad van nikkel ekstraksie te verkry moet die erts onder reduserende kondisies gerooster word. Rooster kondisies moet versigtig beheer word om hoë oplossing van nikkel te verseker. Optimum rooster kondisies om maksimum nikkel oplossing te verkry, moet bepaal word voordat daar met groter hoeveelhede erts gewerk kan word.
